Short-Wave Manual, Sixty Tested Wireless Circuits, Wireless Trans-mission for Amateurs, Radio Training Manual, Superhet Manual and Radio Engineer's Vest Pocket Book, which are uniform in style and obtainable from the publishers of this present volume. F. J. CAMM.
PRACTICAL WIRELESS SERVICE MANUAL. Bridges. —Modern coils are wound to definite inductance values, and it is possible for turns to become short-circuited ...
PRACTICAL WIRELESS SERVICE MANUAL
CA M M
PRACTICAL WIRELESS SERVICE MA NUAL
A Complete Work on the Testing of all Types of Wireless Receivers, and the Remedying of
Faults in Them
EDITED BY
F. J. CAMM
(Fellow. Royal Society of Arts) Editor of " Practical Wireless," " Practical Television," " Practical Motorist," " Practical Mechanics," " Practical Engineering"
and "The Cyclist
3rd Edition
WITH 221 ILLUSTRATIONS
LO N D O N
GE OR GE NE W NES LI MITE D
TO WER HOUSE, SOUTH AMPT O N STREET, STRA N D, W. C.2
I. _A ?
First Published 1938 Second Edition February 1940 Third Edition January 194 1
JOHN RYLPI4 litavERsorry7 U M W at
MA DE AN D PRI NTE D IN GREAT BRITAIN BY JARROLD AN D SONS LTD. NOR WICH
PRACTICAL WIRELESS SERVI CE MANUAL
/ IMP ORTA NT GUIDE t
TO SUCCESSF UL A
RADI O CAREERS W
This modern guide indicates clearly how al engaged in the Wireless industry can equip themselves for safe and highly-paid posts, not only during hostilities, but in the face of the keener competition which will come with a return to peace-time conditions.
"ENGINEERING OPPORTUNITIES" contains 208 pages of highly useful information. It gives full details of our very co mprehensive series of modern WIRELESS COURSES, including General Wireless Engineering, Talking Picture Engineering, Television and Radio Servicing, etc.
RADI O SERVICI N G A N D TELEVISI O N
Our two courses in modern Radio Servicing and Television are particularly popular. The Servicing Course deals with every phase of work covered by %. the modern Radio Service man, both from the technical and purely practical aspects, whilst Radio Engineers who are thinking ahead are giving close consideration to the study of Television. There is no doubt about it that with Peace will come an enormous boom in Television, and the man who isequipped with the most up-to-date knowledge of Television practice and principles will find himself in aposition to skim the cream of this new market.
"ENGINEERING OPPORTU NITIES" also explains the most efficient way of preparing candidates for such recognised Examinations as A. M.I.E.E., A. M.I. W.T., A. M.I.Brit.R.E., City and Guilds, etc.
W e definitely Guarantee --" N O PASS -- N O FEE"
The Wireless industry is pregnant with possibilities. In your own interests you should write for "ENGINEERING OPPORTU NITIES" to-day. There is no cost or obligation of any kind and the book is bound to indicate many opportunities which otherwise you might miss. A postcard would bring the book by return.
BRITIS H INSTIT UTE OF EN GI NEERI N G TEC H N OL O GY
160 Shakespeare House, 11 19 STR ATF OR D PLACE, LO N D O N, W.1
PREFACE
IHAVE received in the course of the last five years many hundreds of requests, not only from the readers of my journals, but also from service engineers all over the country, for a book dealing entirely with the testing of wireless receivers of all types, the tracing of faults, methods of remedying those faults, the elimination of extraneous noises, the tracing of interference, and the hundred and one other matters concerned with ensuring that a receiver functions satisfac-
torily and economically. I have, therefore, produced this volume with the idea of serving alike the needs of the service-man engaged
in the industry as well as the needs of the amateur. Special chapters are devoted to methods of simple testing, suitable
for amateurs who do not possess elaborate meters and testing equipment, whilst other chapters are designed to appeal to the professional
man. I have endeavoured to deal with every aspect embraced by the
title of the book, and the information is drawn from along experience
of the troubles experienced with every type of receiver. The servicing of radio receivers has become arecognised profession,
and good salaries are paid to capable people who are able rapidly to diagnose the faults and to apply the necessary remedies. Ihope that this volume will serve professionals and amateurs as auseful tool to be brought into regular use in the course of their work. I have presumed that readers will be aware of the elementary principles of receiver design, although the instructions are designed to enable faults
to be located and cured without such knowledge. A good deal of the material in this book has been prepared as adirect
result of suggestions made by readers of Practical Wireless, in which jou rnal some of the matter has already been published. It is here
re-issued as a permanent reference and consultant. Readers are reminded that the Free A thrice Bureau of Practical Wireless is available on any subject relating to wireless not covered in this book, which is a companion volume to The Practical Wireless Encyclopaedia, Everyman's Wireless Book, Wireless Coils, Chokes and Transformers, Newnes Short- Wave Manual, Sixty Tested Wireless Circuits, Wireless Transmission for Amateurs, Radio Training Manual, Superhet Manual and Radio Engineer's Vest Pocket Book, which are uniform in style and
obtainable from the publishers of this present volume. F.J. CAMM.
CON TE NTS
CH APTE R
PREFACE .
rAG I
ABBREVIATIONS .
THEORETICAL SYMBOLS
CHOICE AND TYPES OF INSTRUMENTb
FAULT TRACING WITHOUT INSTRUMENTS
D.C.MULTI-RANGE MILLIAMMETER
40
MEASURING RESISTANCE .
46
MEASURING A.C.VOLTAGES
50
MEASURING CAPACITY
55
USING THE UNIVERSAL METER
61
A VALVE-VOLTMETER.
68
IX .CALIBRATING AND USING THE VALVE-VOLT-
METER
73
X.AN L.F.OSCILLATOR . .
78
XI .CALIBRATING AND USING THE L.F.OSCILLATOR 83
XII .A SIGNAL GENERATOR
89
XIII .TRACING FAULTS IN A SUPERHET
99
XIV.TRIMMING AND ALIGNING RECEIVERS
106
XV.TESTING VALVES
13
XVI . VALVE REPLACEMENT
18
XVII .REACTION FAULTS .
123
XVIII .IMPROVING OLD SETS
129
XIX.UNIVERSAL (A.C.--D.C.) RECEIVER FAULTS
132
XX.CHECKING RECEIVER PERFORMANCE . .
135
XXI .DISTORTION --CAUSES AND CURE
140
vii
PRACTICAL WIRELESS SERVICE MANUAL
CHAPTER
XXII .TRACING SOURCES OF INTERFERENCE
.
XXIII .TEMPORARY REPAIRS AND SUBSTITUTION .
XXIV. ADAPTING MILLIAMM ETERS .
XXV. RENOVATING CABINETS .
XXVI .SERVICING WITH THE CATHODE- RAY TUBE
XXVII .TRACING AND ELIMINATING Hum
XXVIII .SIMPLE TESTS FOR COMPONENTS
XXIX .ADJUSTING AND TESTING COILS
XXX.SERVICING COMMERCIAL RECEIVERS
XXXI .SECOND-CHANNEL INTERFERENCE
XXXII .CHECKING PERFORMANCE .
XXXIII .BACKGROUND NOISES
.
XXXIV. LOUDSPEAKER FAULTS AND REMEDIES
XXXV.EQUIPPING A SERVICE W ORKSHOP
XXXVI .W IRELESS CALCULATIONS
XXXVII .COLOUR CODES .
INDEX
.
PAGE
146 156 159 162 165 180 191 204 212 236 241 244 247 251 264 274 285
A B B R E VI A TI O NS
A. --Anode, or plate.
A.A. --Artificial aerial.
A.C. --Alternating current.
AE. --Aerial.
A.F. --Audio frequency.
A.F.C. --Automatic frequency
control.
A.G.C. --Automatic gain control.
A.M. --Amplitude modulation.
A.T.C. --Aerial tuning condenser.
A.T.I. --Aerial tuning inductance.
A.V.C. --Automatic volu me con-
trol.
A.V.E. --Automatic volume ex-
pansion.
B.A. --British Association.
B.C.L. --Broadcast listener.
B.E.M.F. --Back Electromotive
Force.
B.F.O. --Beat frequency oscillator.
B.O.T. Unit. --Board of Trade
unit = 1,000 watt-hours, or
1kilowatt hour.
B.T.U. --British thermal unit.
B. W.G. -- Birmingham
wire
gauge.
C.C.C. --Closed circuit or second-
ary condenser or S.T.C.
C.C.I. --Closed circuit inductance
or secondary tuning induc-
tance or S.T.I.
C.G.S. --Centimetre -gramme -
second system of units.
em. --centimetre.
C.P. --Candle power.
C. W. --Continuous waves.
D.A.V.C. --Delayed A.V.C.
db.--decibel.
D.C. --Direct current.
D.C.C. --Double cotton covered.
D.E. --Dull emitter. Det. --Detector.
D.F. --Direction finding, or direc-
tion finder.
ix
D.P. --Difference of potential. D.P.D.T. --Double pole double
throw.
D.P.S.T. --Double pole single throw.
D.S.C. --Double silk covered. D.X. --Long distance. E.---Earth. E. M.F. --Electro-motive force. F. --Filament. F.M. --Frequency modulation. F.P.S. --Foot-Pound-Second.
G. --Grid. G.B. --Grid battery or grid bias. G.C. --Grid condenser. G.L. --Grid leak. H.F. --High frequency (same as
radio frequency). H.F.C. --High-frequency choke. H.P. --Horse power. H.R. --High resistance. H.T. --High tension. I.0.---Intermittent current. I.C. W. --Interrupted continuous
waves. I.F. --Intermediate frequency. I.P. --In primary (of trans
former) ; start of primary.
I.S. --In secondary (of transformer) ;start of secondary.
kw. Kilowatt = 1,000 watts. L.F. --Low frequency. L.F.C. --Low-frequency choke, or
low-frequency coupling. L.R. --Low resistance. L.S. --Loudspeaker. L.T. --Low tension. mA. --milliampere. M.C. --Moving coil. mfd. --micro-farad. mhy. --microhenry. m m. --millimetre. mmfd. --miclo-micro-farad. 03. --Outside foil. 0.L. --Output load.
PRACTICAL WIRELESS SERVICE MANUAL
THEORETICAL SYMBOLS
71,am
SG
VILQuILS 46, 5 G
pv .,rop ,
Douse t 0, eve reap&
Do w,a Gloom. Ps/. mos
AWNS tivoos
Seti/wAahrosCoo
AaiNS PAW MOZ
VAmmtur Ks Avo ws
*Pra m
.cPerPOPci)f
TiNooe //smoor
/awl 1641.1114144
SO
aveos Paorrnoa O6:1a5r :as
CAA'si r Pavia !JAM
HA"' WAPZ oeferVP/Sit S71a.tz',waPsaw
·7'" "Ildws A rno w,
RANra,,im·trr,c,,err,o,,,
tg
7.ivreafroaatav
NfrWfs_ICtatATraafms
+Hi
§
A rs , C. et
e.,
[Aar a ptr
of
gsocr Ca w.
C
Cd .
ThAvagan...0
A rouna usto art: maga
FAIarl
eaRrit
+>. 1 T
Strin ws
++
J S,
I fl
+ Cc;,:gf.s.r.e
raer° Il;teraay
A wert,
A.·····
Ag.tii;i4
=no.
X
SYMBOLS
0.P. --Out primary (of transformer) ; end of primary. Also output.
0.S. --Out secondary (of transformer) ;end of secondary.
P. Plate, or anode. P.A. --Public address.
P.D. --Potential diference, same as D.P.
P.M. --Permanent magnet.
Pot. --Potentiometer. P.V. Power valve. Q.A.V.C. -- Quiet automatic
dume control.
Q Code. --See p. vii. Q.M.B. --Quick make and break.
Q.P.-P.--Quiescent Push-pull. R.F. Radio frequency (same as
high frequency). R.M.S. Value. -- Root -mean -
square value. Rx. Receiver. S.C.C. --Single cotton covered.
S.H. M. -- Simple harmonic motion.
S.I.C. Specific inductive capacity.
S.P. --Series parallel. S.P.D.T. --Single -pole double
tlirow.
S.P.S.T.--Single-pole single throw. S.R. Specific resistance.
S. S. C. --Single silk covered. S.T.C. -Secondary tuning con-
denser. S.T.I. Secondary tuning in.
Iuctance. S. W.G. --Standard wire gauge. S. W.L. --Short-wave listener.
T.R.F. -Tuned radio frequency. T.S.F. (French abbreviation for
Wireless -- "Telegraphie sans Fils " (telegraphy without wires). T.T. --Tonic train. Tx. --Transmitter.
SY M B O LS
Admittance .
. .
Y
Amplification Factor
IL
Capacity .
C
Current
I
Conductance
Dielectic Constant
Efficiency ·
S .·
Energy
. .
. .
E.M.F. (voltage) .. ·.
Electrostatic Flux Density
Frequency .. Impedance .
·
Magnetic Flux
Magnetic Flux Density
Magnetic Field
Power
· ·
Period Time
Permittivity
Phase Angle
Quantity
Reluctance
Resistance
Resistivity
Reactance
Sel-inductance
UNITS
Ampere
. .
. A
Ampere-hour
Ah
Board of Trade Unit. 1,000
watt-hours or 1 kilo-
watt-hour
. 13. T. Unit
Coulomb
·· C
Farad
·· F
Henry
. H
Joule Kilovolt-ampere Ohm Volt .. Volt-ampere. Watt Watt-hour
PREFI XES
Kilo.. Mega..
.. k
Micro
. M
MUl .
xi
.··kVAJ
.. SI .. V .. VA
W ..Wh
IL
.
CHAPTER I
CHOICE AND TYPES OF INSTRU MENTS
AITHOUGH a qualified service man may carry out all his work with the aid of a single instrument, this involves considerable calculation for various types of test and consequently leaves room for errors to creep in which may nulify the results of his tests. A properly equipped service workshop, on the other hand, wil contain a fair number of instruments, each of which is designed for a specific task and thus servicing amodern receiver is not only simplified, but it is asimple matter instantly to test any component, and replacements are thus made only where essential. In addition to time-saving, the acquisition of acomplete set of test equipment will result in money-saving, and the work will be more eficiently carried out --and a customer's satisfaction will mean more work. A good service-man should build up a business in which goodwill plays a most considerable factor, and recommendations from one customer to another will soon enable the initial cost of equipment to be covered and thus greater profits will accrue.
Fin. r.--This is a typical
Valve Tester --one of the Wearite range of Service
Instruments.
13
PRACTICAL WIRELESS SERVICE MANUAL There are many firms now
specialising in the production of test equipment, and the ilustrations in this chapter show a representative collection of various types. For complete servicing there are various ranges of apparatus from the simple galvanometer to the cathode-ray apparatus by means of which the performance of any receiver may be demonstrated, not only for your benefit, but for the benefit
FIG. 2. -- The Thermionic or ValveVoltmeter has very wide uses. This is a Cambridge instrument of this type.
FIG. 3. --A Ferranti Multi-meter, with accessories.
14
CHOICE AND TYPES OF INSTRUMENTS
of the customer. The galvanometer is merely an indication of current and as such may form the basis of various instruments. A milliarnmeter also gives a reading of current and forms the basis of various test instruments. By a simple application of the familiar Ohms Law this instrument may measure voltages, or resistance in
ohms, and a single
instrument may thus
be built round it as
described in later chap-
ters. In addition to FIG. 4. --k gpneral purpose test instrument trom the
these simple pieces of
Electradix range.
apparatus there are bridges by means of which condensers, coils or
resistances may be accurately balanced against known standards and
their values accurately ascertained. Valves may be tested and in addition
to the location of
faults in the mechan-
ical side it is possible
by means of modern
valve testers to ascer-
tain the characteris-
tics and co mpare
these with the pub-
lished figures and
thus to judge whether
replacement is nec-
Fig. 5. --This is aValve-Volt meter for battery operation.
The General Electric Company supply this model.
essary. A mode rn
superhet
receiver
15
PRACTICAL WIRELESS SERVICE MANUAL
depends for its eficiency upon the accurate lining-up of many tuned
circuits --some receivers perhaps having as many as twenty such
circuits and the slightest maladjustment in one of these may make
all the diference in its performance. A modern signal generator
enables every tuned stage to be properly adjusted, and all guess-work
is avoided. It would be quite true to say that asuperheterodyne receiver
could not be properly adjusted without the use of an instru ment of this
type, and the work involved in endeavouring to make such adjustments
would take many hours.
The charge which a service-man makes for his work must be cal-
culated on an hourly basis,
plus the cost of replace-
ments, and thus it is essen-
tial, if a good trade is to
be built up, that the work
should be carried out
expeditiously. The man-
in-the-street does not fuly
understand the work in-
volved and if you keep a
receiver for a week or more
he doubts your ability to
carry out the work irres-
pective of any argu ment
you may put forward. On
the other hand, if you can
deliver the set properly
working in a day or two he
immediately places your
ability on a high plane and
is not slow to talk about
it to his friends and ac-
quaintances. Cases have
been recorded where a
receiver has been serviced
and after such work has
given abetter performance
than when it was new --
simply because the initial
installation was carried out
by an inexperienced man
and the set was not working
at its best when installed. It should therefore be the
aim of every service-man
F's. 6. --This is atypical set analyser, taken from the range of equipment supplied by the Weston
Electrical Instrument Company.
16
CHOICE AND TYPES OF INSTRUMENTS
to turn out the very best work and for this purpose he must have the proper equipment. Let us deal briefly with each type which he is likely to need for this purpose.
Meters. --Meters may be obtained to read current or voltage, and for those who do not wish to build up complete apparatus on the lines indicated in later chapters, a good multi-purpose meter should be the first item to be obtained. The Avometer is agood example of this type of instrument. A single open dial carries all the markings needed for
FIG. 7. --Here is another Analyser for condensers, a further Wright and Weire product.
this instrument and no calculations are necessary. Selector switches
enable the single pair of testing leads to be connected to any part of a
circuit and the current or voltage to be ascertained at a glance. Resis-
tances or components in which the resistance value is of importance may
also be tested with this instrument, and it may be used on any type of
apparatus. Models are made for D.C. or A.C. use, or for use with Universal (A.C.-D.C.) apparatus. There are, of course, many similar
instruments available, but this particular one is taken merely as an
example. In some cases the meter movement is supplied as aseparate
piece of apparatus, and the additional equipment necessary to convert
it into adevice of the nature mentioned is supplied as separate units, so
that they may be acquired from time to time and eventually all con-
nected together.
2
17
PRACTICAL WIRELESS SERVICE MANUAL
Bridges. --Modern coils are wound to definite inductance values, and
it is possible for turns to become short-circuited with a consequent
modification in the inductance value. This may not afect the working of the receiver but may make it impossible to obtain an accurate tuning
indication or introduce some other dificulty. The inductance of acoil, or of achoke or similar component may instantly be read by connecting
it to an inductance bridge. In its simplest form this will
consist of a galvano-
meter and some fixed components. The coil to be tested is merely
joined to a pair of terminals and aknob turned until the pointer shows the
inductance value. In other instru ments
coils of known value have to be joined to
another pair of terminals. In so me cases a loudspeaker
or pair of headphones
are connected to the apparatus and the
control is adjusted until a hu m or buzz in the speaker ceases,
when the value is
read of. Si milar types of apparatus
are employed for
FIG. 8.--This Cossor Oscilograph isauseful example of the cathode-ray test apparatus,
measuring the capacity of condensers or
the value of resistances, and with the aid of such apparatus unmarked or unknown components may easily be identified without waiting to communicate with the makers of the receiver.
Oscillographs. --Miniature cathode-ray tubes, housed in a neat cabinet are employed for adjusting delicate apparatus such as modern television receivers as well as for checking the performance of either receivers or amplifying apparatus. Contrary to common belief these are not dificult to operate and the makers supply very complete text-books
with them. They enable one to calculate accurately the stage gain of a
18
CHOICE AND TYPES OF INSTRUMENTS
receiver and to see instantly whether any distortion is present. Peculiar faults not revealed by ordinary tests may easily be observed by the aid of this apparatus and leaky transformers, wrong values of by-pass
FiG. 9. --An all-wave oscil-
lator. This is a Radiolab product that may be used for testing down to to metres. The batt,tries are in the small case
components and similar details are revealed in a moment. The fre-
quency-changing stage in a superhet is the most important part of the
apparatus and a
slight fault exist-
ing here can result
in a falling-of in
the performance,
perhaps only over
a very short por-
tion of the scale.
The oscilograph
will enable this
to be ascertained
and the necessary
adj ustments made
to give an equal
perfor mance
throughout the
entire scale.
FIG. to. --For trimming re-
cei r.rs this Ganging Oscillator by Cossor will prove very valuable.
Where acustomer is interested, you can show him the
19
PRACTICAL WIRELESS SERVICE MANUAL
diference when a certain change is made in the circuit, and where a question of expense is involved this demonstration may be most convincing.
Signal Generators. -- Testing has often to be carried out when no suitable programme is on the air, and a local signal is then essential. The serviceman who is not provided with asignal generator has to make use of a gramophone record or some other device and generally is not
12. --To test for insulation, etc., this Wee-Megger, by Evershed and Vignoles, will prove
very popular.
FIG. Ix.--Above isone of the popular Avometers, designed for A.C.
or D.C. use.
able to adjust the receiver on any wavelength. A generator may be used to provide a local signal on any wavelength from the highest now in use down to the shortest. The value of such a device needs no emphasis and this is probably one of the most valuable items of the complete service workshop.
Analysers. --Another type of instrument, which to a certain extent embodies certain of those already mentioned, is the circuit or set analyser. This enables many 20
CHOICE AN D TYPES OF INST RU ME NTS
-
-
--
FIG. 12a. --Two further types of instrument. Above: The Pye " Life Test" instru-
ment (mains operated) for locating intermittent faults, etc. Below: The Mullard
Service Signal Generator (mains operated).
21
PRACTICAL WIRELESS SERVICE MANUAL
component tests to be made without removing the set from the cabinet. By the use of adaptors, plugs or prods it is possible to
ascertain the working of any individual part of the receiver, valve stage or other arrange ment. Some of these analysers are most ingenious pieces of apparatus and often enable a fault to be located more rapidly than by any other means.
Valve Testers. --These are made in various forms, but the principle is the same. The valve is plugged in, either in aholder arranged on the panel to accommodate the particular pin combination employed, or in a special separate adaptor, and it indicates the condition of the valve. In some cases adial is calibrated "Good," "Bad," or "Mediu m " and
thus you can show your customer instantly whether or not the valve is in need of replacement. Other analysers of this type indicate the resistance between elements, show anode current, filament or heater current, and all other essential details regarding the valve.
In addition to the specific instru ments so far described there are nu merous accessories. One or two firms, for instance, make aspeciality
of replacements for any type of receiver. Such items as condensers, for instance, are available in the form of replacement kits for any type of set, whilst special types of transformer may be obtained for use as substitutes or replacements.
Portable Apparatus. --The majority of the equipment so far described is of a type suitable for installation in a workshop and should be fitted as permanent gear, but there are in addition special portable instruments which may be taken out by the service-man so that a receiver may be tested in the customer's house. This will often save considerable
time by revealing a small defect which may be rectified on the spot. Special portable aerials, for instance, by being connected to a set may reveal that the aerial is at fault and thus save the trouble of having the receiver collected or delivered for tests which would prove that everything was in order. Mains-testing equipment should, of course, form part of portable equipment as many cases of breakdown may be traced to faulty mains sockets, and obstinate cases of interference may also
prove to be arriving via the mains. Various types of interference suppressor should therefore be found in the portable test equipment,
and a suitable device will then soon be found for the particular trouble being investigated. Although hardly part of the apparatus of the average service-man, interference-tracing equipment is a definite service device, and by its aid the source of some local apparatus giving rise to interference may be traced. As explained later in this book, a small portable may be employed for this purpose if desired.
The ilustrations in this chapter are merely representative and must not be taken as indicating the only suitable types of equipment. It would
obviously be impossible in the small space available to ilustrate every
manufacturer's products and therefore items which may be regarded 2
CHOICE AND TYPES OF INSTRUMENTS
as ilustrating a particular type of instrument have been selected, but firms specialising in this type of equipment wil be very pleased to render the service-man assistance in making aselection for his own particular requirements.
Multi-Purpose Instruments. --There is no doubt that a multipurpose instrument, reading milliamperes, amperes, volts and oh ms is desirable; but it is far better to purchase a high-grade miiiammeter than acheap and unbranded multi-range afair. If accuracy is the first essential --and it is when anything in the nature of research work is proposed --and when the outlay must be severely limited, ahigh-grade milliammeter cannot be bettered. It wil be of the movingcoil type and will be accurate to within about 2 per cent. This
instrument, although actually a milliammeter only, can be used with the addition of shunt and series resistances to read amperes, volts and ohms simply by buying or making suitable resistance units and connecting them as described later on in this volume.
Generally, it will be found best to obtain a meter with a full-scale deflection of 5 mA., and with a large-diameter scale which is easy to
read. If stil greater accuracy is required, it will be necessary to pay £2
or so, and for this sum a degree of accuracy within approximately 1 per cent is to be expected.
D.C. or UniversaL --The above details assume that the meter is required for measurements of D.C. only, since if it is necessary to measure A.C. as well a more expensive article must be obtained. This is because the two-purpose instrument must contain a rectifier which can be brought into circuit when dealing with A.C., and also a resistance device for compensating for the diference between A.C. and D.C. readings. In consequence, an average price for afirst-grade instru ment is something like £4. In general, however, universal current meters are made only in "multiple" types; that is, the meters are designed to include a switching system whereby various current and voltage readings can be taken. One of the best known of these multiple-range
universal meters for measuring current, voltage and resistance is the Universal Avometer, ilustrated on an earlier page, and there is another universal instrument by the same makers, known as the Universal Avominor. The latter is extremely good, and is suficiently accurate for nearly every purpose, but has fewer ranges than the larger instru-
ment. There are two other reasonably-priced popular multi-range meters
made by the same firm, these being for D.C. only, and known as the Avometer and Avominor; they are made to the same degrees of accuracy as the others mentioned above, and are priced at £8 8s. and £2 respectively. The smaller instrument is to be particularly
23
PRACTICAL WIRELESS SERVICE MANUAL
recom mended to the amateur, whilst the larger is recognised as a standard part of the equipment of the radio test-engineer.
All of the Avo instru ments are made to read current, voltage and resistance by the rotation of a switch, or by the use of alternative socket connectors.
Inexpensive
Eight-Range
Instrument.-- Other combination
instruments which are satisfactory for either experi menter or service-
man are the Pifco Rotameter and Radiometer, the various Ferranti
models at prices from £5 15s., the Wearite (several types) and
the Sifam. This short list is by no means comprehensive, and there
are several other makers of high-grade meters. Of those mentioned,
the Pifco Rotameter is of particular interest in view of the rather
unusual and ingenious method incorporated for changing from one
range to another. There are eight scales mounted on a rotor of octa-
gonal section, and by turning this so that the appropriate scale is ex-
posed the correct shunt or series resistance is automatically brought
into circuit. The instrument incorporated is of the moving-coil type,
whilst readings from 5 to 400 volts and from 10 to 250 mA. can be
taken, in addition to which there is provision for inserting a valve and testing its filament for continuity. There is a cheaper model of the
unit with moving-iron meter costing 29s. 6d.
Another Useful Universal Meter. --In the Ferranti range the
A.C. /D.C. Circuit Tester is worthy of special mention, since it
can be used to cover fifteen ranges of voltage and current. It is
guaranteed within 2 per cent at ful-scale deflection, and is provided
with test cords and crocodile clips as well as necessary shunts: additionally it is complete with neat protecting pocket case.
Choice of Meters.-- In the Wearite range particular interest at-
taches to the Multimeter, which is suitable for either A.C. or D.C., has
ten ranges and reads current, voltage and resistances from 100 to
2,500 ohms and 200 to 500,000 oh ms. Like other meters designed for resistance measurement, it is fitted with aself-contained dry cel which
has a very long life. Additionally, and this applies to all of the higherpriced instru ments, it is fitted with a safety fuse which safeguards the meter against short-circuits which may occur due to applying the
terminals to a high-voltage source without first setting the rotary switch to the correct position. It has ascale which is 2i in. in diameter, and, which is therefore very easy to read.
The Sifam unit mentioned above is for D.C. only, and covers nine
ranges with commendable accuracy. It is listed at £4 5s. and is fitted in a compact case with lid. The full scale milliampere readings are
from 2.5 to 250, and there is in addition arange up to 1amp. and four
voltage ranges from 10 to 500. This instru ment is not designed for resistance measurements, but can be used for this purpose in conjunction with an external battery.
24
CH OICE AN D TYPES OF INSTR U ME NTS
12b.
(Above) The Pye "Tri measy " Signal Generator. (Below) The Pye " Toolkit."
25
PRACTICAL WIRELESS SERVICE MANUAL
Single-Range Meters.-- With regard to meters of the single-
range type there is an extremely wide range of reliable instruments at a low price. In one make there are moving-coil milliammeters reading from 1to 200 mA., and having resistance values from 100 to .5 ohms respectively. The prices range from 35s. for the 1 mA. meter to 80s. for those reading between 20 and 200 mA. For those who require a lower-priced instrument there is a nu mber of moving-iron instruments which can be used for A.C. or D.C. at wil,and which are guaranteed accurate to within plus or minus 5 per cent. The price of any of these meters is 12s. 6d. Incidentally, there are several voltmeters and ammeters in the same range and at the same price. For those who require
a still cheaper job there are some miniature panel- mounting instruments --for volts, amperes or milliamperes --at only 7s. 6d. These are not as accurate as the others, of course, but are nevertheless reasonably good when it is required only to make normal tests.
Other instru ments of similar types are made by Ward and Golstone and Sifam, amongst others, whilst Ferranti are also makers of a very wide range of single-purpose meters of all kinds. These are generally
of the more expensive pattern, and they are made to fine limits of accuracy.
A Few Hints.-- It is obviously impossible to give even brief
details of all the meters which are on the market, so we will give a few simple rules which should be observed in choosing a suitable meter. The first is that the instru ment should be of well-known make --that is a valuable safeguard and ensures a degree of accuracy compatible with the price charged. Buy a moving-coil meter if you can aford it, and if D.C. only is to be measured. If price is an important consideration, and you have to measure both D.C. and A.C., amoving-iron meter must be obtained. In buying a milliammeter get one with a low resistance, but when buying a voltmeter see that it has a high resistance -1,000 ohms per volt is the ideal, but 250 ohms per volt is fairly satisfactory for ordinary test purposes. It is better to buy areally good single-purpose meter than a mediocre multi-range instru ment.
Resistance of Meters.-- There are many diferent types of
meter upon the market, but Iwill first explain why the cheap doublereading meter may prove misleading unless used only for particular purposes. On some of these meters it will be found that one voltage range (generally from 0 to 12) is marked in black figures, whilst the
high range (generally from 0to 120) is marked in red figures, and upon the face of the instrument will appear the letter "R," followed by a figure, on one side of the dial in black, and on the other in red. These figures indicate the resistance of the meter on both ranges, and a typical cheap meter will probably be found to have values of 200 ohms on the
low-voltage range and 2,000 ohms on the high-voltage range, and to many amateurs these figures may appear to be unimportant.
26
CHOICE AND TYPES OF INSTRUMENTS However, from the fa miliar Ohms Law we know that current is relative to voltage and resistance, and, therefore, if we have a meter reading 120 volts with a resistance of 2,000 ohms, we can see that the current passing through that resistance (the meter) will be 120 divided by 2,000, or 60 mA. How many small battery receivers have an H.T. consu mption approaching that value ? Yet, this means that if the meter is joined across the H.T. battery sockets in order to test the voltage of it, a current of 60 mA. will be drawn, and so in avery short
Fig. I2C. A Mullard Master Valve Tester (mains operated). Primarily designed for the testing of al types of valves; it has, however,
numerous other applications. time you will be taking as much from the battery as your receiver would take in many hours of listening. Apart from this, however, you will not get a true idea of the state of the battery for the following reason. Suppose that the battery has been in use for so me long time and is therefore in rather a poor state. Your receiver may consu me, say, 10 or 15 mA., and the battery may be in such a state that it can just maintain this load for another week or so without the voltage dropping unduly. It is presu med, of course, that the reader is aware that current and voltage are related, and that as the current drain rises the voltage supplied will fall off, and vice versa. Wel then, when the
27
PRACTICAL WIRELESS SERVICE MANUAL
meter is applied across such abattery, owing to the much higher current which is taken to drive the meter needle across the dial the voltage which is indicated will be much less than the actual voltage of the battery. It may be argued that this will result in the user throwing away a battery which in any case is nearly exhausted and is thus likely to give rise to trouble, but the main point is that the user is deprived of some hours of good listening due to the sudden load which has been imposed on the battery.
The Grid Battery. --In the case of the grid bias battery it must be remembered that no current is normally taken, and, therefore, if the cheap meter is used at frequent intervals an unnecessary drain is imposed and the life proportionately shortened. In the actual receiver circuit even more misleading results will be obtained, as in many cases
the current taken by the meter will be added to that already taken in that part of the circuit, and thus will give either alower voltage reading than is required, or will fail to give a reading at all. The latter trouble will occur if the voltage to the detector stage is measured, as in this
stage the normal current is only of the order of 1mA. or so, and therefore the addition of a further drain of 80 mA. will show its effect in the voltage dropped across the resistance in the anode circuit, in addition to which the detector valve may have a resistance of such a low value that it will also modify the current taken and so add to the trouble.
Suppose that it is required to know the anode current taken by the output valve. This may be about 9or 10 mA. for agood power valve, and normally included in the anode circuit will be aloudspeaker transformer primary having a D.C. resistance of perhaps 1,000 ohms, across which adrop of 10 volts would occur. If now afurther resistance of 2,000 ohms is included in series (as it would be to read the anode current), a further drop of 20 volts would occur, and this will naturally result in a wrong reading being shown.
The type of meter just referred to will only be suitable for instantaneous readings of voltage as delivered by the L.T. and H.T. batteries,
and must therefore be placed in circuit and removed as quickly as possible, and beyond this they have no further application. On the other hand, if the service-man obtains one of the better class of meters, having aresistance of 1,000 ohms per volt, this may be used to test the batteries or to take actual readings whilst the receiver is working, and
wil give a true indication of the state of afairs. A better scheme, of course, is to obtain only a very low-reading miliammeter and to use this in conjunction with various resistances in order to take readings of voltage, current, and other things, and this will give the purchaser not only much better service, but will provide him with adevice which will give many hours of interesting study in the way of showing how various components function, the efect of diferent methods of coupling, and so on. Practically all the large service testing devices have as their
28
CHOICE AND TYPES OF INSTRUMENTS
basis ameter such as this, and the money required for such an item is well repaid.
Later chapters explain the purpose and use, as well as (in some cases) the making, of other instruments, such as oscillators, resistance bridges, multi-purpose instruments, etc.
The following firms specialise in the supply of Service Equipment of
various types:
AEROVOX RADIO & TELEVISION SERVICE, 2, Sandilands Road, Fulham,
London, S. W.6. AUTOMATIC COIL WINDER & ELECTRICAL EQUIPMENT CO., LTD., Winder
House, Douglas Street, S.W .1. BALDWIN INSTRUMENT CO., LTD., Brooklands Works, Dartford, Kent. BULGIN, A. F., & CO., LTD., Abbey Road, Barking, Essex. CAMBRIDGE INSTRUMENT CO., LTD., 45, Grosvenor Place, London, S.W .1. CHORLMET RADIO ELECTRIC, LTD., Arras Mill, Fitzgeorge Street,
Manchester, 9. COSSOR, A. C., LTD., Cossor Works, Highbury Grove, London, N.5. DIXON, LESLIE & CO., 218, Upper Thames Street, London, E.C.4. ELLIOTT BROS. (LONDON), LTD., Century Works, Lewisham, London,
S.E.18. E.M.I. SERVICE, LTD., Sheraton Works, Hayes, Middlesex. EMS-HARRIS, LTD., 7, Rugby Street, London, W .C.1. EVERETT, EDGCUMBE & CO., LTD., Colindale Works, London, N. W.9. EVERSHED & VIGNOLES, LTD., Acton Lane Works, London, W .4. FERRANTI LTD., Radio Works, Moston, Manchester, 10. GAMBRELL BROS. & CO., LTD., Merton Road, Southfields, London,
S. W.18. GENERAL ELECTRIC CO., LTD., Magnet House, Kingsway, London,
W.C.2. HUNT, A. H., LTD., I3endon Valey, Garratt Lane, London, S.W .18. LELAND INSTRUMENTS, LTD., 46, Bedford Row, London, W .C.1. LEXINGTON ELECTRICAL INSTRUMENTS LABORATORIES, LTD., 155-157,
Great Portland Street, London, W .I. LYONS, CLAUDE, LTD., 76, Old Hall Street, Liverpool, 8. MARCONI-EKCO INSTRUMENTS, LTD., Electra House, Victoria Embank-
ment, London, W.C.2. MULLARD WIRELESS SERVICE CO., LTD., Mullard House, 225, Tottenham
Court Road, London, W .1. PHILCO RADIO & TELEVISION CORPORATION OF G.B., LTD., Aintree
Road, Perivale, Greenford, Middlesex. PIFCO LTD., Pifco House, Watling Street, Manchester, 4. PYE, LTD., Radio Works, Cambridge. RADIOMETERS, LTD., Dunbar Works, Dunbar Street, West Norwood,
London, S.E.27. 29
PRACTICAL WIRELESS SERVICE MANUAL ROSE, NORMAN ELECTRICAL, LTD., Waveband House, 48, Lamb's
Conduit Street, London, W.C.1. SIFAM ELECTRICAL INSTRUMENT CO., LTD., Hollydale Road, Queen's
Road, London, S.E.15. TANNOY PRODUCTS (Guy R. FOUNTAIN, LTD.), Canterbury Grove,
West Norwood, London, S.E.27. TELSEN ELECTRIC Co. (1985), LTD., Fitzgeorge Street, Collyhurst,
Manchester, 9. Tom miN, J., 17, Todd Street, Manchester, 8. UNIVERSAL ELECTRICAL INSTRUMENTS CORPN., 7, Rugby Street,
London, W.C.1. W ARD & GOLDSTONE, LTD., Frederick Road, Pendleton, Manchester, 6. W ARNER & SON, A., 201-207, Forest Road, Walthamstow, E.17. W ESTON ELECTRICAL INSTRUMENT CO., LTD., Kingston By-Pass,
Surbiton, Surrey. W RIGHT & W EAIRE, LTD., 740, High Road, Tottenham, London, N.17.
30
CHAPTER I
FAULT TRACING WITHOUT INSTRU MENTS
EVEN when ameter is available it is often better to make anumber of preliminary tests without using it--even the experienced service-engineer does not take measurements until he has formed
some kind of idea regarding the fault which exists. Moreover, all of
the preliminary tests depend more upon common sense and a general
knowledge of how the receiver operates than upon advanced technical
knowledge. Even the beginner can find out a considerable amount by making simple and logical tests.
When the Set is " Dead." --In the first place, let it be assu med
that a battery set which had previously behaved normally suddenly refuses to "work." This must be due to abroken circuit at some point in the receiver: avalve filament might have burned out; awire might
have come adrift; one of the aerial, earth, or speaker leads might have
broken or slipped away from its terminal; or acomponent might have developed a sudden fault. It is obvious that a start must be made by
carefuly examining the connections, preferably by gently "prodding" each wire with a pencil while the set is switched on. If that does not yield any result, the efect of switching the set on and of should be observed. There should be a click in the speaker when this is done;
if not, the battery connections should be suspected, whilst the efect
of removing and replacing the H.T. wander plugs should be tried. The
accumulator terminals and connections should also receive attention, and any dirt oarwacoyr.rosiSeoen sthhoatuldthebe sscpreaapkeedr
leads are properly connected to the receiver and to the speaker, and try the effect of
I V
"7 fr..eof-:A
Ib i's
FIG. 13. - The on-of switch can be checked by
short-circuiting the ter-
mthirneaels.typTehse ofmetshwiotdchforis shown here.
GB.
short-circuiting the terminals of the on-of switch with a
short piece of wire, as shown in Fig. 13. If the latter
r
causes signals to return it
will be evident that the
switch is faulty, and should
be repaired by bending the spring contacts, or else
r
Z. T
replaced.
31
PRACTICAL WIRELESS SERVICE MANUAL
Fm. 13a. --Many
/:useful tests can
Start with the Last Valve. --
btoeuchmiangde bthye Should none of these tests
grid terminal of give any indication of where
eachholvdalervein- the fault lies, and if there are turn with a no sounds from the speaker
fimnogiesrtentiepd.
when the H.T. wander plugs are pushed into the battery sockets, it
will be fairly safe to assu me
that the trouble lies with the
last valve or with the loud-
speaker itself. It is, of course,
assumed that it is known that
0 the H.T. and L.T. batteries are
in fair condition --if they were
not it is unlikely that the set
would become" dead "suddenly.
In order to test the valve properly meters are required, but a good
indication can be obtained by replacing it with one of the other valves
in the set. Should it be found that the speaker then makes even a
slight clicking noise when switching the set on and of, the valve can
be suspected. Try the efect of substituting a new valve in the
output stage so that afinal check can be made. Assu ming that there
is still no response, the speaker will come under suspicion, and can be
tested by connecting one lead to the positive terminal of the accumu-
lator and touching the other lead against the negative terminal. If the
speaker is in order there will be a fairly loud click, both as the contact
is made and broken; absence of sound suggests that the speaker is
defective. A further test might be made, however, by replacing the
accumulator with a 9-volt G.B. battery.
Systematic Grid Tests. --Despite the absence of signals, it will be often found that a click is heard in the speaker when the set is
switched on, or when the positive H.T. wander plug is inserted. It will then be evident that the fault is somewhere in the set prior to the
anode circuit of the last valve. Moisten the finger-tip and touch the grid terminal of the output valve (see Fig. 18a). A noise should be heard
in the speaker when this is done, but if not, the transformer or grid condenser and leak connected to the grid (according to whether transformer or R.C. coupling is used) is probably at fault. Repeat this test by touching the grid terminals'of preceding valves one at a time until a point is reached at which no sound whatever can be heard in the
speaker. As soon as this valve is reached it will be known that the fault lies in the circuit of that valve; the valve or one of the components connected to it must be defective. The grid-circuit components can be eliminated from the test by removing the wire joined to the grid terminal and repeating the test. If it is found that a sound
32
FAULT TRACING WITHOUT INSTRUMENTS
can then be heard the grid-circuit components are known to be the cause of trouble.
The tests described should make it possible to locate the faulty part of the set, so that assistance can be asked, or so that the suspected components can be tested separately or replaced one at a time. The necessary tests will be explained in a later chapter.
Crackling Noises.-- Instead of the set being completely "dead," we might now consider the procedure when trouble is experienced due to crackling noises. Actually, most of the tests described can be applied in the first place, but others are necessary. The first should be to disconnect the aerial and earth leads and see whether the trouble ceases; if it does the cause must have been interference from outside the set, when more thorough tests must be instituted. But should it
6:e -- Fm. i3b. --Progressive tests can be made by short-circuiting the grld circuits of diferent valves. Methods are shown for transformer and
R.C. couplings.
be found that the disconnection has no marked effect the set should again be examined for bad or loose connections. See that the valves are fitting tightly in their holders; gently press each from side to
side to see ifthe valves are loose, or ifthe holders are damaged. Tighten up the terminals on all components, and clean any that appear dirty. Make sure that the H.T. wander plugs are clean and fitting tightly,
and look for partial short-circuits between any two wires, between a wire and a screen, or between the metallised coating of a valve and a lead running close to it.
The next test should be to connect the grid terminal of the holder for the output valve to its G.B. lead, as shown in Fig. 13b. That the fault was in the output valve or the speaker would be indicated by the fact that the crackling continued, whilst silence 'would mean that the cause of the trouble was in a preceding stage. Repeat this test on other valve holders, working" backward" towards the first valve until the location of the fault is determined. That done, it remains to test
individual components and valves, as before. When this method of procedure does not produce any definite result, a fairly good test can
3
3
PRACTICAL WIRELESS SERVICE MANUAL
be applied to the speaker by connecting it to a battery, as mentioned above, but this time leaving both leads permanently connected for a
few seconds; crackling would at once point to the speaker as being at fault.
Poor Signal Strength. -- Weak signals call for altogether diferent tests, although some assistance might be gained by trying some of those already described. In this case, however, instru ments become practically essential, and it might be impossible to gain any real indication of the fault without taking measurements of voltages at the various valve-holder filament and anode terminals. At the same time it might be helpful to try replacing any resistances in the anode circuits --coupling and decoupling resistances --with others of similar or not widely difering values, since one of these might have developed a partial open circuit, so reducing the H.T. voltage. Also observe the efect of short-circuiting the H.F. choke, especially if it is found that the reaction condenser has little or no efect.
Tracing a Fault. --As an example of what can be done without the use of any instrument, the author had occasion to test aset which had been built to one of his designs. The constructor complained that all he could get was slight oscillation and weak signals with the reaction condenser advanced. The first conclusion to be drawn, therefore, was that the L.F. stages were at least working, if not eficiently. The fact that oscilation was obtained showed that the detector valve was in order, so it was assumed that the fault was somewhere in the H.F. stage. Since the coils and valves were new, it was thought that they could not be the cause of the trouble, although it was possible. On transferring the aerial to the anode of the H.F. valve, no improvement was obtained, so it was concluded that the aerial coil, and H.F. valve, were in order. The only components between the H.F. and detector valves were an H.F. choke, and a small fixed condenser. The aerial was next tried on the actual grid of the detector valve, and good signal strength was obtained. Working backwards from the grid of the detector valve to the anode of the H.F. valve via the feed condenser and the H.F. choke, it was found that signals could be obtained on either side of the condenser, and on the H. T. side of the choke, but not on the actual anode of the valve. Now the purpose of an H.F. choke being to stop H.F. currents from getting into the H.T. system and to block them so that they will be by-passed by the feed condenser to the grid of the following valve, it followed that the feed condenser had been connected on to the H.T. and not the anode side of the H.F. choke. The choke in question was of the screened type with a pigtail connection for the anode, and acommon terminal at the base for the feed condenser, and, on examination of the wiring, it was found to be as suspected, viz., that the terminal common to the pigtail had been connected to H.T. so that the H.F. currents wanted for feeding the
34
FAULT TRACING WITHOUT INSTRUMENTS
grid coil of the detector circuit were blocked by means of the H.F. choke. This accounted for the weak reaction and signals. On connecting the choke in the correct manner, signal strength, reaction, etc., were all that could be desired and the set worked perfectly. Fig. 14 shows the set as it was wrongly connected and Fig. 15 the correct connections.
Checking Connections. --First, therefore, when testing a set without the aid of any instruments, see that all connections are correct and tight, that battery voltages are correct, that wander plugs and spade terminals make good connections, that valves make good contact
Fin. 15 fright). --Showing how the choke should have been connected. The H.F. choke blocks the H.F. currents and a condenser by-passes them to the detector valve in the di-
rection of the arrows.
FIG. 14 (left). --Showing how H.F. choke connected the wrong way round stopped signals from reaching the detector valve. When the aerial was connected to points marked a signals were not obtained, but signals were received when the aerial was connected to points marked b.
NT
r
I C
in their holders (clean the valve pins with emery cloth or a fine file and open them alittle with apen-knife), and that aerial and earth are connected. If the fuse blows when the set is switched on, look carefuly for short circuits between terminals, see that the metal ends of resistances do not touch the chassis, that the H.T. is connected to the anode circuits, and not the filaments.
After giving the set a thorough examination in this way, remove the H.T. wander plug. A click heard in the loudspeaker would indicate
35
PRACTICAL WIRELESS SERVICE MANUAL
that the output valve at least was working. Advancing the reaction control suddenly should cause the set to oscillate, indicating that the
detector anode and the L.F. stages were at least in working order. Now touch the grid terminal of the detector valve holder. If the set howls, you may assume that the detector valve is in order. Similarly, by touching the anode and grid terminals of the H.F. valve in turn, a " plop " heard in the loudspeaker would indicate that these circuits were also working. Also, when touching the aerial terminal, a " plop " should be heard if the set is working. When testing in this manner, better results will be obtained if the finger is moistened.
The H.F. Stage. --If the H.F. stage seems to be at fault, connect the aerial to the anode cap of the screen-grid valve. If signals can now be heard, the fault lies either in the S.G. valve itself, the aerial coil and/or its associated condenser, or in one of the other components in the H.F. stage. If no signals are heard, then the fault must be after the H.F. stage, and the aerial should be transferred to the detector grid terminals as described above.
To test for emission needs instruments, but one can always find out to a certain extent whether the valve is functioning properly in this
respect by inserting a pair of 'phones in between the H.T. feed to the suspected valve and the anode terminal of the valve. There should be a sharp click in the 'phones on connecting them if the valve is in order (Fig. 18).
The blowing of fuses obviously indicates a short circuit or excessive current, due to a leaky condenser, etc. To test for a short circuit remove all valves and, with the H.T. and L.T. batteries connected and the switch in the "on " position, connect a flash-lamp across the filament terminals of each valve-holder in turn. A short circuit of this nature is guarded against in most modern circuits by the fuse. See
that you use one of the correct current rating. Perhaps a better way of testing for H.T. shorts, and one which it
is always wise to carry out before inserting the valves in a newly constructed receiver, is to remove the H.T. battery, and leave the accumulator connected. With one pole of the flash-lamp connected to L.T. plus of the accumulator, touch each anode terminal in turn. If the bulb is iluminated in any case, then that particular H.T. circuit is faulty and should be examined.
The voltages across the filament sockets of the valve-holders can also be tested by connecting aflash-lamp across them. If no ilumination at all is obtained, then the on/off switch may be faulty, or the accumulator discharged. If ilu mination is obtained at one or more of the valve-holders, it is a simple matter to check the remainder of the L.T. wiring.
Testing Components. --If after careful thought and systematic examination of a set as described above, results are still poor or
36
FAULT TRACI NG WIT HO UT INSTRU MENTS
unobtainable, the next step must be to test the components. Having
isolated the fault to some extent, the number of components which
require to be tested should not be many.
Under the heading of " coils," we will consider not only tuning
coils, but also H.F. chokes, transformers, and any component which
utilises a coil of wire, for the purpose of our very elementary tests
without instruments cannot do more than establish whether or not
there is continuity throughout the winding. For such a test, the usual
pair of headphones and a dry battery provides a ready means, but
unless one is rather careful, misleading results may be obtained. For
example, when testing the windings of an L.F. transformer, abreak in
the wiring would not stop a click being heard in the 'phones when they
were connected up as shown in Fig. 19, simply because the two windings
on either side of the break act as a sort of condenser and charge up,
thus producing the click. If, however, the click is quite substantial,
and occurs not only when the 'phones are connected, but also when
they are disconnected, then it is safe to assume that the winding is not
broken. The total absence of a click would indicate that the winding
had either broken away from one of its terminals, or else that the break
was very near one end of the winding.
With Flash-Lamp and Battery. --A better plan is to revert to
the flash-lamp and battery. Due to the resistance of the wire, it
will be necessary to increase the applied voltage and for this purpose
a grid-bias battery can be used, stepping the voltage up gradually until
the lamp is iluminated. The
bulb may not light at all,
owing to the high resistance of the winding, which causes
SI M
a large voltage drop across it.
Look for a small spark as the
circuit is made and broken.
When the resistance is com-
paratively low, such as in
FIG. 18. --Testing for H.T. shorts.
17. --Testing a volume control for continuity, good contact, and noise. 37
PRACTICAL WIRELESS SERVICE MANUAL tuning coils or I.F. transformers, the flash-lamp bulb and battery also ofers a highly effective and reliable means of checking continuity.
H.F. chokes can be further tested after checking continuity by connecting them in the aerial lead to the set. If signals are cut down considerably in strength, then the choke is in order. This test can be amplified by connecting the choke across the tuning condenser in a receiver. The circuit should be retuned to the station being received and the volume compared. A good H.F. choke will not cause much loss in strength.
Other high-resistance components such as volume controls and fixed resistances can also be tested by the 'phones and battery method. With volume controls, the 'phones and battery should be connected first across the two outer terminals, and then between the centre and
fir+
Fig. 18 (above). -- Connect 'phones, as shown, in each anode circuit to test valves.
FIG. 19 fright). -- Testing an L.F. transformer or other high resistance winding.
each outer terminal in turn. Then connect up as shown in Fig. 17, and gradually rotate the sliding contact, at the same time tapping the free tag of the 'phones on to the centre terminal. Then with the 'phones joined to the centre and one outer terminal, gradually rotate the control. A slight scraping noise should be heard in the 'phones, but if crackles are present, then the slider is not making good contact with the resistance proper. Carefuly take out the slider and bend it a little. Smear the element with vaseline and try out again, until no crackles are heard as it is rotated. It may be that, due to continual use, it is not possible to get the slider to make good contact at all positions of its travel. In such a case it is possible, at least where the control is
38
FAULT TRACING WITHOUT INSTRUMENTS
used to vary the bias applied to an H.F. valve, to connect a 2 mfd. condenser across the sliding contact and the top end ± that is, the terminal of the control connected to the voltage dropping resistances --not the end connected to earth) of the control to stop the crackles.
Testing Condensers. --The most common component in a receiver is a condenser. Variable condensers may be tested for shorts and noises by using the 'phones and battery, disconnecting associated coils. Gradually rotate the condenser vanes, and, when a short occurs, a loud noise will be heard in the 'phones. The remedy is either to bend the ofending vane, or to adjust the position of the moving vanes in respect to the fixed vanes by means of the usual bolt fixed at the back of the condenser. If the faulty condenser happens to be part of a ganged unit,
N. #
NFC
\· - -# L. ry·
· N T-· as.
FIG. 20. --A simple three-valve battery circuit in its essentials. Touching the lettered points (beginning at a) in the order shown should result in "plops " being heard in the speaker if the circuit is in order.
however, bending the vanes will affect ganging, and if the fault does not disappear after thoroughly cleaning with a pipe-cleaner, the condenser should be returned to the makers.
Fixed condensers are easily tested by simply connecting the m across an H.T. battery for a moment or two, and leaving them for three or four minutes. Then short the terminals by means of a pen-knife or a piece of stout copper wire. If the condenser is good and will hold its charge, a spark, depending on the capacity of the condenser, should be obtained. Quite a large spark, for example, can be obtained from a 2mfd. condenser which has been charged with a voltage of as low as 50. Small condensers of about -0003 mfd. may be tested by 'phones and battery. A loud click should be obtained, but on touching the condenser with the free end of the 'phones once more, no click should be heard. Before carrying out such a test, however, the condenser terminals must always be shorted to remove any charge. Small fixed condensers may further be tested by connecting them across the terminals of a charged 2mfd. condenser and leaving for a few minutes Having disconnected the small condenser, a spark should be obtained on shorting the terminals of the larger condenser, indicating that the small condenser was not leaky as the larger one had not lost its charge.
39
CHAPTER II
D.C. MULTI-RANGE MILLIAM METER
THE unit described in this chapter is intended as a nucleus, around which it wil be possible through the mediu m of subsequent chapters to form a complete laboratory equipment of test apparatus. Moreover, the construction is not outside the capability of the average mechanic, or service-engineer, as access to precision instruments for calibration purposes is not required. In short, the necessary series resistances and shunts can be chosen, or made at home with the aid of the instrument with which they are to be used, and the simple calibrating apparatus shown. Iwould add, moreover, that this calibrating apparatus will be used as part of the instru ment later to extend further its usefulness.
The parts required are :-- One Moving-Coil Milliammeter --full-scale reading 0.1 mA. (internal
resistance approx. 100 ohms). One Three-way switch, with low resistance contacts. One Ebonite Panel, 7 in. by 5 in. by i in. One Box wi th hinged lid (see Fig. 21). Seven Terminals. One Resistance (fixed), 10,000 ohms (1 watt). One Resistance (fixed), 100,000 ohms (1 watt). One Resistance (fixed), 500,000 ohms (1 watt).
All the above resistances to be of the composition type, and to be selected as described in .the text. Two yards 80 gauge " Eureka" D.S.C. wire. Two yards 20 gauge " Eureka " D.S.C. wire. Two small wooden spools or lengths of Fin. ebonite rod. One Resistance (variable), 5,000 ohms (wire-wound). One " On-of " Switch. One 41-volt Flash-lamp Battery.
Calibrating Apparatus. --The first step is to set up the cali-
brating apparatus, and make the shunt resistances for extending the miliamp scale to 10 mA. and 100 mA. First, connect up the meter, dry battery, switch, and 5,000 ohms variable resistance, as shown in Fig. 22, taking care that the resistance is set so that all the element is in circuit. The meter will then be found to give, with the switch in the "on" position, adeflection somewhat less than full scale. Decrease the variable resistance until 1mA. is accurately indicated on the scale, then bare the ends of the 2 yards of 30 gauge D.S.C. " Eureka " wire and attach these two bared ends across the clips. The reading of the
40
D.C. MULTI-RANGE MILLIAMMETER
milliammeter will then drop to avalue just above 01 mA. The" Eureka" wire must then gradually be shortened, testing across the clips every
inch or so, until 0.1 mA. is indicated accurately. It is the wisest plan to check that the meter still reads 1mA., when the wire is removed from the clips; if it does not, readjust the variable resistance.
When the correct length of wire has been found, double it back on
itself and wind it on one of the small wooden spools or ebonite rods, securing the ends with a binding of thread. The whole shunt may then be given acoat of shellac or immersed in hot parafin wax, and allowed to drain and dry. If the internal resistance of the meter is exactly 100 ohms, the value of the shunt just described will be 1.1 ohms.
A little extra care must be taken when calibrating the 100 mA. range,
and the switch must be opened every time the resistance wire is removed from the clips for adjustment. The procedure is as folows:
-\
connect the resistance we have just made to
- Msf,UcIDO,V)12,
)the clips, then decrease the variable resistance until the meter indicates 1 mA., open the switch \ and remove this resist-
\tance from the clips and 34. replace it with '2 yards
. of No. 20 S. W.G. " Eureka " wire, switch
on and the reading of
the meter should be ofFIG.the21.co--mApleviteedw just about 0.1 mA. Now
multi-range switch off and shorten
meter.
the wire, testing inch
by inch, taking care to
switch of every time
the wire is removed for adjustment, until 0.1 mA. is accurately
indicated. Finish off the resistance as previously, and you wil have
the two shunts extending the milliampere ranges to 10 mA. for the first
shunt and 100 mA. for the second.
Now rearrange the calibration circuit as shown in Fig. 23, and
short-circuit the clips, set the variable resistance " all-in," close the
switch, and reduce the variable resistance until the meter reads 1 mA.
Now open the clips and insert the 10,000 ohms composition resistance;
the reading should now be .1 mA. If it is not, you will probably be
able to find amongst your stock a resistance of a nominal value of
10,000 ohms, which is suficiently low to make the meter give an accurate
41
PRACTICAL WI RELESS SERVICE MANUAL
5,000n VARIABLE RESISTANCE
0-1 MI LIAMMETER
'ON -OFF ' SWITCH
43 /4 v. TORCH BA TT ERY -
··
.····
-
CLIPS FIG. 22. --Calibration apparatus for making miliampere shunts.
D.C. MULTI-RANGE MILLIAMMETER
5,000n VARIA KE RESISTANC
deflection of 01 mA. A little experiment is required to determine the correct value, so run through your stock of
resistances to find one which
will give the desired result. As an alternative, you may
find it possible to remove the outer enamel of the resistance with a pen-knife
at one end, and carefully scrape the resistance coating itself until the desired de-
flection is obtained. This will be the series resistance
for the 10-volt range which
FIG. 23. --How to calibrate th.3 is used to calibrate the re-
meter for voltage ranges.
maining two voltage ranges.
Final Ranges. --To obtain the next range, connect the 10,000
ohms resistance you have obtained in series with the milliammeter and across asource of voltage, say 9volts, from ahigh-tension battery.
If the battery is in a good condition, and it should be for calibration purposes, the meter will read .9 mA. Disconnect the 10,000 ohms
resistance and substitute the 100,000 ohms resistance. The reading of
milliammeter should now fall to -09 mA., or one-tenth of the previous reading, whatever that may have been. If it does not, find a 100,000
ohms resistance, which will give the necessary fall in current. For the final 500-volt range, leave the 100,000 ohms resistance
connected, but increase the voltage applied until a reading of, say, 9mA. is again obtained, i.e., 90 volts. Now substitute the 500,000 oh ms resistance and, if this is suitable, the meter will indicate 0.18 mA., or
one-fifth of the previous reading. Select a resistance which will give this result, and this will be the correct series resistance for the 500-volt
range. We now have all the necessary shunts and series resistances to
extend the meter ranges to 10 mA., 100 mA., 10 volts, 100 volts, and
500 volts. In addition, of course, our meter by itself will indicate 1mA.,
and .1 volt for full-scale deflection.
Assembling ·and Wiring. --All that remains is to fit the meter,
switch, and associated resistances on to the panel and wire them up as shown in Fig. 24. This should need no detailed explanation, especially
ifreference is made to the theoretical circuit in Fig. 25, and no dificulty should be experienced in using the meter if it is remembered to set the
switch always to 1mA. before taking voltage readings, and to be careful
to select the correct ranges when making measurements.
43
PRACTICAL WIRELESS SERVICE MANUAL
500v+ 100v+ 10v+ Fin. 24. --Back of panel wiring diagram.
COMMON --
With regard to the scale, you may be able to inscribe an actual
calibration for each range, but we have found no dificulty in reading
direct from the existing meter scale by multiplying the figures indicated
by the pointer by the full-scale reading of the range in use. For
instance, when on the 10-volt range a reading of .6 mA. equals 6 volts
(i.e., .6 x10), when on the 500-volt range a reading of -4 mA. equals
200 volts (i.e., .4 x500), and when on the 100 mA. range a reading of
mA. equals 50 mA. (i.e., .5 x100).
It should be emphasised that if the manufacture of the resistances
has been carefully carried out, the joints well soldered when wiring up,
and the contacts of the switch are of low resistance, the accuracy of
the complete instru ment will be of the same order as the meter itself,
and the better the meter the higher the all-round accuracy obtainable.
In a subsequent chapter it will be explained how this instru ment may be adapted for use as
adirect-reading ohm meter,
giving accurate measure-
ment of resistance for 500
to 50,000 oh ms, by the
IMA
insertion of the battery
and variable resistance
which have already been
used to calibrate the
MA
CO04011 -
4.10V +100V 4.500v
Fm. 25. --Theoretical circuit diagram of the
unit.
various ranges. Space for this addition has been provided in the box, and
44
D.C. MULTI-RANGE MILLIAMMETER the necessary terminals and means of access for adjusting the variable resistance are shown. If this addition is not required, however, the meter may be assembled as a complete unit, and will be found auseful adjunct to any amateur's or service-man's equipment.
In addition to the above conversion to an ohmmeter, a unit has been designed to attach to the instrument described herein, to enable direct readings of alternating voltage to be made. This will be also described later.
In its present state the meter will prove invaluable when carrying out quite anumber of tests connected with components and receivers, and, before proceeding with the construction of the additional units mentioned in the folowing chapters, it is advisable to carry out a series of experimental tests and thus become familiar with the operation and application of the meter.
The warning given concerning the setting of the shunt selector switch cannot be too strongly emphasised, in fact, it would be advisable to take the slight extra trouble involved and fit a fuse in series with the lead from the common negative terminal. Suitable fuses, to blow at a current value which will protect the instrument, are now obtainable, but on no account should use be made of an ordinary pocket lamp bulb which might serve such purposes in other circuits.
From time to time the switch action should be examined to see that the rotating arm is still making perfect contact with the studs, the surfaces of which should be kept clean and free from oxidisation.
When carrying out current tests, especially in the case of H.F. circuits, care should be taken to see that the connecting leads are not too long and that they do not cross over or come near the L.F. portion of the receiver. If this suggestion is not observed, it is highly probable that instability will be produced in the receiver which, in turn, will have the effect of producing erroneous readings. If such efects are created when short leads are used, then it would be advisable to connect across the input terminals, a fixed condenser of, say, 0.1 mfd.
45
CHAPTER IV
MEASURING RESISTANCE
M OST service-men have experienced at one time or another the annoyance entailed by the lack of a suitable means of determining the value of a resistor which is colour-coded only, when no copy of the standard colour code is to hand or the spot has been rubbed of the body of the resistor. Furthermore, a great many receivers incorporate wire-wound potential dividers which cannot be marked with their ohmic value, and although reference to the circuit diagra m or service instructions will usually give the correct value of the resistance element, very few listeners have any means of determining whether the sections of the potential divider are intact or even completely open-circuited.
The multi-range D.C. meter described in the previous chapter can, by the addition of the components which have already been procured
SA W-CUT IN END OF SPINDLE
5,000n VARIABLE RESISTANCE I8G ALUMINIUM BRACKET
FLEXIBLE LEADS
26. --Fixing the 5,000 oh ms variable resistance in position.
5,00015 VARIABLE RESISTANCE
27. --The extra connections to convert an ohm meter.
and used to calibrate the extended ranges, be easily modified for use as a direct reading ohmmeter, having a range of 500 to 50,000 ohms. This range would seem to be quite suitable for diagnosis purposes, as very few wire-wound resistances lie outside it, and great accuracy is not required when resistances of the magnitude of megohms are to be checked. Actually, it is possible to distinguish between 100,000 ohms and 1megohm by this instrument, but the scale becomes so cramped for these high resistances that an accurate calibration is impossible.
The first step is the inclusion of the 5,000 ohms variable resistance,
46
MEASURING RESISTANCE
and the 4i-volt torch battery inside the instrument case. The resistance is supported on a bracket made from 18 gauge alu miniu m, bent to shape and drilled as shown in Fig. 26, and fixed under the panel in such a way that the spindle of the resistance comes im mediately under the hole which we have already provided through the panel to enable the zero of the resistance range to be set. No dimensions have been indicated on the sketch of this bracket as the actual size will depend on the make of resistance used. Before mounting the resistance, a saw-cut should be made with a fine hack-saw blade across the end of the spindle so that it is possible to insert a small screwdriver through the hole in
the panel to adjust it. The battery can easily be accommodated in the bottom of the instru ment box, and connections made to it by means of flexible leads suficiently long to permit the replacement of the panel after the battery has been slipped into the clip or strap which retains it.
Additional Connections. --The extra connections needed inside the case are clearly shown in Fig. 27, and it wil be remembered that
the terminals for the resistance meter have already been mounted. No dificulty should be experienced in completing this part of the work, and the actual calibrating can now be proceeded with.
Rotate the resistance until the whole of the element is in circuit, and then short-circuit the terminals which have just been connected up. This should be done with a short length of heavy gauge wire, and it will be found that the meter shows areading somewhat less than full scale. Now, with afine screwdriver, engage the saw-cut in the resistance spindle and rotate it carefully until exactly 1 mA. is indicated by the meter. It will now be seen that if we remove the short-circuiting wire from the terminals, and connect a resistance across them, the reading of the meter will be decreased by an amount depending on the value of the resistance we have inserted. Say, for instance, the battery gives exactly 4.5 volts, and the meter has an internal resistance of exactly 100 ohms, then by the application of Ohm's Law we know that the variable resistance must be set at 4,500 minus 100 ohms or 4,400 oh ms to pass 1 mA. round the circuit. Now, if we include 10,000 ohms in the circuit our total resistance will be 10,000 plus 4,400 plus 100 oh ms, or 14,500 ohms. Again applying Ohm's Law, we find that 4.5 volts across this resistance will drive 4.5 divided by 14,500 or 0.81 mA. round the circuit, and this will be the reading indicated by the meter.
It can safely be assu med that aflash-la mp battery will maintain its potential of 4.5 volts for some months when it is only required to give 1 mA. intermittently, and provided the calibration is carried out carefully, and the battery is replaced when the variable resistance has to be rotated an undue amount to make the meter indicate 1 mA., the resistance readings can be relied on for a high degree of accuracy.
Meter Readings. --The next step is to draw up a list of meter readings which will be obtained with various values of resistance in
47
PRACTICAL WIRELESS SERVICE MANUAL
circuit and, fro m the m, plot a graph on squared paper. With a meter resistance of 100 oh ms the list will appear as folows :
500 oh ms -0.9 mA.
1,000 oh ms -0.818 mA.
2,000 oh ms -0.692 mA.
3,000 oh ms -0.6 mA.
5,000 oh ms -0.473 mA.
8,000 oh ms -0.360 mA.
10,000 oh ms -0.31 mA.
15,000 oh ms -0.231 mA.
20,000 oh ms -0.183 mA. 30,000 oh ms -0.131 mA.
50,000 oh ms -0.0825 mA.
and a graph drawn fro m these figures is shown in Fig. 28.
The insertion of these figures is not so di ficult as it sounds, and if reasonable care is taken when re moving the scale fro m the instru ment, no da mage will be sustained. The whole meter move ment must be withdrawn from the case before the scale can be re moved, and the scale
9
50,000 45,000 40 00 35,000 30.000 25,000 20 00 15,000 OH MS
10,000
5,000 1,006 a0(
Fin. 28. --A graph drawn to enable dual readings to be made.
should be pinned down (with the heads of the drawing-pins bearing on the edges of the scale) to so me flat surface, such as a drawing-board, while the figures are inserted. A convenient place is directly underneath the millia mpere calibration, and if this is black, a distinctive coloured ink, such as red, should be used. It will be found that most reputable instru ments have a scale on which it is possible to write with drawing ink, but a slight roughening of the surface may prove advantageous. A light dusting with french chalk will be found to help in getting the
48
MEASURING RESISTANCE
ink to run on smoothly. Fig. 29 will show exactly how the scale looks when the resistance calibration has been included.
Point-to-Point Testing. --The sphere of usefulness of the completed ohmmeter does not end with the measurement of resistance, though this is its primary function. It can, of course, be used as a point-to-point tester, and any bad joints in the wiring of a receiver will be shown by the failure of the meter to read zero ohms. Another application is to use the meter to indicate short circuits in condensers or between the electrodes of valves. A fault of this nature will cause the meter to give a full-scale deflection when the resistance terminals are connected across the suspected component. A quick method of checking for an internal disconnection inside a smoothing condenser is to connect the resistance terminals to the suspected condenser and watch the needle of the meter closely while the connection is being made (Fig. 30). Provided the condenser has a capacity of 1 mfd. or greater the meter will kick, and then retu rn to zero, showing that the · internal battery has charged up the condenser. If the condenser is
Fin. 29. --Showing the resistance calibrated readings added to the
scale of a milliammeter.
30. --The ohmmeter in use testing afixed condenser.
perfect, no further kick should be apparent until the connections to the condenser are reversed, when there should be a further kick owing
to this reversed polarity. Care should be taken when testing electrolytic
condensers by this method, as these condensers are polarised, and must be connected in the correct way. The positive pole must always be connected to the positive terminal of the meter (see Fig. 27), and should not be reversed. When attempting to test high-voltage electrolytic condensers by this method do not be led into thinking that the con-
denser is faulty if the meter reads zero ohms because these condensers sometimes require a somewhat higher polarising voltage than the 4-5 volts which are available from our meter.
4
49
CHAPTER V
MEASURING A.C. VOLTAGES
THE universal meter now covers all the most useful ranges of D.C. measurement met with in radio receiving testing, but there are many occasions when it is wished to ascertain the A.C.
voltages existing in all-mains receivers in order to check such things
as mains transformer windings, filament voltages, and the voltage of
supply mains. Our meter will not, at present, give any deflection when
connected to an A.C. supply, and some means of turning the A.C. into
D.C. and then measuring it will be required.
·
Metal Rectifier Unit.-- The most convenient method of doing
this is by the wel-known metal rectifier unit, and the Westinghouse
Brake and Signal Co., of 82 York Road, King's Cross, London, N.1,
market a special rectifier suitable for meter work. This little rectifier
is supplied in various styles, but the one most suitable for our purpose
is the M.B.S.1.
Now it would seem a simple matter to connect this rectifier to our
meter and connect series resistances to the A.C. side to extend the
range as we did when constructing the D.C. meter, but unfortunately,
there are several peculiarities about metal rectifiers and moving-coil
COMMON 0
, r 006M D(PRE SET)
1.1MFD.
TYPE MEISI R CTIFIER
AC
TO m/A
TERMINALS
ON METER
16v 0
100, 000n
100v
-5 Mn
MAAAA MA,
50 0 000n
.--AW\AAAAAN--,
500v 0
AAAA M MAN
FIG. 32. --The theoretical circuit diagrain of the A.C. unit. 50
JIL
31. --The Westinghouse recti fier. Type M.B.S.1.
MEASURING A.C. VOLTAGES
meters when we wish to read rectified voltages which prevent so easy
a procedure. To explain briefly, the metal rectifier, when connected to a moving-
coil meter, presents a resistance to an external circuit which varies with the amount of current passed through the rectifier. It is not necessary, however, to measure this variation for each individual rectifier and meter, as the manufacturers publish curves showing the variation likely to be experienced with meters of various internal
resistances, and these curves can be relied on to an accuracy greater
than that which we require. The general appearance of the rectifier
is ilustrated in Fig. 81. The second dificulty met with is the fact that amoving-coil meter,
instead of reading "R. M.S." values of A.C. voltage (which are what we require), is suficiently sensitive to read average values which are
lower than "R. M.S." values by 1-11 times. This will, of course,
necessitate series resistances of a value 1.11 times lower than those we
used for our D.C. meter. The dificulties mentioned are not insuperable, and fortunately the
use of a condenser for a series resistance on the lower voltage range will correct the non-linear characteristic of our rectifier and meter.
Most readers will be aware that when a condenser is connected in an A.C. circuit acurrent will flow round that circuit, the amount of current being dependent upon the frequency of the supply, the voltage across the condenser, and its capacity. Thus, a condenser will behave in a
precisely similar manner to a resistance, except that, whereas a non-
inductive resistance will not vary its value with changes of frequency, a condenser of given capacity will pass more current at 100 cycles than
at 50 cycles. To use acondenser as a "series resistance" for our A.C. meter, therefore, means that the meter will only read accurately when
A.C. at one frequency is applied across its terminals. The standard frequency of A.C. mains in this country is 50 cycles, and so our meter
has been designed to read correctly at this frequency.
Now, having decided that our meter shall be designed to indicate
correctly A.C. voltages of 50 cycles periodity, the next step is to
calculate the value of condenser necessary to give a full-scale deflection
of our meter when 10-volts A.C. are applied across the circuit.
· Condenser Values. --As we have now to pass 1.11 times the
current round the circuit to produce the deflection we require, the
impedance
of
the
total
circuit
will
be
equivalent
to
10,000 1.11
or
9,010
ohms.
This value will be made up of the resistance of the meter and rectifier and the reactance of the condenser in series. The resistance of the meter and rectifier is obtained from the maker's curves, and is found
to be approximately 870 ohms for the rectifier we have chosen, and a
meter with an internal resistance of 100 ohms. The current passed by
51
PRACTICAL WIRELESS SERVICE MANUAL
the condenser will be 90 degrees out of phase with the current passed
by the combined resistance of the meter and the rectifier, so it is neces-
sary to use the formula: Z =A/ R i-- R 2,where Z equals the reactance required, X equals the impedance of the total circuit and R equals the resistance. Inserting the values we have obtained we find that
Z= V(9,010) 1-- (870) 1,or 8,969 ohms. We have now to find a con-
denser which will have a reactance of this value at a frequency of 50 cycles, and this is obtained from the formula
C
--
1,000,000 2 x8.14 x50 x8,969
or
0-855
mfd.,
but we are unable to obtain a condenser of this capacity, so we must,
therefore, connect several condensers in parallel to build up the value we require. On referring to Fig. 32 you will see that the condensers
.25 MED.
·006 MED. PRE-SET CONDENSER
·IMED. TO METER
-.A--COMMON i 10v.
WESTINGHOUSE RECTIFIER 100v.
·5Mn. 4 Mn. ·5Mn.. 100,00O n
38.:--.Back of 500 v. fpaot·ntteliewA.iri0n.ugdnliatgram
used are one of 0.25 mfd., one of 0-1 mfd., and a pre-set condenser with amaximu m capacity of 0.006 mfd., this latter being included to enable a final adjustment to be made if it is possible to compare our meter with a meter of known accuracy. Should this not be possible, it will be found that this condenser should be screwed right home and then slacked of about one-quarter of a turn.
We can now turn our attention to the resistances which are used
52
MEASURING A.C. VOLTAGES
to extend the range of the meter to read 100 volts and 500 volts maxi-
mum. Remembering that we already have an impedance of 9,010 ohms in circuit, and that acurrent round the circuit greater than that required to give ful-scale deflection, on D.C. by 141 times will be required, we can start our calculations from a basis of 100,000 ohms, which would be the correct resistance if D.C. were to be passed through the circuit.
Dividing 100,000 by 1.11, we get 90,090 ohms, and subtracting 9,010 ohms from this figure we are left with 81,080 ohms, which is the resistance we require. This, again, is a non-standard value, so two resistances are used in parallel, one of 100,000 ohms and one of 0.5 megohm.
These do not give precisely
the value required, but are suficiently near for our
purpose. The resistances required
for the 500-volt range are
calculated in a similar manner and are found to be 0.5 megohms and 4 megohms, connected in
parallel.
Wiring Connections. --
The actual construction and wiring of the unit
is clearly shown in Fig. 88,
and should present no
dificulty, the only point to
be stressed being that the
connections to the rectifier
Fm. 34 (left). --The complete A.C. unit in its cabinet connected to the meter.
unit should be made in accordance with the in-
structions contained in
the maker's leaflet which will accompany the rectifier, as the latest
type differs slightly from the type actually used by myself in mechanical
construction, though the electrical characteristics are similar. The rectifier, fixed condensers, and resistances are supported by the wiring, which should be of at least 18-gauge tinned copper wire to give the
necessary rigidity. The appearance of the A.C. section when connected to the multi-
range meter is shown in Fig. 84. The use of acondenser in the circuit ensures that the rectifier cannot be
damaged if D.C. is accidentally applied to the terminals of the A.C. section, and also allows the complete meter to be used as an output
meter without extra safety devices. When using the meter for the latter purpose it should be noted that
53
PRACTICAL WIRELESS SERVICE MANUAL the correct range to use will be dependent on the output of the receiver and the strength of the signal being received. It will be advisable to connect the 500-volt range to the speaker terminals if these are brought out for the purpose of an extension speaker connection, and if an insufficient deflection is then given by the meter, the 100-volt range may be tried. Should the extension speaker terminals be arranged for a low impedance speaker, the 10-volt range should be used and the strength of the signal adjusted until the desired deflection
EARTHED SCREW ON METALLISED CHASSIS
35. --Using the complete apparatus as an output meter. is obtained. Obviously a steady reading of the meter will only be obtained when asignal such as the B.B.C. tuning note, is being received as the reading of the meter wil fluctuate rapidly if music or speech is being reproduced. Also the actual voltage developed at the loudspeaker terminals will not be indicated unless a 50-cycle note is being received, but the meter can be used with every success to indicate the eficiency of the receiver when ganging adjustments are being made, if the B.B.C. tuning note or a steady heterodyne note produced by two interfering stations is used for adjustment purposes. In a subsequent chapter asuitable oscilator for producing locally a steady signal of this type will be described.
Should no extension speaker terminals be available, connection should be made by means of crocodile clips to the anode of the output valve and an earthed portion of the receiver (Fig. 85).
54
CHAPTER VI
MEASURING CAPACITY
THE previous chapter gave all the necessary information to enable readers to construct a rectifier unit for use with the Universal D.C. Test Meter and also outlined some of the uses to
which the completed instrument might be put. In this connection it
should be noted that the switch on the panel of the D.C. meter should
always be placed in the 1mA. position before the A.C. unit is connected
and used. If the switch is in any other position, no damage wil be
done either to the meter or to the rectifier but erroneous readings will
be obtained. Also, make certain that the correct range is used for A.C.
measurements, as the rectifier unit may easily be damaged by overload
and, in this respect, is not so robust as the meter movement.
Where A.C. mains of known voltage are available we have at hand a
very convenient method of checking the 500 volt range of the A.C. meter
and the 4 megohm parallel resistance on this range may be reduced
to give the correct deflection when the mains voltage is measured. In some instances a reduction
···· , M MI.
I I
of of
this 8 or
resistance to a value 2.5 megohms has been
4
ACM AIN S
0
200-250v 50--
0 0
d
1 I
I
found to be the reason
necessary, for this
though is not
readily apparent. Of course,
care must be taken to discon-
nect the mains from the
instrument while adjustments
0 0
10v
are made. The voltages are indicated
irl
A.C.
on the 0 to 1 mA. scale in precisely the same way as
UNIT SWITCH
1
when D.C.
the meter is used measurements, i.e.,
for on
COAVAONj
the 10-volt should be
range all readings multiplied by 10,
on the 100-volt range by 100,
and on the 500-volt range by
TEST PRODS FIG. 36. --The theoretical circuit of the
capacity bridge.
5
500. For example, using the 500-volt A.C. range a reading of 0.2 mA. will equal
0.2 x500, or 100 volts.
PRACTICAL WIRELESS SERVICE MANUAL
There is no easy method of extending the current ranges of our A.C. instrument and the construction of suitable transformers for this purpose is rather beyond the capabilities of the reader. However, the occasions on which A.C. current measurements need to be taken for receiver diagnosis are fortunately rare, and the omission of extended current ranges need not be considered a very great drawback or one which unduly restricts the scope of our meter.
Measurement of Capacity. --There is, however, one very useful type of measurement which can be made with our A.C. unit, and
INSULATED TEST PRODS
TO 50^MAINS
SWITCH
-61111111 =.111
20,000n-5watt.
Flo. 37. --The circuit of the capacity bridge in pictorial form.
5,000 n POTS.
/41,2 ,
COMMON
that is the measurement of capacity. For this purpose it is necessary that a source of A.C. is available and A.C. mains will usually be accessible to those readers who have constructed the rectifier unit. In this connection it is assumed that the mains to be used are 200-250 volts,
50 cycles, but any other voltage or periodicity can equally well be accommodated, by suitable modifications to the meter.
It will be seen from the circuit diagram (shown diagrammatically in Fig. 86, and pictorially in Fig. 87) of the additions to be made to the unit, that the A.C. mains are fed to avoltage divider formed by 20,000 ohm fixed resistor, and a wire-wound potentiometer of 5,000 ohms,
56
MEASURING CAPACITY
across which the 10-volt range of the A.C. unit is connected and the condenser under test. The switch is included in the leads to the meter to serve the double purpose of disconnecting the capacity measuring circuit and thus prevent damage to the meter when adjusting for full-scale deflection before taking capacity measurements. From the foregoing it will be clear that although the ilustrations to this part show the capacity unit as a separate piece of apparatus, the inclusion of the extra components can conveniently be made in the same box as the A.C. unit described in Chapter V. If made up as a separate unit, providing the switch is in the "of " position after capacity measurements have been taken, there is no necessity to disconnect the unit in order to use the remainder of the apparatus.
\ A.0 MAINS
00 0
CAPACITY UNIT
A.C.UNIT
MULTI-RANGE DC. METER
FIG. 38. --The unit connected to the universal meter.
The inclusion of insulated test prods is another very necessary pre-
caution because unless these are connected together or bridged across a condenser, fairly high A.C. voltages are present and there is a possibility of an unpleasant shock being sustained if bare test prods are
carelessly handled. The type of test prod in which the connecting plunger is spring-controlled wil be found to be the most suitable as there are no exposed metal parts unless readings are being taken.
A condenser, it has been explained, will permit aflow of alternating current and the amount of current passed wil depend on the capacity of the condenser, the voltage applied to it and the periodicity of the supply. This is the principle governing the capacity measure ments
which we propose to make. Mounting the Components. --No dificulty should be experienced
in mounting the components in the box and the wiring is quite straightforward and may be folowed easily from the drawings,
57
PRACTICAL WIRELESS SERVICE MANUAL
with special reference to Fig. 88. Make sure that the 5,000 ohm potentiometer and switch are constructed in such a manner that the switch is " of " when the knob is rotated fuly in an anti-clockwise direction and that all joints in the wiring are sound. The potentiometer shown in the ilustrations is the Bulgin, Type No. V.S.44, and which wil be found to be eminently suitable.
Now connect the units together as shown in Fig. 89, short-circuit the test prods by extending their points, locking and clamping them together in a crocodile clip and rotate the 5,000 ohm potentiometer until the switch is " off ". Plug into the A.C. mains and rotate the potentiometer until the switch "makes ". The meter should now show a small deflection and further rotation of the potentiometer will increase this until a ful-scale deflection is obtained. We are now ready to commence calibration.
Those readers who have a large assortment of fixed condensers of known capacity will be able to calibrate their meter directly by discon-
necting the mains, removing the short-circuit from the test prods, reconnecting the mains and then touching the test prods to the terminals of the condensers and noting the deflection given by the meter.
The larger the capacity of the condenser the higher will be the reading given by the meter and with the circuit as shown capacities between .001 mfd. and 1.0 mfd. will give readable deflections of the meter. A chart showing the deflections for various capacities can then
be prepared on the lines of the one ilustrated in Fig. 40 and cut out and pasted inside the lid of the D.C. meter for ready reference.
MAINS PLUG 89. --Tbe completed unit ready for calibration.
58
Cali bration
Chart. --This pro -
ced ure, however,
will not assist the
reader who has not
ready access to
suitable condensers,
but a calibration
chart may be pre-
pared by calculation
by the folowing
method.
The formula for
calculating the re-
actance of a con-
denser is React-
ance
=
10 27rf
6C.where
7T 8.14, f fre-
MEASURING CAPACITY
quency of the supply and C = the capacity of the condenser in micro-farads. If we select, for example, a condenser of 0.1 mfd.
capacity we find, on inserting the values in the above formula, that
Reactance =
10 4
or 82,000 ohms. At first we
2 x 8.14 x 50 x 0.1
should be led to assume that we had merely to add this value
to the 25,000 ohms already across our mains supply, which we
will assume is 250 volts and apply Ohm's Law to give us the current flowing round the circuit. We should, however, be wrong, because we had forgotten that the current flowing through acondenser is 90° out of
phase with the current flowing through a resistance connected in series with it and therefore we must use afurther formula to enable us to add
the reactance to the resistance. This formula we have already used when constructing the 10-volt range of our A.C. meter and, you will
re me mber, is Impedance =
V(Reactance) 2
(Resistance) . Re-
writing this with our values
inserted we find that Impedance -
V(82,000) 1 (25,000) 2 or 41,000 ohms approximately. Now we can apply
Ohm's Law and find that the
current flowing round the circuit =
Mains Voltage x 1,000 or 6.1 mA. 41,000
We know that we have set our meter
to read 1 mA. with 10 mA. flowing round the external circuit, so that
our meter will read 0-61 mA, and this is actually the reading obtained when a condenser of 0.1 mfd. capacity is
connected to our test prods.
Ho. 40. -1 he calihration chart showing meter readings and condenser capacities.
Reactance Values. --The calculation of reactances and the transference of the values obtained to the scale of the meter is rather a laborious process, so for the guidance of constructors the following list has been compiled of the more usual capacities showing the meter
reading obtained and though these values may vary slightly with diferent mains voltages they may
59
PRACTICAL WIRELESS SERVICE MANUAL
be relied on for the purpose of reference and will be found su ficiently accurate for all normal uses:
CAPACITY ( W.)
1.00 .75 -50 -25 -20 .15 .10 .09 .08 .07 .06 .05 .04 .08 .02 .01 .009 .008 -007 .006 .005 .004 .003 .002 .001
Reactance at so cycles.
3,200 oh ms 4,300 6,500 13,000 16,500 22,000 82,000 85,000 40,000 45,000 54,000 65,000 80,000 110,000 160,000 820,000 854,000 898 000 455,000 531,000 637,000 797,000 1,061,000 1,580,000 8,200,000
Reactance as,00 ohms summed at sos ..
25,200 oh ms 25,860 25,800 28,200 29,900 84,800 41,000 48,000 47,180 51,400 59,500 69,600 88,700 112,800 161,800 820,700 854,100 898 000 455,000 531,000 637,000 797,000 1,061,000 1,580,000 3,200,000
Meter Reading.
.99 mA.
.985 1/
.965 PP
.89
PO
.885
.718
.61
.58
.53
.485
.42
-36
.30
.22
·155
.078
.07
063
.055
.047
.089
.031
.0236
.0157
.008
The range of capacities covered by our unit is necessarily restricted and some readers may wish to extend this range to suit their own needs.
The methods by which the range described has been prepared are perfectly suitable for calculating different ranges and the main point to be
noted is this. If measurement of larger capacities is to be made the current through the circuit must be increased and if a smaller capacity range is required the current must be reduced. This is most easily done by variation in the value of the fixed resistor and potentiometer. The present range covers the most useful values of condensers and
wil prove avaluable adjunct to the listeners or serviceman's equipment.
60
CHAPTER VII
USING THE UNIVERSAL METER
THE reader having now completed the constructional work, and having calibrated the universal test meter will, no doubt, soon begin to find many uses for it. At this juncture it may not be out of place to outline some of the diferent measurements which can be made, and mention the means which the reader can take to ensure that the indications given by the meter are accurate indications of the currents and voltages which he requires to measure.
It will no doubt occur to the reader that the universal meter cannot be used to take some of the measurements which are necessary for accurate diagnosis in a radio receiver, but these chapters include descriptions of all necessary apparatus to form acomplete testing equipment for the laboratory.
Fig. 41 gives the simplified circuit of a 3-valve A.C. mains receiver which wil be used as a basis for the explanations which are to follow.
Fm. 41. --A simplified 3-valve circuit used as a basis for the tests mentioned in this
chapter. Eli
PRACTICAL WIRELESS SERVICE MANUAL Commencing from the entry of the mains, the first component which may require to be checked is the mains transformer. If any doubt exists as to whether this component has been wired correctly or that the various leads have been identified correctly, the easiest method of checking the wiring is to connect the leads marked A and B, which are easily identifiable because they are much heavier in gauge than the other leads, to a4 volt A.C. supply obtained from the filament sockets of a valve-holder in an A.C. receiver (Fig. 42). This will energise the other windings of the transformer and the voltages developed across other pairs of leads from the transformer can be measured, using the A.C. adaptor. Although these voltages may be slightly higher than normal owing to the tranformer being unloaded, they will serve to indicate the leads which are connected to the various windings internally.
FIG. 42. --Energising the windings of a mains transformer to enable tests to be
carried out.
FILA MENT SOOKET5.-----
TEST C ODS
-·\,,,, .
4-VOLT WINDING
Smoothing Condensers. --The next components to come under suspicion might be the smoothing condensers C and D if the hu m in the receiver were excessive, and though the capacities used for smoothing are usually higher than can be indicated on the scale of our capacity meter, a very useful indication of the capacity available can be obtained by utilising the method outlined in the second article of this series whereby the resistance terminals are connected to the condenser,
62
13IHNI IVSUEAINfl HHI. ONIS11
A
1-
,-- ,A WVW "-- do
8
Avc rntiNECTION
EARTH
LINE
FIG. 43. --Circuit diagra m of diode detector.
PRACTICAL WIRELESS SERVICE MANUAL
and the "kick" given by the meter on connection noted. The resistance meter wil also indicate the presence of a short-circuit in a condenser by giving a ful-scale deflection when connected to the suspected component, although the precautions to be taken when testing electrolytic condensers must be observed.
The smoothing choke has usually a comparatively low resistance to D.C., and although the resistance meter will indicate a complete disconnection it is rather dificult to ascertain whether a short-circuit between individual turns exists. However, if the hu m in the receiver is excessive, and the smoothing condensers are perfect, it can safely be assumed that the inductance of the choke is inadequate and improved results will be obtained if this component is replaced. Care must of course be taken to ensure that the choke is firmly fixed and the clamping bolts holding the core lamination are adequately tightened.
Connecting the resistance meter between the anode of the output valve and the filament circuit of the rectifier valve will indicate that a circuit exists in the smoothing choke, and the primary of the output
tranformer, and the current flowing in this circuit will usually be suficiently large to enable us to measure the voltage at the anode of the output valve with adequate accuracy by connecting the appropriate range of the D.C. voltmeter across points E and F.
Current Taken by Output Valve. --The next measurement to be made will usually be the current taken by the output valve, and the correct position to insert the 100 mA. range of the D.C. milliammeter will be at G in the diagram. Should the current indicated be excessive it will be necessary to adjust the value of the bias resistance (H) which is in the common negative return lead. To adjust this resistance it will be necessary to measure the current flowing through it and then calculate the correct value of resistance by using Ohm's Law which
states that Resistance -- VCoulrtraegnet ,where the voltage required is the grid-bias voltage needed by the valve in question, and the current is the current flowing through the circuit, expressed in amperes.
The decoupling resistance (J) and the secondary of the intervalve transformer can be checked by using the resistance meter, and the anode circuit of the detector valve will be the next circuit which will require checking.
If we attempt to measure the voltage at the anode of the detector valve by connecting the 100 volt range of the D.C. voltmeter across points K and L we shall obtain an erroneous value because our meter
will absorb a certain amount of current and will add to the voltage drop across the components in the anode circuit. A more correct value will be obtained by first measuring the resistance in circuit between H.T. positive and point K. Then break the circuit at point M
84
USING THE UNIVERSAL METER
and measure the current flowing. Applying Ohm's Law again we find
that the voltage at the anode of the detector valve equals the maxi mum
H.T voltage minus the current in circuit expressed in amperes multi-
plied by resistance in circuit.
Checking the H.F. Circuit. --The H.F. circuit of the receiver can
be checked by similar methods, and the only other point at which
erroneous readings are likely to be obtained is the screening-grid con-
nection of this valve. At this point the voltage is obtained from a
potentiometer across the H.T. supply and connecting our D.C. volt-
meter across points N and 0 will increase the current flowing through
the potentiometer and lower the voltage at N. The correct voltage
can only be ascertained by measuring the resistances forming the
potentiometer, adding them together and then finding the current
flowing through the circuit by dividing the voltage across the potenti-
ometer by the value of the resistance so obtained. Now we must
measure the current taken
A
1.1+
by the screening grid of the valve by connecting our
PIESTECTOR
milliammeter between the
junction of our potentio-
meter resistance and the
actual S.G. connection to
the valve and add this to
the current flowing through
the potentiometer. Now if
the value of the resistance
forming the limb of the
potentiometer between the
valve and H.T. positive is
multiplied by the total cur-
rent flowing through it, the
volts dropped by the resistance will be obtained, and
44. --Circuit of a detector stage using a " Westector."
subtracting this voltage from the voltage across the potentiometer,
the actual voltage at point N will be available.
The foregoing procedure may sound rather more complicated than
it actually is, so perhaps a concrete example may be useful as an
ilustration. Suppose that the resistances forming our potentiometer
have values of 20,000 ohm %and 80,000 ohms, the latter resistance
being connected to H.T. positive, our measured screen consu mption
is 0.5 mA, and our H.T. supply is 250 volts. The current flowing
through the potentiometer when the screening grid is disconnected will
be 50,205000- -- 005 amps or 5mA. Add 0.5 mA to this figure which gives
5
65
PRACTICAL WIRELESS SERVICE MANUAL
us 5.5 mA passing through our 80,000 ohm branch. The volts dropped
in this branch will now be equal to 80,000 x .0055, or 165 volts. Sub-
tracting this from 250 (our total voltage) we find that the actual
potential at our screen-grid connection is 85 volts.
Measurement of Grid-Bias Voltage. --The final use of the
universal meter dealt with now is one which many readers find
dificulty in measuring. It is the measurement of voltage of the grid
bias supplied by a Diode Detector or "Westector" when used in an
A.V.C. system. Reference to Fig. 48 will show that this is merely
a skeletonised diagram of a diode detector supplying A.V.C. to
a variable-mu H.F. valve, while Fig. 44 shows an appropriate
circuit utilising a Westector. Inserting our miliammeter at point A
in the circuit will enable us to read the current taken by the H.F.
valve, and on tuning in a strong signal we shall note, if our A.V.C.
system is working properly, that this current decreases and we can use
our meter as a tuning indicator. However, if we attempt to find out
how much extra bias has been introduced by the signal, by connecting
our voltmeter across points B and C, i.e., across the diode load resistance, the current flowing in this circuit will be found insu ficient to
give a deflection of the needle. We must therefore resort to other
means to find this bias voltage and the easiest method is by comparison.
Tune in a strong signal and with the miliammeter connected at
point A, note the lowest current reading that can be obtained by
careful tuning, now disconnect the aerial from the receiver, switch
of and disconnect the A.V.C. connection from the diode load resis-
tance at B. Connect this lead to the 8-volt negative socket of a grid
bias battery whose positive
terminal is connected to the
earth socket of the receiver.
Now, with the aerial still
disconnected from the set,
switch on and vary the
negative voltage applied to
the A.V.C. connection by
moving the plug in the
sockets of the grid bias
battery, until the same read-
ing is obtained on our mili-
ammeter as was previously
noted. The socket on the
battery into which the A.V.C.
connection is now plugged
Flo. 45. --The receiver must be slowly tuned will then be equal to the
through the signal when checking A.V.C.
voltage which had previously
6
USING THE UNIVERSAL METER been supplied by the diode. By measuring the voltage in this way it will be possible to estimate the eficacy of the A.V.C. system to deal with incoming signals of varying strengths and useful data can be compiled on the comparative strength of transmissions received by noting the amount of grid-bias voltage produced by the diode on diferent stations.
Checking A.V.C. --Perhaps, while on the subject of A.V.C., it may not be out of place to indicate an easy way of checking the eficiency of any receiver in this respect. Connect our universal meter as an output meter to the speaker terminals of a set incorporating A.V.C., in the manner outlined earlier. Then tune in apowerful signal (your local station when radiating its tuning note will do --a steadysignal being necessary), and watch the output meter. Now, although we know that as we tune across the powerful signal the voltage applied to the input of the receiver will vary considerably and continuously, yet if the A.V.C. system is functioning properly the reading of our meter should rise quickly as we approach the station and then remain almost steady until we have passed through the exact tuning point of the station to which we are listening and then quickly fall away again. If the reading of our output meter rises to a peak with asharply defined maximum we may be practically certain that our A.V.C. syste m is not doing all that it should. It must be emphasised however that the receiver must be slowly tuned through the signal (Fig. 45) or the flattopped response due to the A.V.C. will be missed.
It wil be obvious to readers that the chief drawback to the universal meter so far described is its inability to measure voltages without extracting amaximum of 1mA of current from any circuit to which it may be connected in order to energise its own movement. There is only one type of meter which will read voltages, either A.C. or D.C., without extracting current for itself and that is the valvevoltmeter.
67
CHAPTER VIII
M A VALVE-VOLTMETER ANY readers regard the valve-voltmeter as ahighly complicated piece of apparatus, so dificult to calibrate and operate that it would be quite out of the question to construct one, far less use it.
This chapter touches lightly on the theory governing the operation of the
type of valve-voltmeter which is to be constructed, in order that this impression may be dispelled and avery useful instrument added to the
equipment of every keen reader. The grid of avalve used as an anode-bend detector is biased negatively
until no current flows in its anode circuit. Now, if a voltage be applied to the grid circuit in opposition to the grid-bias voltage, i.e., positive to grid and negative to filament circuit, acertain percentage of the negative bias wil be cancelled out and current will commence to flow in the anode
circuit. The magnitude of this current will be gove rned by the reversed voltage applied and ifwe connect an appropriate D.C. milliampere range of our universal meter in the anode circuit of the valve we shall be able to measure this voltage by calibrating the milliammeter in terms of the applied voltage.
Alternating Voltage. -- When an alternating voltage is applied to the grid of an anode bend detector valve the negative half-
cycles are suppressed because the valve is so heavily biased that no further change in anode current can take place with an increase of negative volts, but the positive half-cycles reduce the negative bias in precisely the same way as asteady positive direct voltage and the anode current of the valve will rise in sympathy.
It will 'thus be seen that it is quite easy to connect up a suitable valve in such a way that the application of positive voltages to its grid will produce visible indications of current through a meter connected in its anode circuit. When agiven alternating voltage is applied to the grid, exactly identical indications will be obtained on the anode miliammeter, no matter whether the frequency of the applied voltage is 50 cycles from the mains or an extremely high frequency generated by a
wireless transmitter. Of course, there are limitations to the frequency accuracy of avalve-voltmeter but as the limitations are set by the grid filament capacity of the valve used, the shortness of the wiring to it and similar factors, an ordinary receiving valve will operate quite satisfactorily in this respect over a range of frequencies from 50 to 1,500,000 cycles per second, arange more than suficient to meet the needs of the majority of my readers.
68
A VALVE-VOLTMETER
The valve chosen for this valve-voltmeter is the Marconi or Osram
P2,because with an anode voltage of 50 volts and 9 volts on the grid
the characteristics will allow us to measure 5 volts D.C. or A.C.,
without running into positive grid current. It may seem unnecessary
to provide a "grid-base " of 9 volts in order to measure a change of
only 5 volts but my readers will remember that a R. M.S. voltage of
5volts A.C. has peaks which are 1·404 times the magnitude of the R. M.S.
volts which we are to measure, and the voltage will actually rise
to 7 volts peak. Therefore, by allowing a grid base of 9 volts we
effectively prevent the flow of grid current within the range of our
instru ment. If we wished to measure, say, 50 volts directly, we should require
a valve which needed at least 70 volts grid-bias to stop anode current
flowing and a correspondingly higher anode voltage. This would
obviously be impracticable with any battery-driven valve and so we
must incorporate some form of potential divider to multiply the initial
voltage range of our instrument. Reference to Fig. 46 will show that the voltage we wish to measure is applied across a 1megohm potenti-
ometer, the slider of which is connected to the grid of the valve. It will
be seen that ifthe slider is turned to the end of the potentiometer which
is connected to input terminal G, the whole of the external voltage applied is received by the grid of the P2 valve and therefore our cali-
bration wil be direct, but if we set the slider half-way down the
resistance element only half the applied voltage will be received by the
grid of the valve and our calibration will be doubled. This is extremely
convenient and it has been found possible to extend the range of the
instrument to read 50 volts quite easily by this means.
Setting the Zero. --The next point of interest in the circuit is
the method adopted for setting the "zero " of the valve-voltmeter.
It will be appreciated that as
V20A 00zi
METER0--4 0 + we increase the grid-bias on
our P2 valve its anode current
will fall, and as it will be very ta50v dificult to read on the mili-
1
w.f.
ammeter in its anode circuit
nos
INPUT
· the exact point when the + current falls to zero it will be
2v more convenient to select an arbitrary point on the scale,
oi red
say, the first small division,
or -01 mA and set the meter
SCREENING
_ 801 8 PANEL Fin. 48. --The theoretical circuit of the
valve-voltmeter,
to read this value before we commence to take readings. For this purpose we must use a grid-bias battery and
69
PRACTICAL WIRELESS SERVICE MANUAL select a negative tapping which will reduce the anode current to approximately the value we require and then make our final adjustment by rotating the 25 ohm potentiometer across the filament circuit of the valve which gives us a final control over a range of 1 volt positive to 1 volt negative. Fortunately a P2 valve with 50 volts on its anode and 9volts negative on its grid is almost biased to "cut-of" and the filament potential divider will be found to give ample adj ustment.
Coming next to the anode circuit of the valve we see that avariable resistance of 20,000 ohms is connected in series with the millia mrneter
FIG. 47. --The compsplaecteedleftinstforruthmeentcalibinratiitsonscrcheaernt.ed box showing the to 50 volts positive H.T. This resistance provides a means of limiting the current taken by the valve when grid-bias is removed or neutralised by an applied voltage. If we look at the characteristic curves of a P2 valve we shall find that with no volts on the grid and 50 volts positive on the anode, the anode current will rise to approximately 12 mA. This would obviously be greater than we could read on the 0to 1mA scale of our universal meter and furthermore would be a fairly heavy load on the small capacity H.T. battery we intend to use. If, however, we include a resistance in the anode circuit of the valve we can cut down the anode current which will flow for low values of grid-bias without efecting the grid circuit or the maximu m range of
70
A VALVE-VOLTMETER
our meter in any way. Also, by making this resistance variable we have an easy method of arranging that our milliammeter shall read exactly 1 mA. when the maximu m voltage we wish to measure is
applied to the grid circuit of the P2. As will be seen from Fig. 48 the 20,000 ohm resistance is mounted on a bracket on the baseboard as it is unnecessary to adjust this continually and provided it is set at the commencement of calibration or measurement, all further
adjustment is carried out by the external controls.
25n POTENTIOMETER UPPER
LOWER
'ON -OFF' SWITCH
I
L
IMn POTENTIOMETER
I
UPPER
L
LOWER
UNSPILLABLE 2v ACCUMULATOR
HT -
H.T.BATTERY
GB -
HT + GB+ G.B BATTERY
FIG. 41.--The full wiring plan for the valve-voltmeter.
Constructional Details. --The foregoing explanation deals with all the major technical details of the meter and we can proceed with the
constructional details. The whole apparatus is mounted on a metal panel and wooden
baseboard which should be inserted, as shown in Fig. 47, into a metal screening box in order that the readings obtained on the meter are unafected by stray H.F. fields. The size of the screening-box is governed by the size of the H.T. battery and unspillable accu mulator which the reader wishes to incorporate, and hard and fast dimensions are therefore not given. At this juncture it may as well be
71
PRACTICAL WIRELESS SERVICE MANUAL mentioned that the valve-voltmeter will work quite satisfactorily without the complete screening box provided that extremely high accuracy is not required, but in all cases the metal panel should be retained. This introduces slight complications because three of the terminals to be mounted on this panel are not at earth potential, and it will therefore be necessary to insulate their stems by suitable insulating bushes. Also the 1megohm and 25 ohm potentiometers should be of the type whose spindles are not connected to the rotating contact. Alternatively, the spindles could also be insulated by suitable bushes, but this system is not quite so satisfactory, as the screening is impaired by bringing "live " spindles through the panel.
The remainder of the constructional work and wiring is straightforward, and if the explanatory drawings are followed carefuly,. no dificulty should be experienced in constructing the valve-voltmeter.
List of Parts Required for Valve Voltmeter 1 -- Screening Box (see text) 1 -- Panel (metal) and baseboard to fit above. 1 -- 60-volt H.T. Battery. 1 -- 9volt G.B. Battery. 1 -- 2-volt unspilable Accumulator. 1 -- 1megohm Potentiometer (ungraded and with insulated spindle). 1 -- 20,000 ohm. Wire-wound Variable Resistance (with insulated
spindle). 1 -- Mounting Bracket for last above. 1 -- 25 ohm. Filament Rheostat. 1 -- 4-pin baseboard-mounting Valve Holder. 2 -- 0.01 mfd. Fixed Condensers. 1 -- " On-off " Switch. 4 -- Terminals (three with insulating bushes for metal panel).
Connecting Wire, Screws, etc.
72
CHAPTER IX
CALIBRATING AND USING THE
VALVE-VOLT METE R
IT wil be observed a space is provided on the front panel of the instrument for a calibration scale to be gu mmed on, and that a circular white paper scale is to be gummed to the panel under the
knob of the 1 megohm multiplier potentiometer. Alternatively, if the surface of the panel is matt finished aluminium, it is quite possible to omit these paper scales and draw the calibration points on the metal
itself in Indian ink, afterwards covering the markings with a coat of colourless cellulose lacquer.
Before we commence calibration it will be necessary to procure a
6-volt accumulator, and a 250 ohm variable potentiometer. If it is inconvenient to get a 6-volt accumulator, four large-capacity dry
cells may be used to supply the calibration voltage, as their voltage
will remain reasonably constant with a current drain of 24 mA, over
the short period which we shall use to make measurements. The accumulator or dry cels and the 250 ohm potentiometer should
be connected up as shown in Fig. 49, but the actual connection to one
terminal of the voltage supply should only be made when calibration
is in progress in order to limit the current drain imposed and thus maintain the voltage as stable as possible.
Connections and Adjustments. --Now connect the H.T., L.T., and G.B., batteries to the valve-voltmeter, making sure that the
switch is "of "before doing so, and connect the milliampere terminals of the universal meter to the
"meter " terminals of the
valve-voltmeter. The positive terminal of the universal meter should be connected to the "meter "terminal of the valve-
voltmeter, which is connected 6v
internaly plug, and
thtoe thsewitcH.hT.shopouslidtivbee
ACCLIR
in
c") I vie
set at 1 mA. The multiplier potentiometer is then rotated in a fuly anti-clockwise direction, i.e., to ratio 1, and the 20,000 ohm variable anode
resistance mounted on the
INPUT OF V/VOLTMETER
.
I E
Fin. 49. --Theoretical circuit of the call-
bration apparatus.
73
PRACTICAL WIRELESS SERVICE MANUAL
baseboard set until the whole of the resistance element is in circuit. Now switch on the valve-voltmeter, and it will be noticed that the pointer of the universal meter gives a little " kick " and then returns to a very low reading on the scale. It might be thought that the 25 oh m zero-adjusting potentiometer on the front panel should now be set so that zero was actually indicated on the miliammeter, but this is not the case owing to dificulty in ascertaining the exact setting at which the milliam meter reads zero. A much more satisfactory method, and the one adopted in this case, is to adjust the 25-oh m potentiometer until the pointer of the milliammeter reads 02 mA. This point will now beco me the zero point for the voltage scale of the valve-voltmeter, and the meter should always be checked before making measure ments to ensure that, under the foregoing conditions, a true zero is indicated.
The valve-voltmeter should now be switched of, the multi-range meter disconnected, and its 10-volt D.C. range connected across the
VALVE VOLTMETER
MULTI -RANGE METER
6v ACCUMULATOR
Fro. SO. --Adjusting the zero reading.
leads from our calibrating voltage supply marked G and E, and the 250-oh m potentiometer varied until exactly 5 volts (or half-scale deflection) is registered. Then re-connect the universal meter to the
"meter " terminals of the valve-voltmeter and the calibration leads G and E to their appropriate imput terminals as shown in Fig. 50. Switch on the valve-voltmeter, and it will be found that the pointer of the universal meter will now give a reading somewhat higher than the zero reading which we previously selected. By rotating the 20,000 ohm resistance on the baseboard until the universal meter reads exactly 1mA, the scale will be set to read 5 volts between .02 mA and
74
CALIBRATING AND USING THE VALVE-VOLTMETER mA. It may be advisable now to disconnect the calibration voltage by removing one of the leads from the accumulator, or dry batteries, and to note that the pointer of the milliammeter falls immediately to our previously selected zero position. If it does not do so, a slight readjustment of the zero-adjusting potentiometer will be necessary. The internal 20,000 ohm resistance will not need re-setting unless the characteristics of the P2 valve change considerably, and with the limited periods of use which we are likely to give the instru ment, the life of the valve should be almost indefinite. It has also been found that the replacement of the valve has very little effect on the calibration of the instrument, provided the instrument is re-set for zero and maximu m readings whenever it is necessary to make this replace ment.
Checking the Deflections. --All that remains now is to make a note of the deflections given by the milliammeter for various voltages
VOLVATLMVEETE'R FIG. 51.--Method of measuring the voltage
developed across atuned circuit. between 0and 5volts applied to the input terminals, and to obtain these readings the required voltage is selected by adjust ment of the 250-ohm potentiometer with the 10-volt range of the universal meter connected across leads G and E and then noting the deflection given by the milliammeter when this voltage is applied to the valve-voltmeter. Some readers may care to plot agraph of these readings, and so be able to measure intermediate values directly, but the valve-voltmeter
75
PRACTICAL WIRELESS SERVICE MANUAL
calibration is nearly linear, and a chart is prepared in the same way
as the capacity chart described earlier.
Having prepared the graph or chart it only remains to calibrate
the multiplier potentiometer. This is quite asimple matter, and should
present no dificulty. First apply 5volts to the input terminals of the valve-voltmeter so that a full-scale deflection is given by the mili-
ammeter, then rotate the multiplier knob in aclockwise direction until the milliammeter indicates 2.5 volts in accordance with our calibration
chart. Mark the position of the multiplier pointer and call it X2, then
if the pointer is left in this position all the readings obtained on the
valve-voltmeter will be multiplied by two, and we have extended our
range to 10 volts maximu m. Again rotate the multiplier knob until 0.83 volts is indicated and mark this point X6. This extends the range
to 30 volts maximum, and all readings obtained with the multiplier
knob in this position must be multiplied by 6. Finally, carefuly
rotate the multiplier until 0.5 volts are indicated, and mark this point
X10. We shall now have extended the scale to 50 volts maximu m, and all readings must be multiplied by 10.
Using the Valve Voltmeter. --The calibration is now complete, and we can start to make use of the valve-voltmeter. It will find its major application in the measurement of voltages in circuits where the
current flowing is too small to actuate any other type of voltmeter;
for instance, it will measure the voltage developed by a steady signal across the load resistance of a diode detector --a very useful guide to
I.F.TRANSFORMER
A.V.C. eficiency. Also it will
measure accurately the voltage developed by a signal of
any frequency across the
secondary circuit of an output
transformer.
To measure the radio-
frequency voltages across
tuned circuits it will be
necessary to prevent the H.T.
voltages from reaching the
grid of the P2 valve, and
mica condensers of -01 mfd.
capacity should always be
connected in series with the
input terminals before mak-
ing measurements in any
circuit where D.C. currents
are also flowing. When
62. --The meter is connected to the points XX, across the secondary of the I.F. trans-
former nearest the second detector.
measuring A.C. voltages existing across, say, the anode
76
CALIBRATING AND USING THE VALVE-VOLTMETER resistance of an L.F. amplifier, the capacity of the condensers should be increased to as large a value as possible, though care should be taken to see that the insulation of the condensers chosen is high. .Fig. 51 will show the connections to the valve-voltmeter when measuring the H.F. voltage developed across a tuned circuit, in this case the tuned-grid coil of an H.F. amplifier, and it will be noticed that the " G " terminal is connected to the high potential end of circuit and the "E" terminal to earth. This procedure should always be followed wherever possible, because it is advisable to keep the metal case of the valve-voltmeter at earth potential to eliminate stray pick-up efects. The valve-voltmeter connected as shown in Fig. 51, will actually measure the H.F. voltage presented to the grid of the detector valve if the set is tuned to apowerful local station, and by experimenting with the voltages applied to the H.F. valve and, incidentally, by making any modifications to the aerial coupling coils, etc., it is possible to see the result of such experiments as a gain or loss in voltage. The I.F. and frequency-changer circuit of asuperheterodyne could also be experimented with using the meter connected across the secondary of the I.F. transformer nearest to the second detector (Fig. 52) and, if the magnification is not too great, it is possible to measure the amplification of a valve, and its coupling components, by first adjusting the voltage across its grid circuit by an H.F. volume control to 0.5 volts, then transferring the valve-voltmeter to the coil in the anode circuit of the same valve and again measuring the voltage. If the reading given is, say, 45 volts, then the stage gain
45 is --.5 or 90.
7
CHAPTER X
AN L.F. OSCILLATOR
pARTICULARS are given here of an instrument which wil fulfil the needs of both the service-man and the listener when investigating problems connected with L.F. amplifiers; the apparatus is quite simple if the principles governing its working are understood.
All my readers have, no doubt, heard the weird whistles and groans which can be produced by an oscillating detector valve when tuned to acarrier-wave, and will remember that as acarrier-wave is tuned in the pitch of the note varies from ahigh-pitched squeak to adeep bass note, vanishing at "silent point" and then rises again, finally to vanish as a high-pitch squeak again. This heterodyne note as it is called would therefore appear to cover all the audio frequencies, and should be ideal for loudspeaker testing as we can pick out whatever note we like from the whole musical scale, and reproduce it through our loudspeaker for as long as we like while we make adjustments.
Using a Fixed " Carrier Wave." -- What we must do is to provide our own fixed "carrier wave" and mix it with a "carrier wave" whose frequency can be varied to produce the heterodyne note we require in a way which will not interfere with our neighbours. Those readers who have constructed superheterodyne receivers will know that when we mix in the grid circuit of a detector valve the output from two valves oscillating at radio frequencies we produce a third frequency, i.e., the intermediate frequency which is equal to the diference between the two separate frequencies. For instance, a 200-Kc. carrier-wave mixed with a 810-Kc. carrier-wave and then rectified will produce a frequency of 10 Kcs., which is known as the intermediate frequency, or I.F.
Now suppose we make our two "carrier-waves" oscillate on frequencies comparatively close together, say for instance 800 Kcs. and 801 Kcs., we shall produce a frequency of 1 Kc. or 1000 cycles which will, after rectification, be audible in a pair of headphones or a loudspeaker. Suppose now we wish to alter the pitch of this note, and increase its frequency to 2,000 cycles, we must merely re-tune one of our "carrier waves" so that the diference between the two is 2,000 cycles.
Hartley Oscilators.--:-In the instrument described in this chapter the two "carrier-waves " are produced by two Hartley oscillators, one with its frequency fixed at approximately 300 Kcs., and the other variable between 300 Kcs. and 810 Kcs., thus giving us an audible
78
AN L.F. OSCILLATOR
range of frequencies between 0 and 10,000 cycles which will cover the band of audio-frequencies usualy considered adequate for good reproduction.
Reference to the circuit diagram (Fig. 53) will show that the Hartley oscillators are constructed from centre-tapped plug-in coils tuned in one case by fixed condensers of .001 mfd. and .0005 mfd. connected in parallel, and in the other by apre-set condenser of .002 mfd. maximu m capacity with avariable condenser of .0001 mfd. in parallel. The output from the anodes of the two oscillators is taken via air-spaced trimming condensers of 7mfd. maximu m capacity to the grid circuit of adetector
J. CAS WELL COIL TYPE'S'
(CENTRE·TAPPED)
I L.GCHOKE
GAMBREL! COIL TYPE'S' (CENTRE·TAPP
dik+HT ITP6OY
0004ed
5 Ma
1.
'SCREENING BOX (SHOWN DOTTED)
J
EARTH
OUTPUT
Flo. 53. --Theoretical diagram of the L.F. oscillator.
valve in whose anode circuit the audio-frequency note appears. This audio-frequency component is developed across an L.F. choke and fed through a 2 mfd. by-pass condenser to a 25,000 ohm, potentiometer which acts as a volu me control, and enables us to adjust the audio-
frequency output to the voltage required. The values of the components have been so selected that the range
of 0 to 10,000 cycles is covered by rotation of the .0001 mfd. variable condenser, and any deviation from the specified coils will result in failure to obtain the full range of the instrument. A further reason for the choice of these coils is that their construction renders the m reasonably free from inductance variation owing to temperature rise and fall,
79
PRACTICAL WIRELESS SERVICE MANUAL
and this helps to prevent the oscillators from wandering from their pre-selected frequency.
Screening Box. --The three-compartment screening box is not a standard component, and must be made in accordance with the dimensions shown in Fig. 55. The provision of ascreening box exactly as shown is an essential in order to prevent interaction between the two radio-frequency oscillators, which would otherwise prevent extremely low frequencies from being produced, owing to the tendency for the two oscillators to pul into step when tuned to nearly the same frequency.
There are two other points to be watched when constructing the
FIG. 64. --The type B coil and holder.
FIG. 5.-- The complete L.F.
oscillator and screening box.
apparatus, viz.: the necessity for insulating the components which
are brought out through the screening box. Suitable insulating bushes should be procured for the spindles of the -0001 mfd. variable condenser, the output volume control, and the output terminal which is connected to the slider of the volume control. Actually, in the original model both output terminals have insulating bushes, but this is purely for the sake of uniformity of appearance. It wil be seen that none of the aforementioned points is at earth potential, and, in the case of the .0001 mfd. variable condenser, failure to insulate the spindle will short-circuit the H.T. battery.
Checking Connections. --Reference to the drawings, and particularly to the wiring diagram in Fig. 56, should enable the constructional work to be completed, and then the connections should be
80
AN L.F. OSCILLATOR
carefully checked over with the aid of the resistance meter, previously described, to ensure that everything is in order before connecting the batteries. Emphasis is laid on this point because the oscillator valves are self-biased by their grid-leaks, and failure of these valves to commence oscillating when switched on may cause damage to their filaments. It is also worth while to make certain that the insulating bushes previously mentioned are really eficient --and this can be done by placing the filament switch in the "on" position and joining the terminals of the resistance meter, firstly, between the H.T. positive lead and the screening box, and then between the junction of the 2-mfd. output condenser and the 25,000 ohm, volume control and the screening
HT+ HT--
LT +
LTEr EARTH
02urfd PRE
ARIL WEB :OIL ,Cli nNiNG
2mn
7muld
c
25, 00 CI
% .1 !H ·
OUTPUT CONT0.01.
A16511 TYPED COtl
.0001nrEd ARIARLE
OUTPUT
56. --The wiring diagram of the L.F. oscillator.
box. In the first instance there should be no deflection at all, and in the second the meter should read 25,000 ohms if the bushes are satisfactory. Before the second test is made the volume control should be rotated to the maximum position, or fully in a clockwise direction.
List of Parts Required for the Low-Frequency Oscillator.
2--Inductances, Range B
1-0001 mfd. Variable Condenser, with slowmotion drive
1-002 mfd. (maximum) Pre-set Condenser
2-7 mmfd. Air-dielectric Pre-set Condensers
6
81
PRACTICAL WIRELESS SERVICE MANUAL
1--.001 mfd. Fixed Condenser
1--.0005 mfd.
2 -0001 mfd. ,,
St
8--.0003 mfd.
1-2 mfd.
PP
Of
1 -5 megohm Fixed Resistances
1-2 megohm ,,
.1
1 - 2 5, 0 0 0 oh ms Output Control
1--L.F. Choke, 40 Henries
1--Toggle " On-off " Switch
2--Three-compartment Screening Box
1-4-pin Baseboard Mounting Valve Holders,
Wire, Baseboards, Systoflex, etc.
3--Valves, Type L210
82
CHAPTER XI
CALIBRATING AND USING THE L.F. OSCILLATOR
THE first thing to make sure of is that both the radio-frequency oscillators are working. As will be seen from the values of the components chosen, the approximate wavelength of the fixed oscillator should be somewhere near 1,000 metres. Should any reader have incorporated centre-tapped coils other than those specified, he can, provided the inductance of the coils is known, calculate the wavelength of his fixed oscilator by employing the formula -- Wavelength
in metres =1885 V L C, where L is the inductance in circuit expressed in microhenries, and C is the capacity in circuit expressed in microfarads. In the case of the inductance coils, which have an inductance of 200
microhenries, the formula will read -- Wavelength =1885 V200 x.0015, or 1,080 metres.
Tuning the Fixed Oscilator. --Most broadcast receivers wil tune to 1,000 metres, but it will be necessary to make the receiver oscillate if we are to hear anything from our local oscillator. Presu ming a suitable receiver is available, tune it roughly to 1,000 metres, which wil be approximately the lowest wavelength receivable with a normal receiver switched to" Long- Waves," increase reaction until the receiver oscillates, and then disconnect the aerial lead, replacing it by a short length of wire. Now insert an appropriate valve into the valve-holder in the left-hand compartment of the beat-frequency oscillator, switch on and, with the cover removed from the screening box, and the short length of wire from the aerial socket of the receiver lying across the top of the oscillator, tune the broadcast receiver until quite a loud "carrierwave " is heard (Fig. 57). If this can be heard we can quite easily prove that our fixed oscilator is working by switching it off and noting the disappearance of the heterodyne note in the loud speaker of our receiver. Should it be impossible to tune in the fixed oscillator on the receiver, do not jump to the conclusion that the fixed oscillator is not working, but carefully check over all the connections, and connect the 10 mA. range of the multi-range meter in the H.T. positive lead to the oscillator. Switch on again and a reading of 2 or 3 mA. should be obtained which, if the valve is oscilating, will rise to 4 or 5 mA. when the grid connection is touched with a wet finger. This test is infallible for checking oscillation because there will be no change of anode current unless oscilation is taking place. No difficulty has been experienced in obtaining oscillation with this type of Hartley circuit,
83
PRACTICAL WIRELESS SERVICE MANUAL and failure of the valve to commence oscillating immediately it is switched on has always been traced to a faulty component or a wrong connection.
Checking the Variable Frequency Oscillator. --The next step is to check our variable frequency oscillator in a similar way, and to do this the valve should be removed from the fixed oscillator and inserted in the valve-holder in the right-hand compartment. Set the vanes of the -0001 mfd. variable condenser to minimu m and screw up the -002 mfd. pre-set condenser to maximu m. We should now hear the carrier-wave from this oscillator in the receiver at aslightly higher setting than the first oscillator gave, but if unable to locate it, satisfy yourself that the valve is oscillating by the "wetted finger" method. Having satisfactorily proved that both our radio-frequency oscillators are working, we can put all three valves into their sockets and try to mix their outputs to produce the L.F. component we require. Set the. 7m mfd. trimmer condensers to minimu m capacity, connect a pair of headphones to the output terminals, turn the volu me control to maximum, switch on and slowly unscrew the -002 mfd. pre. set condenser. Now, if all is in order, we should hear quite a strong whistle in the
OSCILLATING RECEIVER TUNED TO 1000 METRES 57. --Connect alength of wire to the aerial terminal of an oscillating receiver, and lay it across the open top of the oscillator. 'phones, commencing with an extre mely high-pitched tone and coming down the scale til a silent point is reached. What we want to do is to set the pre-set condenser at the silent point and then replace the lid on our screening box. When this has been achieved we shall find that rotating the dial of the -0001 mfd. variable condenser (which has been in its minimu m position until now) causes the note to reappear and rise gradually in pitch over the whole scale.
84
CALIBRATING AND USING THE L.F. OSCILLATOR
The components incorporated in the beat-frequency oscillator have been so selected that the rotation of the variable condenser produces an audible note which rises from 0 to approximately 12,000 cycles per second, and, if we find during our checking operations that the wavelength of our fixed oscillator is not reasonably close to 1,000 metres ,we may expect to find that the range of audible frequencies is restricted, owing to our parallel condenser being low in capacity value. That is why it is advisable to check the wavelength of the oscillators rather than rely entirely on the "wetted finger" method of checking for oscillation.
Setting the Trimmers. --The next step is to set the 7 mmfd. trimmer condensers for maximu m audio-frequency output. In this connection it has been ascertained that the maximu m output will be delivered when the radio-frequency output from the oscilators is mixed at the detector grid in the ratio of 10 to 1. In order to achieve this
1 II 1 11 1 1 !I "I I I I I
"MIDLE C" FREQUENCY
FIG. 58. --The piano keyboard showing corresponding frequencies.
result the valve-voltmeter should be connected across the output terminals of the beat-frequency oscillator in parallel with our headphones and, with the -0001 mfd, variable condenser set so that an easily audible note is produced, the trimmer condensers should be adjusted until the valve-voltmeter gives the highest reading. A word of warning is necessary here, however, and is briefly this. Although it may be possible to boost up the audio-frequency note by setting the trimmers to maximum capacity, we shall find that our lower frequencies will have vanished owing to the oscilators having afected each other and pulled into step. The procedure recommended is as folows: Leave the trimmer condenser in the right-hand compartment set at minimu m and rotate the trimmer in the left-hand compartment until maximu m output, as indicated on the valve-voltmeter, is obtained. Then turn the .0001 mfd. condenser to minimum, set the .002 mfd. pre-set condenser to silent point and ascertain, by swinging the variable condenser, that the lower audio-frequencies are stil audible. Then increase the capacity of the right-hand trimmer a little and repeat the routine.
85
PRACTICAL WI RELESS SE RVICE MAN UAL
Continue increasing the capacity until the output is greatest, and the low-frequencies are still present. Careful adjustment of my own instrument has enabled me to get nearly 8 volts of audio-frequency output while still producing notes as low as 20 cycles per second.
Calibrating the DiaL --Having made the best possible adjustment to our instrument, we can proceed to calibrate the dial of the variable condenser in frequencies. Very few people have access to a calibrated L.F. oscillator or series of tuning forks, but all my readers wil undoubtedly be able to find a piano which has recently been tuned, against which to compare their oscillator. Fig. 58 shows which notes on the piano correspond to various frequencies, and if the output from
TO VALVE -
NT+
VOLTMETER
LOUDSPEAKER IN RECE IVER UNDER TEST
Imfd FIXED CO NDENSERS
CHOK E FROM ANODE I VII H.T.+
FIG. 59. --Disconnect the speaker from the set and substitute a resistance equal to the optimum load of the output valve in
parallel with an L.F. choke.
the beat-frequency oscilator is connected to the pick-up terminals of a receiver, an appropriate note struck on the piano and the variable condenser rotated until a similar note is heard in the loud-speaker, the dial setting can be noted down to correspond with the frequency selected. By repeating this process with various notes on the piano, a graph may be drawn showing frequencies against dial settings. As will be seen, the notes on the piano do not reach such high frequencies as our oscillator will produce, but by listening carefully it is possible to tel when the oscillator is producing a note an octave above the note struck on the piano, and ifitis remembered that this wil correspond to a frequency twice that of the note struck, a calibration of the whole scale will be possible. Readers who are not musical may think it
80
CALIBRATING AND USING THE L.F. OSCILLATOR
advisable to enlist the services of a musician (a violinist for preference)
to assist them in plotting these higher frequencies.
While rotating the variable condenser two things may be noticed:
(1) The appearance at several points on the scale of subsidiary" chirps,"
and (2) the valve-voltmeter will give diferent readings for diferent
frequencies. In the case of (1), the "chirps "are caused by harmonics
beating with the fundamental to produce further audible frequencies,
but as they are relatively weak, they may be disregarded rather than
that elaborate filter circuits to eliminate them shall be included. Case
(2) explains the necessity for an output volu me control, because when
it is desired to take the response curve of the L.F. portion of areceiver
or amplifier it is necessary to make the voltage input equal at all
frequencies in order that variations in the output at the speaker terminals may be measured.
The L.F. Response Curve. --To take the L.F. response curve of
a receiver or amplifier, connect the output of the beat-frequency
oscillator to the input or pick-up terminals of the receiver under test, and connect the valve-voltmeter across these terminals as wel, now
rotate the variable condenser and select the lowest voltage obtained as
a basis for comparison. Quite alow voltage, say, .5 volt, will be quite
suitable if the amplifier has
a high gain. Now disconnect
the speaker from the set and
substitute an output choke
with a resistance equal to
the optimum load of the out- Ii
put valve, as shown in the Ii manufacturer's valve data
I
INPUT
I
1 mfd.
OF VALVE-
VOLTMETER
charts, in parallel (see Fig. 59).
Set the beat-frequency oscil-
lator at 50 cycles, adjust its output with the valve-volt-
meter to the pre-determined level, and then transfer the
valve-voltmeter to the output
·OUTPUT
FIG. 60. --The valvecvoolntnemcetteerd amcraoyss bea split optimum load
resisretaadinnceg douanbdled.the
end of the amplifier by connecting it across the load resistance
which we have just inserted. Measure the output volts at this frequency and repeat the procedure at anu mber of points in the range of audible
frequencies. Then plot a graph showing the variation of these voltages
with diferent frequencies and the response curve of the amplifier will be available. We can, of course, if we want to be more technical, reduce
these voltage ratios to decibels, and plot the result on logarithmic graph
paper. This has the efect of telling us more exactly what our amplifier will sound like to the ear, because quite large differences in output
voltage may be inaudible to the ear which is itself logarith mic in its
response to sound intensity variations.
87
PRACTICAL WIRELESS SVRVICE MANUAL However, ifwe suspect that our amplifier has apronounced resonance at 5,000 cycles the figures obtained will soon prove our suspicions, and we can safely assu me that, if our amplifier shows a reasonably flat response, then our loud-speaker is providing the resonance. When testing amplifiers with a high gain our output voltages may lie outside the range of our valve-voltmeter, even with the smallest input we can measure. This, however, is not important, because we can quite easily split our load resistance into two halves, and double the voltage readings obtained on our valve-voltmeter across one-half of the resistance. Fig. (30 will show how to do this. Apart from the uses of the instrument as a speaker-rattle locator etc., it provides a source of low-frequency voltage for modulation purposes, and in Chapter XII a radio-frequency oscilator will be described, which can be made into a modulated signal-generator by attaching it to the beat-frequency oscillator.
88
CHAPTER XII
A SIGNAL GENERATOR
THE instrument about to be described produces, in conjunction with the beat-frequency oscilator already dealt with, a steady signal over the whole range of approximately 150 metres to 3,500 metres, which can be modulated to produce an audible note in headphones or loud-speaker of any frequency within the range of the beat-frequency oscillator. Not only can it be used to check alignment of circuits, but an idea of the selectivity and sensitivity of any receiver can be gained, and comparisons made between individual receivers. This will, of course, be invaluable to aservice-man, for he can compare the performance of a receiver which he knows is up to standard with one which he suspects is failing, and carry out adjustments to render the poorer receiver equal in performance to the standard model. Furthermore, he can settle, once and for all, the relative merits of any two receivers by taking readings of their performance and comparing them.
81. --The complete instrument in screening box.
The Hartley Circuit. --On examining the circuit used, shown in Fig. 62, readers wil see at once that it is the familiar Hartley oscillator using centre-tapped coils of the plug-in variety to cover the wave-range required, and tuned by a
89
PRACTICAL WIRELESS SERVICE MANUAL
.0005 mfd. variable condenser. As neither set of vanes of this con-
denser is at earth potential, it is necessary to insulate the m fro m the panel and, for this purpose, the condenser is mounted on a " universal " bracket screwed to the baseboard, and provided with an insulated extension spindle which passes through the panel, and engages with the vernier dial which is mounted externally.
In order to modulate the oscillator, terminals are provided on the panel which are to be connected to corresponding terminals on the beat-frequency L.F. oscilator, and the voltages available from the latter source are injected into the grid of the H.F. oscillator. Grid circuit modulation is not always suitable, but in this case we are not dealing with apower oscillator, and the output from the beat-frequency L.F. oscilator would not be large enough to modulate the H.F. oscillator if anode modulation were used. A switch mounted just above the modulator terminals serves to short-circuit them when not in use and the -001 mfd. condenser acts as an efective by-pass for the highfrequency component when the modulator is in circuit.
Turning now to the anode circuit of the valve, it wil be seen that this is connected via an H.F. choke to H.T. positive, and the plug-in coil is parallel-fed through a .0003 rnfd. pre-set condenser. The reason for this is that when a Hartley circuit is set up with a direct drive the grid of the valve is sometimes overrun and "squegging " co mmences. By varying the feed by means of the pre-set condenser we can control the oscillation of the valve and arrive at asetting which will give smooth oscillation over the whole wave-band.
Output. --The output from the oscilator is taken from the anode end of the plug-in coil, and fed to a 5,000 ohm potentiometer through a .0001 mfd. condenser. This form of feed is open to criticism, as the condenser does not allow aconstant load on the oscillator at all settings of the potentiometer, and will be found to exercise some control over the wavelength emitted by the oscilator. This is not serious, however, and, provided the potentiometer is set at a fixed point while making a wavelength calibration, no trouble should be experienced. From the
slider of the potentiometer a screened lead is run through the side of the screening box, and care should be taken to make a good connection between the screening braid on this lead and the case. It is suggested
that a 4 BA clearance hole is drilled close to the hole through which the screened lead passes and a soldering tag secured by a bolt and nut
fixed in place. The screened lead should then be bound with some thin tinned copper wire and soldered to the tag. At the other end of the screened lead two crocodile clips should be attached, one to the central wire itself and one to a wire soldered to the screening braid. The length of screened lead outside the box should be suficient to enable connection to be made to components inside areceiver, but not so long that undue losses are introduced. About 2 feet should be ample.
0
10IVUHNHO IVNOIS V
GAMBRELL C.T. COIL. (SEE
± -00Diuf T
;CD VD.
F
003 PRE-SET
H F CHOKE
4005,ed (WITH EXTENSION
r-ZS
SPINDLE &
s.,4 DIAL)
HIVAC
· L2I0
M
I M MO
2ttfd
C'
---
OUTPUT I
0
1
--
(5,WI0T00H n INSULATED SPINDLE)
1"l
SWITCH . _411
·00 IA.sfd
E0 MODULATDR G
"ON-DEPSWITCH
62.--Theoretical circuit diagram of the signal generator.
HI0
0+ L T. 0
PRACTICAL WIRELESS SERVICE MANUAL
The size of the screening box will be governed by the sizes of the accu mulator and H.T. battery which readers select, and, therefore, no hard-and-fast dimensions are laid down. The panel and lid should be a good fit, and all the joints should preferably be made with brass angles or welded throughout, if possible.
Choice of Coils.-- One point regarding the coils is worthy of men-
tion. No doubt readers will have suitable plug-in coils, but care should be exercised in the selection of these, as the whole frequency, stability, and calibration of the oscilator is dependent upon the coils used, and it may be impossible to cover the whole wavelength range with but three coils of other make. Nevertheless, before buying special coils it may be as well to try those which are available, and the nearest sizes in ordinary plug-in coils are Nos. 50, 150, and 500. Readers may wonder why it is considered necessary to cover the whole wave-band between 150 metres and 8,500 metres, when broadcast receivers usually tune between 200-550 metres and 1,000-2,000 metres. The reason is that superheterodyne receivers have intermediate frequency stages which often require adjustment, and are tuned to 465 Kcs. (645 metres) or 110 Kcs. (2,727 metres). Also, when testing very sensitive receivers it may not be possible to reduce the output from the signal generator su ficiently when working on its fundamental, and the second harmonic, equal to half the wavelength, may be selected and used for adjustment, as it will be much weaker.
The construction of the apparatus should not be dificult if Figs. 62 and 68 are studied closely. Fig. 61 ilustrates clearly the screened lead passing through the side of the box.
Final Adjustments.-- First of all check through al the circuits
with the resistance meter of the multi-range tester, and pay particular attention to the insulation of the anode circuit of the oscillator valve. With no batteries or valves in circuit there should be no deflection apart from an initial "kick " of the needle, due to the charging of the 2 mfd. condenser when the resistance terminals are joined between anode and negative L.T. If all is in order connect up the batteries to their appropriate leads, and insert the valve in its socket. Unscrew the pre-set condenser to minimu m capacity, and close the switch shortcircuiting the modulator terminals. Then, with the B coil in the coil holder, and the -0005 tuning condenser at minimu m capacity, connect the 10 mA. range of the D.C. meter between the 80-volt tapping on the H.T. battery and the H.T. positive lead. If the oscillator is now switched on a current of 8 to 5 mA. will be indicated, and no change in the current should be noted when the grid terminal of the valve is touched with a wet finger.
Now commence to screw up the -0008 mfd. pre-set condenser, and it will be noticed that at one point the anode current falls to a lower value, which rises again when the wet finger is applied to the grid.
92
ACCU MULATO
MODULATOR TERMINALS SWITCHES
LOWER' UPPER.'"
0000
SLOW MOTION DIAL
;/,5,POT00E0NTnIOMETER,
CLIPS
EXTENSION SPINDLE
INSULATED SLEEVING
10IVUHNHO "IVNOIS V
30v HT BATTERN,
GAMBREL CENTRE TAPPED
Colt
·0003rnfd PRE 5E1
FIG. 63. -- Wiring diagra m of the signal generator.
PRACTICAL WIRELESS SERVICE MANUAL
The instru ment is now oscilating and the .0005 mfd. variable condenser should be rotated over the whole of the scale to ensure that oscillation is maintained. If oscilation ceases at any point the -0003 mfd. pre-set condenser should be screwed up until oscillation commences again.
List of Parts Required for the Signal Generator
8--Centre-tapped plug-in Coils (Sizes B, El,
and G)
1--Coil-holder (with provision for centre-tap
terminal connection)
1--Potentiometer, 5,000 ohms (ungraded) with
insulated spindle
1-4-pin Valve-holder
2--.0001 mfd. Fixed Condensers
1--.001 mfd. ,,
SP
2 mfd.
,,
1-100,000 ohm i-watt Resistance
1--H.F. Choke
1--.0008 mfd. (max.) Pre-set Condenser
1--.0005 mfd. Variable Condenser
1--Micro-vernier Dial
2--" On-of" Switches
2--Terminals (one with insulating bush)
1--Screening Box (see Text)
1-2-volt Unspillable Accu mulator
1-80-volt H.T. Battery
1--L210 Valve
Sleeving, Wander Plugs, Screened Lead, 2 Crocodile Clips, Insulated Spindle, and Universal Bracket.
The easiest method of carrying out calibration is to press into service a heterodyne wavemeter of known accuracy, but this method is not suitable. We shall, therefore, have to fall back on broadcast transmitting stations whose wavelengths we know, in order to do the necessary calibration.
Having ascertained that our signal generator is oscillating satisfactorily, we must now select the conditions under which our wavelength calibration is to be made, and the instru ment should be calibrated with the modulation terminals short-circuited and the output potentiometer set about half-way in. With most receivers this wil provide a
94
A SIGNAL GENERATOR
useful signal, and if the
potentiometer is always set
to the position we select we
shall be able to rely on the
accuracy of our calibration.
Using a Broadcast SignaL --Now connect the
output leads from the signal
generator to the aerial and
earth terminals of your broad-
cast receiver via a -0002 mfd.
condenser, as shown in
Fig. 65, leaving the normal
aerial and earth connections
in place. Then tune the
broadcast receiver accurately
to a station whose wave-
length is known, say, London
National (261.1 metres), then
switch on the signal gener-
ator and with Coil B in the
socket, and the lid of the screening box firmly closed,
FIG. 64. --Details of the wiring connections to the switches of the H.F. oscillator.
slowly rotate the dial of the tuning condenser fro m minimu m
to maximu m. Now, if the signal generator is oscilating satisfac-
torily, we shall hear at one point on the scale a beat note in the
loud-speaker of our receiver very similar to that which is produced if
we make our set oscillate and then tune in to a station. This beat note
always commences as a high-pitched whistle, which falls in pitch to a
silent point and then rises again and vanishes as the tuning controls
are rotated. The beat note produced by our signal generator will behave
in a precisely similar way, and all we have to do is to set the signal
generator at the silent point midway between the two audible notes
and jot down the dial reading. Now re-tune the broadcast receiver to
another station and repeat the process until a reasonable nu mber of
points have been logged down corresponding to wavelengths between
200 and 450 metres. The El coil should then be inserted in the coil
socket, and the process repeated with stations whose wavelengths lie
between 400 and 550 metres, also tuning the receiver to 900 metres
which is usually easily identifiable as the wavelength used by com-
mercial aircraft stations, and taking a reading of the dial setting for
this wavelength. Finally, insert the G coil and take readings against
broadcasting stations whose wavelengths are known between 1,000 and
2,000 metres.
95
PRACTICAL WIRELESS SERVICE MANUAL
Calibrating Graphs. --This wil give us a nucleus for drawing calibration graphs on each of the three coils, but it will be noticed that there is a gap in the graphs between 550 metres and 900 metres, and that no readings have been obtained above 2,000 metres. However, these are quite easy to fill in ifit is remembered that our signal generator not only produces a fundamental frequency, but also a nu mber of harmonics, the loudest of these being the second harmonic corresponding to a frequency twice that of the frequency to which the generator is tuned. If readers are not used to thinking in frequencies, the second harmonic will be found at a point equal to half the wavelength of the setting of the generator.
This makes it easy to fill in the gaps in our calibration chart, for if, for instance, we tune our broadcast receiver to London Regional (842.1 metres) and rotate the dial of our signal generator fro m mini mu m
TO AERIAL
TO SIGNAL GENERATOR
TO EARTH
BROADCAST RECEIVER
FIG. 65.--Connecting the signal generator to the receiver with a.002 mid. condenser in the aerial lead.
to maximu m we shall hear abeat note when it is tuned to 842.1 metres, and if we continue to rotate the dial, a second beat note of slightly smaller strength, when it is tuned to twice this wavelength or 684.2 metres. Thus, by selecting known stations between 300 and 450 metres and putting down the dial settings obtained on the signal generator when its second harmonic beats with these stations we can fill in the gap between 700 and 900 metres, thereby making known the settings for 645 metres which is equivalent to 465 Kcs. intermediate frequency.
Turning now to the wavelengths above 2,000 metres, we can take readings in precisely the same way. For instance, tune the broadcast receiver to Droitwich (1,500 metres) and rotate the dial of the signal generator with the G coil in circuit until the beat note produced by its
90
A SIGNAL GENERATOR
second harmonic is heard. The dial setting will then correspond to 3,000 metres. Similarly Radio-Paris, on 1,648 metres, will give us the setting for 8,296 metres, and so when we have plotted our graph we shall be able to locate 2,727 metres, which is equivalent to 10 Kcs. intermediate frequency.
Beat Note Andibility. --So far so good, but there is one point on which we have yet to touch. If our broadcast receiver cannot be made to oscillate, we shall not hear abeat note from our signal generator unless the receiver is tuned to astation, so we must find some means of superimposing an audible note on the output of our signal generator if meare to use it for adjustment purposes. This is the purpose of the modulation terminals, and we shall find that if we connect the output
1/2/EBONITE ROD WOUND' WITH 39 TURNS OF 366 'EUREKA' WIRE.
SOLDERED TO BRAID
TO CLIPS
·0002mfd 0
SOLDERED TO BRAID
TO SIGNAL GENERATOR
SMALL TIN BOX WITH WELL -FITTING LID.
60. --Tbe components for a" du m my aerial "mounted in asmal tin box.
terminals of the beat frequency oscilator to the modulation terminals of the signal generator and open the modulation switch we shall hear the audible note produced by the L.F. oscillator in the loud-speaker of a broadcast receiver which is connected and tuned in to the signal generator. We shall be able to vary the pitch of the note produced by varying the setting of the dial on the L.F. oscilator and also vary the strength of the note by adjustment of the output potentiometer on this instrument. A word of warning may be necessary here, viz., the output from the L.F. oscilator should never be increased to a point where the beat note produced by the signal generator and a broadcasting station when listened to on a receiver, sounds harsh. If insuficient volume is produced in the loud-speaker of the set, always increase the
output from the signal generator, not the L.F. oscillator, otherwise the signal generator may be over-modulated.
7
97
PRACTICAL WIRELESS SERVICE MANUAL
Using a "Dummy" AeriaL --The simple .0002 mfd. coupling
condenser used for calibration purposes will usually be found a satisfactory means of connecting the output fro m the signal generator to a receiver under test, but some readers may wish to construct a more complete "du mmy aerial." As will be seen on reference to Fig. 66, it consists of asmall brass or tin box, into which the output lead from the signal generator is led at one end, and the shielding braid connected to the box to earth it. Inside the box are mounted a .0002 mfd. fixed condenser, and a small inductance wound with 86 gauge " Eureka " wire. These are connected in series, and brought out to a further short length of screened lead, the screening of which is again secured to the box. The short length of lead is terminated by crocodile clips in the manner shown. This little accessory serves to reproduce more or less accurately the conditions to be expected when a broadcast receiver is connected to an outside aerial, and if the signal generator is used to calibrate a receiver for wavelength it will be found that the readings obtained wil be maintained when the receiver is actually used on a normal aerial and earth system.
Some readers may be interested to know exactly what the sensitivity of their receivers is in actual terms of microvolts, etc., fed into the aerial circuit. I must make it clear that this equipment will not do this. Measurements of this description require very costly and complicated apparatus, quite beyond the capabilities of most amateurs to construct, but these instruments wil provide a means of tracking all the trouble which an amateur or serviceman is confronted with, and if used intelligently are capable of giving eminently satisfactory results within the sphere of their capabilities. It should be understood that although each of the instruments which have been described will operate separately, and, indeed, have been designed with that end in view, yet their major usefulness will be achieved if they are used in conjunction with one another.
98
CHAPTER XIII
TRACING FAULTS IN A SUPER HET
IN principle, the location of faults in a superheterodyne receiver is the same as with a"straight "receiver, but there are many additional actual tests which should be applied. When a multi-range meter is available it is, of course, a fairly simple matter to check the various components; but before doing this it often saves time to trace the fault to the particular portion of the circuit in which it exists. For example, if it is first ascertained that it lies in the intermediatefrequency amplifier, comparatively few components have to be tested to find the defective one; but if it is simply known that the fault is in a part of the circuit previous to the L.F. amplifier, a good deal of time might be wasted in testing every individual component in that part of the set.
Process of Elimination.-- It is fairly common knowledge that a
" straight " receiver, employing both high and low frequency stages, can be tested stage by stage by connecting the loud-speaker, or a pair
1.-ftVi·
HT
FIG. 67. --This skeleton circuit shows how the frequency-changer of atypical
superheterodyne can be cut out of circuit, so converting the set into a "straight " arrangement. Broken lines indicate original connections, and
heavy lines the new connections. 99
PRACTICAL WIRELESS SERVICE MANUAL
of 'phones, in the anode circuit of each of the L.F. valves and the detector, and by transferring the aerial lead from the first valve to each consequent H.F. valve, and then to the grid circuit of the detector. Up to a point, the same idea holds good with a superhet, but it is obviously impossible to eliminate H.F. stages, because there probably are none. The I.F. valves correspond to the H.F. valves in a" straight" arrangement, but they are not tuned to the signal frequency, but to another frequency outside the range of broadcast wavelengths. For this reason it would be useless to transfer the aerial lead-in to the grid terminal of one of the I.F. valves.
Converting to a " Straight " Circuit. --The simplest procedure in a case like this is to transfer the lead which normally goes from the grid terminal of the aerial tuning circuit to the grid of the first detector or frequency-changer to the grid of the I.F. valve, and to break the connection from the latter to the I.F. transformer. This is shown in Fig. 67, where the original connections are indicated by broken lines and the new ones by heavy lines. When this has been done it is evident that the I.F. valve is used as an ordinary H.F. amplifier, its input circuit being tuned to the frequency of the signals to be received.
This is not suficient to provide reception though, because the anode circuit of the I.F. valve is still tuned to the intermediate frequency, and so nothing --or very little --in the way of signals would be passed on to the detector. The next step, therefore, is to modify the anode circuit arrangement so that it will respond to the frequency of the received signals. One method would be to replace the primary winding of the L.F. transformer with an H.F. choke; but a simpler idea is merely to transfer the grid-condenser connection from the secondary winding of the transformer to the anode terminal of the pri mary winding; this also is indicated in Fig. 67.
After making the simple alterations described --they may be made very roughly, and without disturbing the rest of the set --the receiver becomes asimple "straight " three-valver, comprising an H.F. amplifier, choke coupled to a leaky-grid detector, and L.F. amplifier. As such, it should be capable of providing good reception from at least half a dozen stations, assu ming that the original fault was in the frequency changer, as had been assumed. Super-selectivity should not be expected, nor should long range, because there are only two tuning circuits (the original band-pass tuner) and reaction is not employed. Nevertheless, should it be found that a new component or valve is required, and this cannot be obtained immediately, the receiver can be used as a reasonably eficient three-valver for an indefinite period.
A Precaution with Mains Sets. --In making the above suggestions, it has been assumed that the receiver was afour-valve battery-operated instrument of the simplest type, embodying a pentagrid frequencychanger, folowed by a single I.F. amplifier, a leaky-grid detector, and
100
TRACING FAULTS IN A SUPERHET
a pentode output valve. The general idea holds good, however, for
most other types of set, but in the case of a mains receiver it is better,
after changing the connection from the band-pass filter, to connect the
grid of the first valve direct to the lower end of the tuning circuit, so
that it receives its usual bias voltage. If this were not done the valve
would pass more than its normal H.T. current, and if the valve were
removed from its holder the H.T. and L.T. voltages applied to the other
valves might become too high. The method of "earthing " the grid
is shown in fig. 68, where it will be seen that the grid must be connected
to the A.V.C. line (where provided), to the variable bias supply, or
directly to the earth line.
When there are two I.F. valves the anode circuits of both must be
modified, whilst a.0002 mfd. grid condenser and a2-megohm leak should
be included in the grid circuit of the first, as shown in Fig. 69. The
object of these components is to prevent the grid from being biased
positively by the H.T. voltage, and to ensure that the bias voltage
remains as before. If the set still refuses to function after making these minor altera-
tions to the wiring, it can be treated in asimilar manner to a" straight"
arrangement by taking the lead from the grid terminal of the band-pass
tuner to the grid of the second I.F. valve, and then to the grid condenser
of the detector. Even with the latter connection the set should operate,
although signals will naturally
be rather weak; nevertheless,
it is possible in this way to
check the I.F. valves and their
corresponding circuits.
Checking Diode Second
Detector. --The position is not
quite so straightforward when
a double-diode-triode or similar
valve is used as second detector,
but even then the main principles
may be applied, as shown in Fig.
70. In this case one end of each
winding of the I.F. transformer
which feeds into the diode is dis-
connected and replaced by a
double-wound coil and a -0005 E
mfd. variable condenser. The coil may be of any type having primary and secondary windings, one of the type used for aerial circuit tuning being quite satisfactory. The corresponding
68. --In the
BIAS OR
case of a mains-
A.VC.
operated set the detector grid of the
frequency-changer should be connected so
as to receive the normal bias, as shown,
when using the modified connections shown
in Fig. 67.
101
PRACTICAL WIRELESS SERVICE MANUAL
tuning condenser has to be operated at the same time as the original gang condenser, of course --but no dificulty should be found in obtaining some sort of reception.
Gang-condenser Connections. --The tuning circuit is the part which probably causes more dificulty than any other; this is to be expected in view of the fact that it is this which differs most from the corresponding part of the older types of receiving set. Sometimes it is found, for instance, that nothing but the local stations can be heard, the signals from these being much weaker than they should be, and the dial reading
being entirely diferent from what would be anticipated. Such a fault generaly points to the fact that the gang condenser has been wrongly connected, the oscillator portion being joined to one of the signalfrequency coils instead of to the oscillator coil. This mistake can easily be made because there is no standard position for the oscillator section (which has quite a diferent capacity and a diferent "law " fro m the others), and it is sometimes placed at the end nearest to the operating spindle, and sometimes at the other end. Very often the condenser is marked in some way, but this is not universal.
The Oscillator. --Should there be any doubt concerning which is the oscilator section it is easy to check up by removing the screening cover and examining the vanes. In most cases the moving vanes are of diferent shape, whilst the fixed vanes are cut away more in the centre where the spindle passes them. When the condenser is of the three-gang type it is easy to find one section which is diferent fro m the other two, but this is not possible when using a two-gang component. Fig. 71, however, shows the general shapes of the vanes in both sections, and this should clear the dificulty.
Incorrect Intermediate Frequency. --Another trouble which is experienced by those who do not follow any published design, or who
do not employ the specified components, issimilar to that just described, but normally less severe. This is due to using a condenser designed for an intermediate frequency of one figure with coils designed to work at a diferent I.F. This is a trap which is very easily fallen into by those who build from "junk "parts picked up at acheap store I Yet another similar trouble is due to using I.F. transformers of the wrong frequency calibration; in this case the dial readings will be approximately correct, but all signals will be very weak --if received at all.
When tuning is not perfectly sharp --as it should be with asuperhet
--it is generally indicated that the trimmers on the intermediatefrequency transformers have not been correctly adjusted, although attention should also be directed to the trimmers on the gang condenser. But the latter rarely cause flat tuning; they are more likely to reduce signal strength, because the tuning nearly always" follows "the tuning of the oscillator condenser.
Interference. -- When long-wave interference is in evidence, however,
102
TRACING FAULTS IN A SUPERHET
Fm. 80. --A skeleton circuit showing the connections used in eliminating the frequencychanger when there are two I.F. valves
PRACTICAL WIRELESS SERVICE MANUAL
the setting of the gang-condenser trimmers should be checked, whilst
it should also be seen that the I.F. transformers and their connecting
leads are properly screened, especially if any of the connecting wires
are more than a few inches long. Another possible reason for this
interference, or for so-called second channel interference, is due to
signals leaking across the capacity between the electrodes of the
frequency-changing valve. The amount of leakage is nearly always of
very small proportions, but it might be suficient to allow signals from
IFTiZAN5FOR MER (DISCONNECTE D
the local stations to pass. And when this does occur
whistles wil be heard at
various parts of the tuning
scale, which cannot be
eliminated properly by alter-
ing the settings of the I.F.
transformer trimmers.
One simple modification
which is often successful in
removing the trouble con-
sists of inserting an H.F.
choke of low inductance
between the anode of the
frequency changer and the
anode terminal of the first
I.F. transformer, as shown
in Fig. 78. A small, in-
expensive and unscreened
FIG. 70. -- W hell a double-diode-triode type of valve is used as second detector the I.F.transformer which feeds it should be replaced by a double-wound tuner and .0005 mid. variable
condenser as shown here.
"reaction " choke can often be used, but the inductance of abetter-class component is so high that it will probably silence the set,
or otherwise cut down signal strength. A choke can be made by
winding about 200 turns of 86-gauge enamelled wire on a bobbin of
in. inte rnal diameter. It is not essential to split up the winding,
because the small self-capacity which results from pile-winding is not
suficient to have any serious efect. The bobbin can be made by
screwing two cardboard end cheeks 1in. in diameter to the ends of a
wooden rod about 1in. long, as shown in Fig. 74. The rod and ch eks
should be given a couple of coats of shellac or celulose varnish, and
connections can be made by soldering short lengths of flex to the ends
of the winding.
Instability. --The same cure is often satisfactory in the case of
instability or persistent oscillation, which renders reproduction poor
and causes the set to " howl " at each side of the tuning point of a
104
TRACING FAULTS IN A SUPERHET
signal. Another reason for this instability is too close a coupling between the primary and secondary windings of the I.F. transformers. When the positions of the windings are variable it is an easy matter to efect a remedy by moving them farther apart. When they are fixed, a similar result can be obtained by connecting a fixed resistance of between 200 and 1,000 ohms between the ends of the primary winding
CARDBOARD CHEEKS
SCREW,
WOOCEN ROD.
FIGS. 71 and 72. --The general shape of the vanes in a plain and oscillator section of a
superhet-type gang condenser.
74. --Details of the spool for the special choke indicated
in Fig. 73.
HT 1,000n. FIG. 73.--Interference GBand certain forms of instability can be cured by connecting asmall choke
as shown here,
IIII LET.
r---G.
Flu. 75. -- Instability
which ,occurs on long
waves only or on the
G B. local station, can often be
cured by resistance
asconshnoewctninghere.a
of one or both of the transformers. In most cases it will be suficient to join a 500-ohm resistance in parallel with the primary of the first
I.F., as shown in Fig. 75. It is very often found that the instability is experienced on long
waves only, or when receiving one or two powerful transmissions. When
this is the case it is a good plan to connect an on-of switch in series with the resistance, as in Fig. 75, because the resistance does actually reduce the sensitivity of the receiver to a certain extent, and thus
impairs the reception of weaker signals. Leads to the switch should be short and screened.
105
CHAPTER XIV
TRI M MING AN D ALIGNING RECEIVERS
W HEN two or more circuits are tuned by the use of a ganged condenser, it is necessary to trim each section of the condenser to counteract the efect of the diferent stray capacities across each section caused by the added capacity of the aerial, wiring, valveholders, etc. This is accomplished by adding asmall adjustable amount of capacity to each of the tuned circuits, and is usually carried out by means of a trimmer for each section of the ganged condenser.
A properly ganged receiver is one in which the ganging holds good over the whole of both wave-bands. The first essential is that the coils shall be accurately matched, that is, the inductance of each must not vary by more than 0.5 per cent. The second requirement is that the matching of the various sections of the ganged condenser shall be accurate at all settings. Actually, of course, this is not possible as a commercial proposition, and it is suficient for the capacity of each section of the condenser to be exactly the same at about six points on the tuning dial.
Ganging Straight Receivers. --The procedure of ganging a straight receiver is to arrange the trimmer of the tuned circuit with the greatest
capacity due to wiring, etc., at a low value, and then to adjust the trimming condensers of the other tuned circuits until the capacities across the coils are the same in all cases. This is shown by the receiver being more selective and tuning sharply. The tuner is now correctly adjusted and the circuits should, if coils and condensers are accurately matched, remain in tune with each other over the whole of both wave-bands.
Before dealing with actual ganging, it will, perhaps, be as well to consider the causes of the added capacity and how to reduce it to a mini mum. An aerial, for instance, has alarge capacity to earth, which must be compensated for by the action of atrimmer on the section of the condenser which tunes the aerial circuit. This capacity is greatly reduced, however, by the use of an H.F. transformer in the aerial circuit, but it is always as well to re-trim the circuit when the receiver is used with a different aerial.
The position of the reaction condenser afects the tuning of the detector circuit. The capacity across this circuit varies with the by-pass capacity in the anode circuit of the detector valve, but this efect can be obviated by the use of adiferential reaction condenser. When using an ordinary reaction condenser, however, it is best to gang with the
106
TRIMMING AND ALIGNING RECEIVERS
detector circuit just on the point of oscillation, since the receiver will then be accurately ganged when the transferred capacity in the grid circuit of the detector valve corresponds to the most selective condition
of the tuned circuits. A screen grid-valve or an H.F. pentode acting as adetector transfers
but little capacity to its grid circuit, but, nevertheless, it is still advisable to gang the receiver with the detector just short of oscillating point,
as the set is then in its most selective position. Modern ganged condensers are fitted with trimmers which are
controled either by ascrew or star wheel situated at the side or on top of the condenser cover. The latter is more convenient, since it is very accessible. To gang astraight receiver, first set the trimmers to about their midway position and then tune in aweak station of as low awave-
length as possible. Always gang on a weak station, since the ear is very much more sensitive to small changes in volume when the total
volu me is small than it is when the total volume is large. Now adjust the trimmer of the detector tuned circuit for maximu m volu me by moving it slightly in or out. At the same time" rock" the main tuning
condenser, and gradually bring the dial reading to the correct setting for the station being received. The detector trimmer should be gradually rotated to keep step with the movement of the main tuning condenser, and, when the correct setting of the latter is obtained, afinal adjustment should be given to the trimmer to bring the station up to maximu m volu me.
Band-pass Tuners. -- When a band-pass circuit precedes the H.F. valve, attention should next be turned to the circuit of the band-pass tuner, which feeds the grid of the H.F. valve, i.e., the second band-pass circuit. Adjust the trimmer of the condenser section which tunes this circuit for maximu m volume, and then carry on with the aerial circuit. Always co mmence at the detector stage and work backwards, no matter how many tuned circuits are incorporated in your receiver.
If one of the trimming condensers has to be screwed right in, the other trimmers should be unscrewed alittle and the whole procedure of ganging recommenced. Similarly, if one of the trimmers has to be set at its minimum value, then the other trimmers should be increased a little and the receiver re-ganged. If this is not done, it is impossible to tune the receiver correctly. If one of the trimmers has to be fuly out it indicates that the stray capacities associated with that particular
circuit are too great and the capacities of the other sections of the condenser must be increased. If one of the trimmers has to be screwed right down tight, then the stray capacities across that circuit are lower than those associated with the other circuits, and the latter must be lowered by unscrewing the trimmers alittle. If it is impossible to gang the receiver without one of the trimmers being either all-in or all-out,
then the fault is, assuming the coils and condensers to be correctly
107
PRACTICAL WIRELESS SERVICE MANUAL
matched, due to excessive capacities across one or more of the other tuned circuits, and these should be examined, as explained later.
When ganging, try to arrange matters so that the capacity of the trimming condensers is at a low value, so as to avoid restricting the tuning range of the receiver. If the trimmers are set at a high value, then, when the tuning condenser is near its minimu m position, the capacity across each coil wil be large and the receiver will not tune to a low wavelength. Similarly, the increase of wave range at the upper end of the scale will be small, since the capacity of the trimmers will then be only a very small percentage of the capacity of the condenser.
Having ganged the receiver at the lower end of the mediu m waveband, a weak station at the upper end of the tuning scale should be tuned in, and the trimmers varied to see if any improve ment can be obtained. If the coils and condensers are in good order, there should be no necessity to alter the values of the trimmers.
Now return to the lower end of the wave-band and finally, and accurately, gang the receiver on a very weak station of as low a wavelength as possible. Ganging should now hold good over the whole of both wave-bands.
Using aMeter. --The use of ameter greatly simplifies ganging, since it is now possible to see what is happening, and to obtain far greater accuracy than by ear.
The meter should be inserted in the detector anode circuit, and, in order that the meter shall not cause instability, it should be inserted in a position as far from the actual anode of the valve as possible. Between the decoupling resistance and the H.T. supply is a good place, but where the valve is of the indirectly heated type, the meter may be inserted in series with the cathode lead.
Where a diode is used as a detector, and A.V.C. is employed, the meter should be inserted in the anode circuit of one of the controlled
valves. With a receiver using a leaky-grid detector, a station is tuned in
and the trimmers adjusted for minimu m reading on the miliammeter. Gang on a fairly powerful station, as otherwise the efect of trim ming will not be noticeable, but take care that there is no overloading of the
detector, as this will result in "double hump" tuning and fictitious results will be obtained. If the detector works on the anode bend principle, maximu m signal strength is indicated by amaximu m reading
of the milliammeter. Never be satisfied until you can obtain a definite decrease in signal
strength on either side of the optimu m point. This should be indicated by the reading of the milliammeter increasing greatly immediately the
station being received is slightly of-tune, that is, directly the tuning condenser is rotated slightly.
108
TRIMMING AND ALIGNING RECEIVERS
When using a meter in the anode circuit of a valve controlled by an A.V.C. system, accurate ganging is obtained when the deflection is
at aminimum. Where amiliammeter is not available, avoltmeter may be connected
across a voltage dropping resistance in the anode circuit, and the receiver ganged by adjusting the trimmers for minimu m voltage drop
across the resistance. Uneven Ganging. --It may happen that a receiver gangs perfectly
wel at the bottom end of the scale, but not at the top. This is bound to occur to a slight extent in all sets, but may be overcome if the condenser is fitted with a concentric trimmer which can be operated
from the front of the panel. If the ganging becomes more and more out of step as the condenser is advanced, then the fault is definitely in the coils. On the other hand, the condenser is at fault if the ganging is uneven, that is, the set is properly ganged at the bottom of the scale, is out of gang at the middle, and then falls into gang again at the top. A condenser may be corrected by bending one of the segments, but this is not a job which the average constructor can carry out, and the condenser should either be returned to the manufacturers or the work entrusted to a skilled service-engineer.
If the ganging is out only at one particular point, it is in all probability due to the resonance efect of the reaction choke, or an H.F. choke used for parallel feeding in one of the anode circuits. This efect is not so important, however, as it will only upset ganging at one definite point, and can usually be remedied by substituting adiferent choke.
A change in the position of the reaction condenser alters the stray capacity across the detector circuit, and it is not possible, therefore, to gang a receiver so that the matching is accurate for all positions of the reaction condenser. It will be appreciated that ganging must be most critical when trying to receive a foreign station, and that in such a case use wil have to be made of the reaction control. As explained above, it is important, therefore, to gang areceiver with the reaction condenser
advanced so that the receiver is just short of oscillation. Afect of Trimming on the Wave-range. --It will be appreciated that,
by altering the trimmers, a station can be received at several points on the tuning scale. For example, it would be possible accurately to gang a receiver so that London Regional, instead of being received at a dial reading of 842 metres, was received either at 880 or 350 metres, depending on the setting of the trimmers. But, in the former case, it
would not be possible to receive stations which would normally come in at the 200-210 metres mark, and, in the latter case, the top end of the wave-range would be curtailed. Therefore, having trimmed a receiver correctly, all trimmers should be screwed out and the receiver
re-ganged with the minimum capacity across each circuit. Any slight
109
PRACTICAL WIRELESS SERVICE MANUAL
Idivergence in the position of stations as determined by the tuning dial can generally be put right by altering the position of the dial. Ganging on the Long Wave-band. --A receiver which is correctly ganged on the mediu m waves should also be correctly ganged when the receiver is switched over to long waves. With modern coils, arrangements are made so that the stray capacity due to the aerial is constant on both wave-bands. Most of the individual stray capacities are bound
to be the same whatever wave-band the set is switched for, but, on the other hand, some, notably switches, self-capacity of the long wave windings of the coil, leads, etc., may greatly difer. Assu ming that the
coils are correctly matched for the long wave-band as well as the mediu m wave-band, the most usual cause of mis-ganging on the long waves is due to the self-capacity of the switches. It is important, therefore, always to use switches of as low a selfcapacity as possible.
Reducing Stray Capacities. --Stray capacities include the grid to filament capacity of valves, the anode capacity of valves, the capacity of the valve-holders, the capacity of screened H.F. leads, the capacity of chokes and condensers, reaction coils, etc.
Every efort should be made to keep these stray capacities as low as possible, in order to increase the wave-range of the receiver. When using screened wire in H.F. leads, always use sleeving of large diameter with as thin a connecting wire as possible, so that there is a good space between the metal braiding and the wire.
Iron-cored H.F. chokes have a much higher capacity than air-cored, while A.V.C. or grid bias decoupling condensers must be made as large as possible to avoid their having too great an effect on the tuning of the circuit in which they are used.
Similarly, the wiring can introduce quite alarge capacity, and it is advisable to keep all wires carrying high frequency currents as short as possible. Another mistake which is commonly made, especially in mains receivers, is to use a screened wire for connection to the grid of
the detector valve. This should always be avoided as the capacity of such a lead is bound to afect ganging.
Clanging Superheterodyne Receivers. --Ganging asuperhet is slightly different, but is no harder than ganging a simple straight receiver with only two tuned circuits, provided the folowing procedure is carefully followed.
First, adjust the trimmers of the I.F. transformers to about their midway position, and unscrew the trimmer controlling the condenser tuning the oscillator coil. The remaining trimmers should be set to their midway positions. Now tune in aweak station at the bottom end of the mediu m wave-band and adjust the trimmers of the I.F. transformers, beginning with the anode trimmer of the first I.F. coil and working backwards to the grid trimmer of the last I.F. coil, for
10
TRIMMING AND ALIGNING RECEIVERS
maximum signal strength. Now adjust the trimmers of the aerial coil, or
band-pass coils, if the latter are used, and endeavour to tune in astation at the top end of the scale, and adjust the trimmer of the oscillator tuning condenser for maximu m volume, at the same time gently "rocking" the ganged condenser. Now adjust the I.F. tri mmers for maximu m volume and return again to the bottom end of the scale. The I.F. trimmers are again adjusted for maximum signal strength, and areturn made to the top of the scale, where the I.F. tri mmers are again
adjusted. This procedure is carried out until a position is found for the I.F. trimmers which is constant for both ends of the scale. The trimmer of the aerial circuit, or band-pass circuit, is then adjusted for maximum signal strength. It will be appreciated that it is possible to
set the trimmers of the I.F. coils so that all the I.F. circuits are accurately in tune, but at a frequency of 100 or 120 Kcs. instead of the 10 Kcs. required. If the stations are received at their correct wavelength as determined by the dial, then it is safe to assume that the intermediate frequency is correct. Otherwise, the intermediate frequency is not operating at its correct wavelength, and the whole procedure must be repeated.
Having ganged correctly for the medium wave-band, attention should now be turned to the long waves. A separate padding condenser in the oscillator circuit is included for this purpose. Tune in a station and adjust this condenser, which is usually of the pre-set type and mounted on the baseboard, for maximu m signal strength, gradually rotating the main tuning condenser until the station is received at its correct wavelength. The receiver is now correctly ganged for both wave-ranges.
In carrying out the operation of ganging a receiver it is essential to tune in a station which is not subject to fading. The performance of the operations in daylight will ensure that no fading is occurring, and it is better to use astation trans mitting speech, since music varies so in volume.
A good plan is roughly to gang the receiver during the evening, and then to carry out the final ganging in daylight. If it is not possible to find a weak station at first, especially in the case of a superhet where there is no manual pre-detector control, the local station may be used for ganging by cutting down the signal strength by using a very short indoor aerial. Gang up roughly on this and then connect up your outdoor aerial. You should now be able to find aweak foreign station on which to carry out ganging. With astraight receiver using variable-mu valves, the signal strength of the local station can, of course, be cut down by increasing the bias, but even then it is better to turn the volume full on and endeavour to find aweak foreign station.
The use of a meter greatly facilitates ganging a superheterodyne receiver. A milliammeter is inserted in the anode circuit of the I.F. valve,
11
PRACTICAL WIRELESS SERVICE MANUAL which is, of course, controlled by A.V.C., and the foregoing adjustments are made until a minimu m reading is obtained on the meter.
The operation of ganging a receiver may perhaps appear a little formidable from the foregoing, but it is in practice quite simple and interesting if the work is carried out systematically. To start of with a receiver on which it is just possible to receive the local station, and then to" hot it up" by ganging, so that upwards of fifty stations may be received, is work which is well worth doing, and amply repays all the time and trouble spent on it.
Once the ganging and aligning has been accurately and carefully carried out, leave wel alone. It is practically an impossibility for the settings to alter, and therefore only harm can result from further adjustment.
A good plan is to run a couple of drops of sealing wax over the trimming screws to prevent them from turning due to vibration. If adjustment is required at a later date --because a new valve is fitted, for example --the wax can easily be chipped away with the blade of the screwdriver.
12
CHAPTER XV
T ES TI N G V A L V ES
THE standard of performance and consistency reached by modern valves is very high, and the troubles which occur are mainly the result of using old valves which are past their prime, although,
of course, there are instances in which the valves are definitely faulty.
This suggests that the first stage in testing out a receiver for the
eli mination of faults should be to examine each valve separately, and that amateurs should be acquainted with the methods of conducting such tests. There is a quite mistaken notion that valve testing always
involves the use of alarge quantity of expensive equipment. While it
is true that certain types of test can only be performed with the use of a considerable amount of very accurate gear, it is possible to carry out
all the essential tests required by the amateur with quite simple equipment such as may be rigged up at home for avery modest expenditure of time and money.
Two Distinct Types. --It should be remembered that there are two distinct types of tests on valves. The first are on the valves outside the set, and are intended to indicate their condition and eficiency. The other tests are those conducted on the valves under the operating conditions obtaining in the receiver, and are, therefore, of more consequence in afording a check on the working conditions which may or
may not be afected by circuit faults or valve faults. The advantage gained by carrying out the initial type of tests is that, once the valves have been proved to be in good order, any abnormalities discovered during the course of the second series of tests will indicate that the trouble, whatever it may be, is due to the circuit and not to the valves. Moreover, if the first tests reveal that one or more valves are faulty, replacement will, in most cases, effect a complete cure, and much valuable time will have been saved.
In examining valves independently of the receiver, there are two
alternatives. One is to conduct acomplete series of tests which approximate to those adopted by valve- makers during and after manufacture, and the other is the application of a simpler series of tests calculated
to determine whether the valves are in reasonable serviceable condition. The simpler testing technique to be described is quite adequate for
the amateur and, indeed, for the service departments of dealers.
Inter-electrode Insulation. --The first test is acheck on inter-electrode insulation. Sub-normal insulation between electrodes is one of the most common defects found in valves, and asource of many troubles which,
8
13
PRACTICAL WIRELESS SERVICE MANUAL
without some means of checking, may appear very obscure. For
example, low insulation between the heater and the cathode of an
indirectly heated valve is afrequent source of hum, and is more likely to occur in valves of the " universal " type, although by no means
unknown in A.C. valves. Grid-to-cathode shorts or partial shorts are
also not infrequent and, in general, the risk of poor insulation between
electrodes is more likely to occur in the more complex types of valves,
such as frequency-changers, and the various types of multi-function
valves such as double-diode-triodes.
The second essential test is on the emission of the valve, and will
show whether a valve which will pass the first test is serviceable or
whether, by reason of age or mishandling, it has lost its emission and
requires to be replaced. There are several ways in which such tests
can be conducted, and numerous types of suitable equipment are dealt
with in earlier chapters. This explains methods to be adopted when
such apparatus is not available. For the tests on insulation be-
tween electrodes quite simple apparatus will sufice. The simplest
is to connect asuitable voltage and aneon lamp in series between each
pair of electrodes in turn. A high tension battery or the output of a
mains unit, a neon tube of the type used in tuning indicators with
a suitable series resistance, and a pair of test prods are all that are
really essential (see Fig. 76), and will sufice when only very occasional
tests are required. But for those who are likely to be doing a fairly
large amount of testing, or who take a pride in the appearance of their
equipment, it may be worth while constructing a more impressive
apparatus consisting of a panel with a line of valve-holders of diferent
types -4 and 5-pin, 7-pin,
9-pin, and side contact --with
the neon lamp fitted behind a
neat escutcheon, and aswitch-
ing device to permit the
various pairs of electrodes to
be selected for testing.
A Suggested Tester. --It will be found that two rotary
nc'esisz-4/vce
switches connected in accordance with the diagram shown
in Fig. 77 will achieve this object, and if ten-way switches
which are standard products,
are used, any combination of
two electrodes in any standard
type of valve can be tested.
It is advisable to nu mber the
FIG. 70. --A general method for testing interelectrode insulation.
switch positions from 1to 10 consecutively, and to connect
114
TESTING VALVES
them to the sockets of the valve-holders bearing the same nu mbers according to the standard B.V.A. convention, reserving No. 10 for the top cap. A list of the standard connections for various types of valves should also be obtained, and mounted on a card for ready reference.
Tdo C.4-
4t- - n(4y
- -R---0
--v w\AAAAAA,
/Pr
W72e4a9n/E
Fro. 77. --A layout for a tester of inter-electrode insulation.
As an alternative to using two switches, an arrangement of nu mbered
sockets may be used, and for this purpose a single 9-pin valve-holder of the chassis-mounting type is particularly handy, each socket being connected permanently to the socket bearing the same nu mber in every other holder, and the test connections being made by means of two plugs attached to short flexible leads. In this test low insulation is, of course, indicated by ful ilu mination of the neon tube, and it will be necessary first to adjust the value of the safety resistance to a value
15
PRACTICAL WIRELESS SERVICE MANUAL
suitable for the test voltage used. Valves should be tested when hot, i.e., after the heater or filament has been in circuit for some time, as
poor insulation is often the result of expansion, and is only revealed after the valve has warmed up.
Checking Emission.-- Next for consideration is the means for taking
the emission test. The simplest, and possibly the most convenient, is to test the anode current when the valve is operated at its correct filament or heater voltage, and the voltages at the other electrodes are maintained at some standard values such as anode volts 100, and grid volts zero, or such other values as are indicated in the maker's catalogue or characteristic curves. Care should be taken that if the test conditions are such that the anode current is likely to be substantially greater than the normal anode current under working conditions, the valve is not in circuit for longer than is necessary to take the readings. In the
case of output valves it is generally advisable to select testing conditions in which the anode voltage is less than the normal rated anode voltage of the valve, or in which a negative bias is applied to the grid to limit the anode current to a safe value, otherwise there is risk that even in testing the emission of the valve will be impaired. But whatever the test conditions are, they should be values for which adequate data as to the approximate anode current for a normal valve is available. Even so, it should be remembered that valve- makers allow a fairly wide margin of working " tolerances," that is to say, a valve may be passed as satisfactory if the anode current is somewhat less or greater than the average value. If, therefore, a variation from the expected test value to as much as 50 per cent is observed, it does not necessarily signify that the valve is unfit for service, more particularly if it is a fairly old valve. The normal emission of modern valves is on a very generous scale, and quite good results may often be obtained in practice with valves which appear to be considerably sub-normal with respect to emission. Nevertheless, if a valve is found to be of low emission it should be dee med a possible suspect if other tests on the receiver as a whole reveal no other faults.
Test equipment for checking emission can vary between the most
simple, and the very elaborate. A panel with a range of valve-holders and a selection of split-circuit adaptors is adequate for occasional testing, or for stil less frequent use a row of diferent valve-holders of the baseboard type with short leads and wander plugs connected to each terminal can be used in conjunction with arow of sockets connected to sources of high tension, low tension, and grid bias. There are, of course, many ingenious adaptors of the universal type which permit every type of valve to be tested in this way, or the ingenious reader may like to spend an hour or two devising switching arrangements which will simplify operation, and minimise the nu mber of loose leads and connections.
16
TESTING VALVES
If tests on inter-electrode insulation and emission indicate that the valve is in reasonably good condition, no further tests on the valve, as a valve, are necessary. It is, of course, quite an easy matter to check the mutual conductance of the valve by noting the anode current under two diferent conditions of grid voltage (the diference should be small, say 1 volts), and then dividing the change of anode current by the
-
_ 11_
.141000.
/1:1ir
11- i iiii 10111 11i1
II P
r I
78. --An interesting commercial all-valve test panel. This is a most useful accessory for the keen experimenter and service engineer.
diference in grid voltage, but the value of such adetermination, being the "static" characteristic and not the "slope" under working conditions, is very problematic.
But when the valves have proved to be good or reasonably good, it is advisable to make further tests of the valves in position in the set, checking voltages to each electrode and currents in each circuit. Since the valves are above suspicion, any abnormalities must be due to circuit defects, and the nature of the irregularities usually furnishes some clue to the actual trouble.
17
CHAPTER XVI
VALVE REPLACE MENT
IT is in some respects unfortunate that present-day valves continue to function for such agreat length of time before burning out. The reason isthat in many instances listeners" make-do "with reception
which is not nearly as good as it should be, due to the valves being
well past their prime. A dificulty arises because it is not always an
easy matter to tel whether or not the valves are " up to standard."
This can be done quite easily, of course, by means of a tester of the
type which enables the mutual conductance to be checked, or even by
taking anode-current readings at various bias voltages and comparing
these with the figures given by the makers for new valves.
Lost Eficiency. --
a4TrTRiEkORDYE
OR PE NTOD E
The main point to bear
in mind, however, is that valves do fre-
quently become less
TO Hz *
eficient after they have
been in use for some
time; often there is no
sign of a falling-off in
performance for a few
years, but in other
instances the emission
Pf/LLLA ME TE R
begins to get less after
OSE. A NOOS
ro sr*
Flo. 79. --
Methods
of
checking anode
current of dif-
ferent
types
of valve.
Pf/LLIAPIE TERS
18
VALVE REPLACEMENT
a year or so. It is wise to try the efect of replacing the valves by new ones or by others which are known to be in perfectly good
condition. A prelimary test of the "integrity " of the circuit should be made
before inserting the new valves. Anode Current Tests. --If, for any reason, this procedure is incon-
venient, the next best thing when a valve tester is not available is to measure the anode current of each valve in turn at intervals of a few months and compare the readings. A drop in current will immediately
indicate that the particular valve is becoming less eficient, provided, of course, that the H.T. voltage is the same as when the previous test
was made. The method of testing is perfectly simple and is as shown in Fig. 79; amiliammeter is connected between the anode terminal of the valve and the lead to high-tension positive. The position of the anode connection varies according to the type of valve and holder employed, but connections for the simpler types are shown in Fig. 79.
Using Higher Grade Valves. --After it has been decided that it would be worth while to replace a valve, the question of the type of valve to
be used in its place arises. It might be considered an obvious procedure to use a new valve exactly like that which has become faulty, but in many cases this would not be the ideal method, and in a few instances it would even be impossible. Valves have been improved very considerably during the past few years, and it would be foolish to buy an obsolete type of valve simply because it happened to be like the old one. As an example of this, suppose it were intended to replace asmall power valve in an S.G., det., P. receiver. The power valve originally used would probably be of a type which provides a low amplification,
and, consequently, only amodest volume for agiven input. That being the case, it would definitely be worth while replacing it by either a pentode of the high-eficiency, low anode-current type, or by a highamplification triode, such as the Cossor 220 P.A., Osram L.P.2, and Mulard P. M.2A, to mention afew. These valves give agreater degree of amplification than ordinary small triodes, and are intended for use when only a single L.F. stage is used and when it is not desired to employ a pentode; their consumption of L.T. and H.T. current is no more than that of ordinary triodes.
Substituting Pentodes for Triodes. --In using valves of this type for replacement purposes, no alterations need normally be made, unless to the G.B. voltage. The valves are provided with 4-pin bases of standard type. Rather more care must be exercised when replacing a triode by a pentode, however, for the connections must slightly be modified, and if a valve of incorrect type is chosen, the high-tension current may be agood deal greater than the supply unit can satisfactorily supply. The following are a few of the pentodes which can normally be used to replace small power valves without any dificulty arising: Cossor 220
19
PRACTICAL WIRELESS SERVICE MANUAL
H.P.T., Tungsram P.P.222, Hivac T.220, Osram P.T.2 and P.T.2- K., Mazda Pen. 220, and Mulard P. M.22A. Each of these valves has a maximu m undistorted output in the neighbourhood of 500 miliwatts, and the combined anode and screening-grid current does not exceed about 10 mA. at 120 volts H.T. In other words, although they provide a greater output than a small triode, they are equally economical of current.
In using a valve of this type it is necessary to replace the existing 4-pin valve-holder by one of the 5-pin type; the connections are as before, with the exception that an additional lead must be taken from the screening grid socket (centre) to a tapping on the H.T. battery (see
in otioos nowsone
/A4N9O111D. E
FILA MENT -
CONTROL GRID
SaRRRESSO R CONTROL
GRID
a
FILA MENT
LC/5VAC
SCREEN GRID
BLA NK
SCGRRIEDEN
80. --When replacing a smal power valve by a pentode, a 5-pin holder must be used, alead being taken from the centre socket to H.T. + as shown, and a fixed condenser joined between the anode socket and earth.
NT + FIG. 81. --These two valve-holder ilustrations clearly show the altered connections required when replacing a 4-pin H.F. valve by a 7-pin H.F. pentode.
Fig. 80). There is another point to re member, which is that the optimu m load of the pentode will be much higher than that of the triode which it replaces. This means that for the best results different tappings on the speaker transformer must be used. If the speaker is not of the "universal" type it might be worth while to have the transformer
changed by the makers, but if this expense is objected to it will be better to use a high-amplification triode, as mentioned above.
Tone Correction. --A pentode has a tendency to give emphasis to the higher musical frequencies, and it is generally recommended that a tone-correction device be used. But despite this, many listeners prefer the tone provided by the pentode, so that in practice the extra fitting is not always required. Details of the condenser and resistance
required are given on the makers' instruction sheet, but it wil very often be found quite suficient to connect a fixed condenser between
120
VALVE REPLACEMENT
the anode of the valve and earth, as shown in Fig. 80. The value of
the condenser can be varied to suit individual requirements, but
402 mfd. frequently provides adequate correction.
The H.P. Stage. -- When replacing a high-frequency valve of the
S.G. or variable-mu type it is worth while to use avariable- mu pentode
in its place. This type of valve can be used either with or without the
variable-mu control, and the latter can be added later on if desired.
Modern variable-mu H.F. pentodes can be obtained with 4-pin or 7-pin
bases, but it might sometimes be found easier to obtain a 7-pin valve
when a replacement becomes necessary. This will involve the replace-
ment of the existing valve-holder, and the connections must be changed
as indicated in Fig. 81. Where
chassis-mounting valve-
holders are used it will be
necessary slightly to enlarge
the hole in the chassis, and
this can be done with a rasp
or coarse file. Representative
variable-mu H.F. pentodes
are: Cossor 210 V.P.T., Hivac
V.P.215, Osram V.P.21, Mazda
V.P.215, and Mulard V.P.2.
Loss of Stability. --It is
possible that the change may
at first cause the set to become
unstable --this is because of
Flo. 82. --Fixed bias can be applied to an H.F. pentode by using the simple connections
shown here.
the higher eficiency of the new valve --but a few slight modifications should clear this
dificulty and make the set more sensitive than it ever was before. When
the set originally had avariable-mu control, stability should be secured
by working with alower setting of the potentiometer and by varying the
screening-grid potential. Where such a control was not provided, it
might be added, or a fixed bias may be applied, as shown in Fig. 82,
by inserting a 4002 mfd. fixed condenser in the lead between the tuning
coil and the grid, and taking alead from the grid through a 1megohm
grid leak to the 11 or 8-volt tapping on the G.B. battery. Even when
using this fixed bias the new valve will nearly always give better results
in the way of increased range and greater selectivity than the old one.
The Detector. -- With regard to the detector valve, it will rarely be
desirable to change to atype diferent from that previously employed,
unless it is considered worth while to modify the circuit to take an
H.F. pentode. The modificat ion is comparatively simple, and has been
dealt with at length in these pages before.
When it is not proposed to make any circuit modifications the best
121
PRACTICAL WIRELESS SERVICE MANUAL type of valve is nearly always one of the super-detector class, which is represented by the Cossor 210 Det., Hivac D.210, and Mulard P. M.2 D. X. These valves have acomparatively low amplification factor, but they are very stable and provide a very smooth reaction control. They are very suitable for use with transformer or parallel-fed transformer coupling, especially when the coupling component has a step-up ratio of 1.5 or more. When greater amplification is required of the valve -- for example, when a low-ratio transformer or resistance-capacity coupling is used --somewhat better results can be obtained by using areplacement valve of the H. or H.L. class; these have an amplification factor of about 25, compared with one of about 15 for the super detectors.
Care with Maine Sete. --The same general rules as those outlined above apply also to mains sets, but greater care must be taken with those because the power-supply unit is invariably designed to give an output which is just right for the valves originally used. Additionally, valves of diferent type will gradually call for diferent bias resistances, and so on.
122
CHAPTER XVII
REACTION FAULTS
pRACTICALLY every "straight" receiver employs variable reaction. Before dealing with reaction faults, it may be as well to consider what reaction is, how it is obtained, and why it
is used. A simple reaction circuit, devoid of all but the bare essentials, is
shown in Fig. 88. When signals are received by a radio set, they are
first of all amplified and selected by the high-frequency stage and then
passed on to the grid of the detector valve via the tuned circuit, L.1 C.1,
in Fig. 88, where they are rectified (or detected) and passed on as amplified signals to the L.F. stages of the receiver. :These rectified
currents appear at the anode of the detector valve, but despite the
detecting properties of the valve, and its associated grid leak R.1 and
condenser C.2, high frequency potentials are also superimposed on the
grid, and appear at the anode to be passed on to the L.F. stages with
the amplified rectified signals. Now these high-frequency currents are
of no use to us in the L.F. stages. In fact, they can do a great deal of
harm, but if we include areaction coil, L.2, in the anode circuit, we can
arrange for at least the greater percentage of these H.F. currents to
return to earth through the coil. If this coil is brought close to the
tuning coil, L.1, the H.F. currents flowing through it will induce further
H.F. currents into coil Li, so that the original H.F. currents appearing
at the grid of the detector valve wil be greatly strengthened. The
closer the two coils are brought together, the greater the induced
currents in coil Li, and thus the greater the signals will be increased. If brought too close together, how-
?, F
STAGE
ever, the valve will oscillate of its own accord, independent of the in-
coming signals. In current practice,
the reaction coil is fixed in relation to
its closeness to the tuning coil L.1,
and the degree of feed-back of the
H.F. currents for reaction purposes
L2
is controlled by a variable condenser
in series with coil L.2, but, what-
ever method is used, one thing is
common to all, that is, the H.F.
Flo. 83. --Theoretical diagram of a standard reaction circuit.
currents appearing at the anode of the valve, and which would otherwise
123
PRACTICAL WIRELESS SERVICE MANUAL
be wasted, are fed back to the grid circuit so that the currents already there are strengthened.
Advantages of Reaction. --The benefits of reaction are not merely confined to increasing volume, for its effect is the same as reducing the resistance, and thus damping, of the circuit. However good modern tuning coils and their associated condensers may be, a certain amount of loss is bound to exist. Since reaction helps to reduce the resistance of the circuit, it follows that the eficiency of the circuit is improved by its use, and therefore it is made more selective and eficient.
Reaction may be obtained from adetector valve or an H.F. valve, and, in some extreme cases, even aseparate valve may be employed; a moving coil or a fixed coil may be used, and, in the latter case, the degree of reaction obtained may be altered by means of a variable condenser or a variable resistance, and the coil may be in series or in parallel with the anode circuit of the valve.
For the purpose of dealing with reaction faults, we will consider the most popular method in use to-day, i.e., the choke-fed syste m, with fixed coil and variable condenser in parallel with the anode circuit.
A typical reaction circuit is shown in Fig. 84, with indications showing why reaction may be faulty. In this particular circuit, the H.F. currents appearing at the anode of the valve are prevented from getting into the low frequency circuits by means of the H.F. choke, H.F.C.1, and they are by-passed to earth via the reaction coil, L.2, and condenser, C.2, the latter controlling the degree of reaction obtained. A small fixed condenser, C.4, is also connected in parallel with the anode circuit, the purpose of this being to by-pass any H.F. currents not wanted for reaction purposes, and to prevent "Miller efect" when reaction is at zero.
The following are the faults most commonly met with in reaction circuits :-
1. Reaction not suficient. 2. Reaction too fierce. 8. Reaction only efective on one wave-band. 4. Reaction only efective over one part of a wave-band. 5. Reaction flattens tuning and reduces volume. 6. Hand-capacity efects. There are, of course, many other reaction dificulties, but they are supplementary to, and will be dealt with, under one of the above headings. Insufficient Reaction. --Every set should, if working properly, be capable of being made to oscillate at all settings of the tuning condenser by means of increasing reaction. If this is not so, there are a variety of reasons and remedies. There may be aloose or dry joint in one of the connections, the grid leak and/or condenser may be of the wrong value, or the valve may not make good contact in its valve-holder.
124
REACTION FAULTS
We will presume that all connections have been examined and that,
except for reaction, the set is working eficiently. Probably the greatest
cause of inadequate reaction, presuming the valve is good, the coils
eficient, and the right value of reaction condenser is being used, is low H.T. on the anode of the valve, or a wrong value of by-pass condenser
(C.4, in Fig. 84). A common cause of inadequate high tension is too
high a value for the decoupling resistance, R.2, in the anode circuit. If, say, 80 volts are recommended, the constructor should not try a
drastic increase, even if this does seem to improve reaction, as this is
more than likely not the real cause of failure, the large increase in high
tension merely enabling reaction to be obtained while hiding the true fault. A small increase of about 10 volts can only be of benefit, but
more should not be tried. If the high tension applied to the valve is too low, the anode resistance, R.2, is either too high or else the de-
coupling condenser, C.5, is leaky. The voltage, both sides of the
resistance, should be measured with a high-resistance moving coil
voltmeter (a cheap moving iron instrument is of no use for reasons that have been pointed out in these pages
oFifo. fa84.ult--yIndirecaatcitioonsn R2
shown on a standard circuit.
R ES/S TA NGE
roo H/OH
/. LEAKY
more than once). If there is
a very great diference (say, 120 volts at the battery end, and only 20 volts on the
fC I UE
,NECEFvF,Ocn'/LE-Nr
OR SHORTED W RONG VALUE
anode side), the resistance is
obviously dropping too many volts, and may be conveni-
vAL
DE TEN/ORATI NG
ently reduced in value. On
the other hand, if this re- ca
sistance is being used as a
coupling resistance to the next stage of the receiver,
TOO HIGH OR TOO
Low cRxw ory
care should be taken not to reduce its value too much, as this will
cause the amplification of the stage to be lowered. In such a case, the resistance should never be lowered in value to less than twice the
impedance of the valve. If this slight increase in H.T. has no beneficial result, the by-pass
condenser, C.4, should be reduced in value. The next step should be
to increase the capacity of the reaction condenser, C.2, by fixing asmall condenser of, say, -0001 mfd. capacity across its terminals.
An ineficient or shorted H.F. choke will cause insuficient reaction,
as this will allow the H.F. currents required for reaction to get into
the L.F. stages. Such a state of afairs would generally be easily traceable, as it would be accompanied by distortion due to the presence
of the H.F. currents in the L.F. stages.
125
PRACTICAL WIRELESS SERVICE MANUAL
If reaction works properly when a new H.T. battery and a fuly charged accu mulator are in use, then nothing can be gained, as it is obvious that the receiver has been correctly designed, and that the reaction fault is due to either arun-down H.T. battery or accu mulator.
Reaction too Fierce. --This state of afairs is just as bad as insuficient reaction. Probably worse, because a very slight increase in reaction causes the set to become unstable and to burst into oscillation. Because reaction is too fierce, it must not be assu med that the H.T. on the anode is of too high a value. Reducing the H.T. severely is just as bad as increasing the value of grid bias applied to an H.F. valve to prevent instability. Both are but half-measures, and will greatly afect the sensitivity. If a few volts reduction does not smooth out reaction, a fixed resistance of about 250 to 500 ohms should be connected between the anode of the valve and the reaction coil. The value of this resistance depends a good deal on the value of the by-pass condenser and a series of diferent values for both these components should be tried. Wrong values of grid leak and/or condenser will also cause excessive reaction, and these may conveniently be lowered.
A great deal of fierce reaction may also be caused by instability in the high-frequency stage of the receiver. If the latter uses avariable-mu valve, the volume should be reduced so as almost to cut out the signals. This H.F. stage will then be biased to such adegree as to prevent any instability in it and reaction should be tried. If it is now quite smooth, the fault obviously does not lie in the reaction circuit, but in the H.F. stage, which should be tested for instability.
Reaction Only Effective on One Wave-band. --This fault may be remedied in the same manner as described for insuficient reaction, but it may also be due to afaulty coil, especially where acommon reaction coil is made do for both medium and long waves. At any rate, some improvement may be obtained by treating as outlined above, but perhaps at the expense of rather fierce reaction on the band which was normally working satisfactorily.
With highly eficient coils, especially the modern ones using iron cores, parasitic oscillation is likely to be generated in the detector valves. This is shown by the set going into oscillation on the long waves with the usual slight " pop," but without giving the usual squeal on the received carrier, and before the full amplification from reaction has been obtained. This is due to the detector valve going into oscillation at a frequency determined by the combined constants of the reaction circuit and that portion of the grid circuit between the grid, and the earth-line of the receiver.
Parasitic oscillation is easily cured by including astopping resistance in the reaction circuit, as explained earlier. By making this resistance high enough, the long-wave band may be made to work with satisfactory reaction, but it may then be found that at the lower end of the
126
REACTION FAULTS
medium-wave band reaction has become unsatisfactory. Fro m our
experience we have found that the values which give the best results are 250 ohms for the resistance, a by-pass condenser of .0002 mfd. capacity, and a reaction condenser of .0002 mfd. capacity. These will give satisfactory operation over the whole of both wave-bands, provided of course, the rest of the circuit is in order.
Reaction Only Efective Over One Part of a Wave-band. --This is another fault which may be due to a variety of reasons, and the constructor will have to carry out all the tests already described if he wants to put matters right. Probably a slight increase in high tension, and an alteration of the value of the by-pass condenser will be found to be most beneficial, but it must always be remembered that, even in the best of sets, there is bound to be a slight diference in the degree of reaction obtained at the bottom and top of awave-band.
Reaction Flattens Tuning and Reduces Volume. --This fault is obvious --either the reaction coil is connected the wrong way round or else the reaction condenser (if a diferential condenser is being used) has been wrongly connected. It is thus a very simple matter to put things right.
Hand-capacity Effects. --A set that "squeals " every time a hand is brought near the reaction condenser can never work at its best. It usually happens that, even if the set does not actually oscilate as the hand approaches, the volume is reduced considerably when the hand is taken away from the reaction control after the latter has been used
to bring up the volume of aforeign station. This fault only appears when the reaction condenser is connected
between the coil and the anode of the detector valve itself. Some coils have the "earth " side of the reaction coil permanently joined to a common earth terminal on the coil base. In such cases, the reaction condenser can only be connected between the coil and anode, and the only remedy is to fix an aluminium or copper shield behind the panel, but in front of the reaction coil. When this shield is joined to the earthline of the receiver, it prevents the capacity of the hand and body from afecting the reaction condenser. An improvement may also be made by making sure that the fixed vanes of the condenser, and not the
moving ones, are connected to the anode. If the coil has both ends of the reaction winding free, the reaction
condenser should be connected between the" earth "end of the reaction coil and the earth-line of the receiver. Here again, make sure that the moving vanes of the reaction condenser are connected to earth, and
the fixed vanes to the coil. Other faults which appear in reaction circuits are " back-lash,"
where the set goes into oscillation at one setting of the reaction condenser, and comes out of oscillation at another; growling, where an increase in reaction causes the set to" growl "and not oscillate properly;
127
PRACTICAL WIRELESS SERVICE MANUAL
and oscillation with the reaction condenser at zero. Al are, however, closely associated with one or more of the faults dealt with above, and a cure can generaly be afected by reducing H.T. slightly or by increasing the value of the by-pass condenser.
Another reaction fault sometimes met with is "crackling," as reaction is advanced. This can only be due to a dirty reaction condenser, or else to one or more of the plates being slightly bent and alowing H.T. to momentarily short across them as they are rotated.
SU M MAR Y
Fault
Remedy
Reaction insuficient.. Increase H.T. on anode slightly.
Decrease value of by-pass condenser.
Increase value of reaction condenser.
Test H.F. choke and decoupling condenser.
Alter values of grid leak and condenser.
Replace valve.
Reaction too fierce
Decrease H.T. on anode slightly.
Increase value of by-pass condenser.
Add small fixed stopping resistance in reaction
circuit.
Decrease value of grid condenser.
Connect grid leak to negative instead of
positive side of filament.
Reaction only efective Treat as for insuficient reaction. on one wave-band Carry out cure for parasitic oscillation, i.e. add
stopping resistance in reaction circuit. Reaction only efective Increase H.T.
over one part of a Try diferent values of by-pass condenser.
wave-band
Reaction flattens tuning Reverse connections to reaction coil, or, if a
and reduces volume
diferential reaction condenser is in use,
reverse connections to the two sets of
fixed plates.
Hand-capacity efects Connect condenser between " earth " side of
coil and earth-line of the receiver.
If condenser must be connected between coil
and anode, use larger knob and shield the
condenser.
Crackling
Clean reaction condenser.
Test for shorts between fixed and moving
vanes.
128
CHAPTER XVIII
IMPROVING OLD SETS
IF the set is of the commonly used three-valve type, having one H.F. amplifying stage and one L.F. stage, it may have an ordinary S.G. valve as high-frequency amplifier, with volu me controlled by means of the reaction condenser. This type of receiver is often prone to H.F. instability, and in cases where the aerial is situated near a transmitting station overloading of the detector valve may occur. An improvement may be efected by fitting a variable-mu pentode valve in the first stage. The degree of amplification obtainable fro m this type of valve can be reduced practically to zero by applying a bias voltage to its control grid, and this voltage can be varied according to the volu me required by means of a variable potentiometer.
The easiest method of adding this type of control to an old receiver is as folows: Disconnect the lead at present joining the grid terminal of the S.G. valve-holder and connect it to one terminal of a .0005 or
Afeezi Cbabir9 k tiv-th 1Pronnal.
Control Crue O·i.'Re m nv
Concienver-
-$27ren Qrrot 50/90
A oprersor 40,-.vC lb Earth 7i."- ro morl
Cat`hocte.7b BAirs 42eristance (n w uspet On Battery/Po,
f iler anent d0 Z Orrsivreer/Onc6ry
A ir roent 72/ L TA httzte?-10na wsc
FIG. 85. --Standard connections for a 7-pin H.F. pentode.
.001 mfd. fixed condenser, and then connect the other terminal of this extra condenser to the grid terminal of the valve. The centre terminal (connected internally to the moving arm) of the variable potentiometer
should then be connected via a fixed resistance of approxi mately 500,000 ohms to the grid of the S.G. valve-holder, the two end ter minals of the potentiometer being joined to H.T.- and G.B.-9 respectively. When this modification is made it is also advisable to substitute a three-point on-off switch for the two-point type; one terminal of this
should be connected to earth and H.T. --, the second terminal to G.B.4-,
and the third terminal to L.T. --. Some of the modern H.F. pentodes have seven pins, and if one of this type is fitted the method of connection
shown in Fig. 85 should be adopted.
9
129
PRACTICAL WIRELESS SERVICE MANUAL
( 04Z 6e Ari'lf/SICZ 7 C e
/YT-74
Improving Selectivity. -- The selectivity of most old
0/A1
type receivers is inadequate for present-day requirements,
but this may be improved
without making drastic alter-
ations to the circuit arrange-
ment.
The most efective method
is to fit an extra tuned stage
between the aerial terminal,
and the existing first tuned
stage. This extra stage should
consist of acoil similar to the
ones in the receiver, and a
-0005 mfd. tuning condenser.
Unless the existing gang con-
denser is replaced by a new
88. --This method of connecting an L.F. transformer (parallel-fed) gives im-
proved bass response.
one having an extra section, the addition of the extra stage introduces the disadvantage
of an extra tuning control. The easier method of improving selectivity
by fitting amore selective coil between the H.F. valve, and the detector,
may therefore be adopted. An H.F. transformer having astep-up ratio
of approximately 3to 1can be relied upon to provide reasonably good
selectivity if awel-designed reaction circuit is fitted. The cap terminal
of the H.F. valve should be joined to one terminal of the primary
winding of the coil, and the
other primary terminal to
ZS
H.T.
the secondary wind-
ing being connected to the
tuning condenser in the usual manner.
The L.F. Amplitier. --After
Gfre
c'fc-7 z.ve. (e
s."-0000 n ·)sorter
the H.F. amplifier has been
satisfactorily modified atten-
tion should be paid to the
L.F. stage or stages. The Are,"
L.F. amplifier has more efect
on quality of reproduction
than the H.F. stages and
therefore, if quality is inferior
or--
particular attention should be
Zjerth
paid to the couplings and valves used in the former. If
FIG. 87. --A simple and useful L.F. volume control.
130
IMPROVING OLD SETS lack of bass isexperienced the L.F. transformer should be parallel-fed as shown in Fig. 86. With this type of coupling no direct current is passed through the primary winding of the transformer and therefore the pri mary inductance is higher than it would be if the primary winding were connected between the valve anode and H.T. +. Apart from improved bass response this method of connection also obviates the possibility of a primary burn-out.
L.F. Volume ControL --If two L.F. stages are used the addition of an L.F. volu me control is very desirable in order to prevent overloading of the last two valves. The control should have aresistance of between 250.000 ohms and 1megohm and should be wired as shown in Fig. 87. This control is particularly useful if apentode valve is used in the output stage, as this type of valve is easily overloaded. It can also be used as avolume control when the receiver is being used in conjunction with a pick-up for gramophone record reproduction.
When locating the control, in a new design, it is always advisable to select a spot as close to the grid of the associated valve as possible to reduce the risk of an interference or interaction being introduced by a long grid lead.
As the control is directly across the grid circuit, it is essential to select acomponent of a reliable make otherwise severe crackling noises might be set up in the output stage by adefective resistance element or afaulty contact between the moving arm and the element.
To obtain the most even control, it is advisable to use acomponent having a graded resistance, as this allows smooth and progressive variation of the signal strength.
If metal panels or chassis are used, care must be taken to see that the spindle of the control is in itself insulated from the moving arm, or that the fixing arrangements of the components are such that contact is not made between the arm and the chassis or panel.
131
CHAPTER XI X
0UNIVERSAL (A.C.-D.C.) RECEIVER FAULTS WING to the gradual changing over of D.C. supplies to A.C., the demand for receivers that will work equally well when supplied from either A.C. or D.C. mains has increased. There is no doubt that they have met with agreat measure of success, and have overcome the dificulties which were experienced with this type of valve when it was first introduced.
A.C. /D.C. Valves. --A transformer cannot be used on a D.C. supply to step the mains voltage down to the value required by the valve heaters, and the current passed through the heater circuit must be taken direct from the mains through adropping resistance or lamp. In the interests of economy this current must be kept low, and therefore valve-heater resistance is made reasonably high --the actual value difers with diferent makes of valve, but 60 ohms is about the average. The average A.C. type of valve, on the other hand, has a resistance of only
88. --Heaters wired in series showing points at which breakdown can occur.
4 ohms, and therefore the A.C. valve should be more robust than the A.C. /D.C. type. In practice, however, there is little to choose between the two types in this respect nowadays --an A.C. /D.C. valve can be relied upon to give auseful life of about eighteen months.
Insulation Breakdown. --A more common trouble than a heater burn-out is a breakdown of the insulation between the heater and the cathode. This sometimes happens in A.C. valves, but is more co mmonly experienced with the A.G. /D.C. type, owing to the greater potential diference which exists between cathode and heater in the latter. Referring to Fig. 88, and assu ming that the valves are of the 16-volt type, there wil be apotential diference of 82 volts between the positive end of the heater of the centre valve and the negative line. Again, assuming that the bias voltage obtained by means of the resistance R is
132
UNIVERSAL (A.C.-D.C.) RECEIVER FAULTS
2 volts, there will be a potential diference of 80 volts between points
A and B --the cathode and the heater. In some cases the potential
diference between these two points can be much higher, and, therefore,
it will be readily realised that a breakdown can occur at this point
unless the insulation between cathode and heater is efective. It is
in this respect that modern A.C. /D.C. valves are greatly superior to the
earlier models.
Consumption. --The consumption of an A.C. /D.C. receiver is higher
than that of an A.C. receiver having a similar undistorted wattage out-
put at the speaker. This is due to the higher heater circuit consu mption
of the A.C. /D.C. set. The average three-valve A.C. set has a heater
circuit consumption of 12 to 16 watts, whereas in athree-valve A.C. /D.C.
type using -2 amp. valves, with a supply voltage of 240 volts, the con-
sumption is 48 watts. The H.T. consumption of the two receivers will
be somewhat similar --approximately 80 watts for a table model --and,
therefore, the total consu mption of the A.C. /D.C. receiver will be more
than one and ahalf times that
of the A.C. set. This does not
materially affect the lighting
11,
bil, however, as the difference
is less than the consu mption
of one lamp of normal wattage.
89. --Showing method of checking heater circuit continuity of,A.C./D.C. set.
Dropping Resistance. -- When constructing an A.C. /D.C. set, a reliable component should be used for limiting the current passed through the heater circuit. Some manufacturers use aspecial lamp known as a barretter, whereas others use an ordinary resistor. Most home constructors will probably use a resistance, and, therefore, great care should be taken to choose the correct type. It should have the correct resistance value and must also be capable of carrying the required current without becoming overheated. For example, if the valves used are of the .2 amp. type, the current must be limited to this value and
133
PRACTICAL WIRELESS SERVICE MANUAL therefore, if the mains voltage is 240 volts the total resistance in the heater circuit must be 1,200 oh ms. Assu ming that five .2 amp. valves are used their total resistance will be 500 oh ms, and therefore, the dropping resistance must have a value of 1,200 ohms less 500 ohms, i.e., 700 oh ms.
Pick-up and A.B. Connections.-- As the H.T. --line of the A.C. /D.C.
set is in direct connection with one of the mains leads, it is necessary to connect a condenser between H.T. -- and the earth socket, otherwise there is the possibility of the mains being short-circuited when the earth lead is joined to the set. It is also advisable to adopt the same precaution with the aerial, as the aerial socket is generally connected direct to H.T. -- through the aerial coil, and, therefore, if the aerial is not well insulated ashort-circuit can occur as in the case of the earth lead. When a pick-up is used, it is advisable to connect the leads to the grid circuit of the first valve of the amplifier via fixed condensers, otherwise a shock can be obtained when the pick-up is handled.
The most common trouble with A.C. /D.C. receivers is a burnt-out valve or dropping resistance. If no signals are obtainable, and the receiver seems quite dead, the first test should be that shown in Fig. 89. The multi- metre should be set for measuring ohms, and its two leads should be connected across the mains plug. If a valve or the dropping resistance is burnt out no reading will be obtained; in most sets a reading of between 500 and 1,000 ohms is obtained across these two points, but the value varies slightly with the valve types used, of course. H.T. and H.F. tests should be conducted in the same manner as in A.C. main sets.
134
CHAPTER XX
CHEC KING RECEIVER PERFORMANCE
IT might be thought that it would be a perfectly easy matter to decide how good or how bad a receiver is simply by connecting it to aerial and earth and listening to it. This would be a
suficiently good method of testing when only rough comparisons are to be drawn, but it is useless as a means of checking the results of
experiments. The alteration might be so small that it is not audible in average conditions, in which case the time spent in experimenting
would probably be wasted.
·
It is not suggested that the service-man and experimenter should
provide the mselves with expensive calibrated valve-voltmeters, cathode-
ray oscillographs and the like, but he should have a small amount of
simple apparatus, most of which can be made at home quite cheaply.
Obtaining a "Test" Signal. -- One of the first requirements is a device for
generating a steady signal; this is because the signals sent out by a broadcasting
station --with the exception of the tuning note --are quite useless for fairly accurate 1 test purposes. The more
> LT+ > LI-
usual apparatus and tests
90. --Circuit of asimple and efective
applied ard dealt with in a previous chapter. As an
local oscillator suitable for test purposes and for accurate trimming of a receiver.
example of what is meant it can be explained that if an atte mpt were
made to measure the output of a receiver, no matter how good the measuring instrument happened to be, it would be impossible to gain even arough idea when, say, a band was playing. The reason is that
·the intensity of sound at the transmitting station is constantly varying between extremely wide limits, and corresponding variations
must occur in the sound reproduced by the speaker, or in the output
from the last valve. A simple and reliable miniature transmitter --for that is what an
oscillator comprises --can be made by using asingle valve in the circuit shown in Fig. 90, and using the connections indicated in the wiring plan in Fig. 91. It may be seen that a four-pin plug-in coil is used in a
standard regenerative circuit with series reaction. The valve is an
135
PRACTICAL WIRELESS SERVICE MANUAL ordinary triode of the L. or H.L. type and is connected in the same way as a leaky-grid detector. Reaction coupling is practically " fixed," but slight variations can be made by means of the -002-mfd pre-set condenser during use.
Using the Oscillator. --In order to use the oscillator it is necessary only to plug in the valve and a standard four-pin coil (details for construction will be given later), and to connect a2-volt accu mulator and a 60-volt H.T. Incidentally, it is sometimes possible to use the same batteries as are used to feed the receiver, but the separate ones are to be preferred. After connecting it is necessary only to place the oscillator a few feet away from the aerial lead-in wire, and then to operate the
Fm. 01. --Pictorial wiring plan for the circuit shown in Fig. 90. Two valve holders are shown, that on the left being for
the four-pin coil.
oscillator tuning condenser until a high-pitched whistle is heard in the speaker. This is obtained only when the set and oscillator are tuned to the same frequency, and consequently the four-pin coil must be of such a size that it is appropriate to the wavelength to which the set is tuned.
When the oscillator and receiver have been adjusted in this manner the efect of making the various alterations to the set can be noted by comparing the strengths of the note emitted by the speaker in various circumstances. In the same manner, experiments in selectivity can be carried out by noting the "spread " of the received note over the receiver tuning scale. Incidentally, this "spread " can be varied very
136
CHECKING RECEIVER PERFORMANCE
considerably by altering the position of the oscillator in respect of the
aerial lead-in, or of the set, so it must re main in the same relative position throughout the tests. It will generally be found best, however,
to move the oscillator so far away that the whistle is very sharply
tuned. When first using the oscillator, adjust the pre-set condenser
until the note is perfectly clear.
Oscilator Coils. -- With regard to the coils, as mentioned above it is possible to use standard ready-made components, but those who wish
to make their own can use co mmercial four-pin formers, allowing approx-
imately 75, 110 and 220 turns for the grid winding (between pins
1and 4), and 40, 50 and 80 turns for reaction (between pins 2 and
5) for mediu m waves, 465 Kcs. I.F., and long waves, re-
spectively. The windings should
be about * in. apart and should consist of 80-gauge enamelled
GRID.
4
wire for the two smaller coils, and of 88-gauge enamelled for
%ACTION.
j
2
the largest. In each case the
5
windings are arranged side by
side, and both windings are
wound in the same direction;
details are given in Fig. 92. If it is wished to use the
oscillator for short waves suit-
able coils can be made, using approximately 9 and 6 turns
for 80 metres; other ranges in
proportion. Clanging I.F.'s. --It will be
appreciated that the oscillator
FIG. 92. --Details of an easily-made type of four-pin coil. Winding data is given in the
text.
can usefully be employed for
ganging and trimming, by adjusting the tri mmers until the received
whistle is at its loudest. In the case of a superhet the inter-
mediate-frequency transformers can be adjusted in a similar manner, although some form of "artificial " coupling might be neces-
sary due to the components being fuly screened. One method is to wind asingle turn of wire round the oscillator coil, and then to wind the other end of this wire round an unscreened lead to the primary winding of the first I.F. transformer. When this is done the oscillator must be placed near to the set so that the coupling lead is short. For 110 and
150 Kcs. the long-wave coil should be used, whilst for 465 Kcs. the
middle coil will be suitable. In the case of the lowest frequency
137
PRACTICAL WIRELESS SERVICE MANUAL mentioned it wil also be necessary to connect a .0005- mfd. fixed condenser in paralel with that used for tuning the oscillator in order to tune to the corresponding wavelength of about 2,700 metres.
Visual Output Checks. --In many cases it is not suficient to judge the output of the set by ear, especially when more than a few seconds must elapse between making the necessary adjustments. The reason is that it is impossible to" remember "sounds, and therefore even large variations are not easily recognised. A better method is to use so me form of output meter, the simplest of which is an A.C. volt meter (such are
HT.+
OUTPUT VALVE
MC.VOLTMETER .
WIRE -WOUND POTINTIOMETER
FIG. 93. --A simple and accurate method of comparing outputs in various circumstances, by connecting an A.C.voltmeter and
potentiometer in place of the loudspeaker.
quite cheap) connected as shown in Fig. 93. The purpose of the 5,000-ohm potentiometer is to provide a reasonable load in the anode circuit of the output valve, and to vary the proportion of the output applied to the voltmeter so that a convenient portion of the scale can be used. It must be remembered that the meter does not read actual output volts, or output watts, but merely enables comparisons to be made. In practice the potentiometer should first be set so that it is short-circuited, the reading being zero. The potentiometer can then be adjusted until about a one-quarter-scale reading is obtained. After making a note of the exact reading the potentiometer should not be altered during the course of the experiments.
Any adjustments which result in an increased reading will be known to have resulted in a greater output being obtained and hence in increased efficiency, whilst the opposite is also true.
This system of measurement is not ideal because the opti mu m load of the output valve will probably not be provided, unless especial care istaken in this direction. To alarge extent this objection can be overco me
138
CHECKING RECEIVER PERFORMANCE
by using a sensitive miliammeter (reading up to not more than 2mA.) in conjunction with a.005-mfd. condenser and variable resistance, as shown in Fig. 94. The resistance is first set to its maxi mu m value, and then adjusted until a convenient reading is obtained on the meter scale. After that the procedure is the same as described above.
Another Method. --Another method, which has the advantages of
being inexpensive and rather fascinating, is where aminiature neon lamp is used in place of the voltmeter shown in Fig. 93. In this case, increased output is indicated by increased brilliance of the neon, although
TO SPEA KER
OUTPUT VALVE
MILL i--MM ETE
WESTECTOR .
FIG. 94. --A better method of comparing outputs --a milliammeter and Westector are connected between the anode of the output valve and earth. The speaker in this case
remains in circuit.
most accurate results can be obtained by setting the potentiometer until the light is just extinguished. Any adjust ment which causes it to glow again will be known to have resulted in greater output. After this adjustment has been made the light can be extinguished again and further adjustment tried.
139
CHAPTER XXI
DISTO RTION --CAUSES AN D CU RE
W HEN reproduction is not as good as it should be, and ifdistortion is suspected, it is usual to lay al the bla me on the low-frequency
portion of the set. In many cases this is quite wrong, for there are many other sections of the receiver which may be responsible for the trouble. Let us follow through a typical H.F.-Det.-L.F. circuit with a view to considering the possible sources of distortion.
The Aerial Tuning Circuit. --It is quite safe to assu me that the signals
which reach the aerial from any of the B.B.C. transmitters are as free
fro m distortion as is pos-
CARR/ER WAVE ,..cc.\...cecci5.0n00 _2D.1 l 1 2. 75.000
sible, although the same cannot be said of two of
, FIG. 95.--In asharply-tuned circuit the higher frequencies may be lost as indicated here, where broken lines indicate the amount of movement of the tuning pointer befweoraeker.signals rapidly become
three of the Continental stations. For this reason it is best to make any tests which may be necessary on the nearest B.B.C. transmitter, although weaker signals from other British stations may also be used for purposes of certain comparisons. The signal striking the aerial might possibly be "mutilated" immediately due to the aerial-tuning circuit being
of ineficient design, but this is unlikely. Never-
theless, if reproduction is low pitched the input
tuning circuit might co me under suspicion. The reason is that if, for so me reason, the circuit tunes very sharply the higher musical frequencies may be " lost". This is best understood by considering the musical or audible frequencies as extending on each side of the exact tuning
point of the carrier wave, as suggested diagrammatically in Fig. 95. High-Note Cut-of. -- When the tuning is exceptionally sharp it might
happen that the input circuit will not pass on frequencies above, say,
2,000 cycles; the result is bound to be boomy reproduction. Should the aerial circuit be suspected of causing trouble in the manner which has
140
DISTORTION --CAUSES AND CURE
been suggested it would be well to note the efect of connecting a fixed
resistance of about 2,000 ohms either between the aerial and earth
terminals, or between the ends of the aerial coil, as shown in Fig. 96. The resistance will certainly cause a reduction in selectivity, but if
better quality is obtained it will be known that the coil is unsuitable
when good reproduction is required. An improvement would in most
cases be obtained by fitting a band-pass filter in place of the single-
circuit coil. This provides a more uniform response as indicated in
Fig. 95a.
Local-Distance Switching. --The latter test does not indisputably
prove that the original
CARRIER WAVE
coil was in some way
,
ni ws s.000-,...2.100(i4 2I,.000er-.5.000 ----
unsuitable, because the
main efect of the redstance might simply have been to cause a reduction
I a
li
':
in eficiency of the input circuit, and thus a lower input to the grid of the first valve. Thus, if the
valve were previously being overloaded it might
operate well within its capacity with the resis-
tance in circuit. This
could be proved most easily by noting whether or not
Flo. 95a. -- When aband-pass tuning circuit is used, the full range of audio-frequencies can be "covered," since signals vanish suddenly at the
two extremes of the tuning band.
the resistance had any efect on the quality of reproduction when listening to weaker signals. In either
case, the fitting of a " local-distance " switch to put the resistance in or out of circuit at will would be justified. The simplest arrangement
is that shown in Fig. 96a, where an ordinary on-of switch is wired in
series with a fixed resistance between the aerial and earth terminals. When the switch contacts are closed the set is most suitable for
reception of powerful signals, and when they are open distant reception can be obtained without any loss of eficiency. The most
satisfactory value of resistance is best found by trial, but it is
often found that one of 500 ohms is just right. Self-Oscilation. --Another common source of distortion is in the
circuit of the H.F. valve itself. Probably the valve tends to oscillate in certain conditions; and distortion is an inevitable result of oscillation
at either high or low frequency. Quite often the only alteration required
to produce a remedy is in respect of the H.T. voltage applied to
141
PRACTICAL WIRELESS SERVICE MANUAL
"ri
Flo. 96. --Too-sharp tuning can be modified by connecting afixed resistance in one of the positions
indicated here.
the screening grid. The
optimu m voltage can be
found only by trial,because someti mes it is considerably higher than that usually suggested by the makers, and frequently it
is lower. Sufice it to say, however, that oscilation
can often be prevented by making the adjustment referred to. This is easily
done in the case of a battery set with separate tapping, but there is a
little more dificulty where a mains set is concerned, or where the supply is by means of a potentiometer consisting of a pair of fixed resistances, wired as
shown in Fig. 97.
Screening-Grid Voltage. --
In the latter case, the applied
voltage can be reduced by
using a resistance of higher
value in the position marked
A, whilst it can be increased
by using a lower resistance.
The simplest and most
accurate method of checking
is to replace the two resis-
tances by a potentiometer,
wired as in Fig. 98. The
resistance rating of the
potentio meter should be approximately equal to the
Flo. 96a. --Showing the simplest system of
local-distance switching.
combined value of the two
resistances in series. For example, if the resistances have values of
10,000 and 15,000 ohms, a25,000-ohm potentiometer would be suitable.
Self-oscillation of the first valve can also be due to the fact that the
decoupling condenser between the screening grid and earth (or cathode
in some mains sets) is defective or of incorrect capacity. A 0.1- mfd.
condenser is generally most satisfactory, but it should be agood-quality
142
DISTORTION --CAUSES AND CURE
non-inductive type. Condensers of higher value may possibly be less
efective due to the very small inductance (even so-called non-inductive
condensers have aslight amount of inductance) being greater than that
of the lower-capacity component.
H.F. Coupling Too Tight. --Oscillation can also be due to the use of
HT-- & EARTH
an unsatisfactory H.F. choke, or to the coupling
between the windings
of an H.F. transformer
being too tight. The
method of checking and
correcting the choke is
obvious, but there is
not much which can be
done with regard to the coil, unless the user feels
Arr+
97. -- When afixed potentiometer is used to apply the screening-grid potential, the applied voltage can be increased or reduced by using alower or higher value for the resistance
marked A.
competent to remove a few turns from the prim-
ary winding --assu ming that the secondary is tuned. Another method which sometimes yields satisfactory results is
to shunt a fixed resistance across the primary winding, as shown
in Fig. 99. The resistance reduces eficiency to a certain extent, but if it cures the oscillation trouble its use is justified. The value of resistance required must be found by trial, but the highest which has the desired effect should eventually be used. In passing, it is worth mentioning
that this idea is very often applicable to superhets, especially those in which the I.F. transformers (which correspond with the H.F. transformer just mentioned) have separate windings which can be moved in
relation to one another. In more than one instance it has been found possible to remove all traces of distortion and instability simply by
moving the windings slightly farther away from each other. The alteration, besides giving greater stability also sharpens tuning, although
it is inclined to cause aslight loss in sensitivity. The Detector Stage. --In most cases the detector valve is responsible
for very little of the distortion which might take place in the set, but there are a few points which should be observed. For example, if the
valve is preceded by more than one H.F. or I.F. stage there is adanger of its being overloaded unless power-grid connections are used --the main considerations are that the anode voltage should be not less than
100, and that the grid leak and condenser should have values of approxi-
mately .25 megohm and .0001- mfd. If the valve is being overloaded it
is always worth while to try the efect of increasing the anode voltage,
143
PRACTICAL WIRELESS SERVICE MANUAL
or of using a valve of the L.F. type. If this does not overcome the
trouble it might be necessary to replace the triode with a double diode or with a " Westector," but this is necessary in very few cases indeed,
HT-- & EARTH
98. --Easy S.G. voltage adjust ment by means of avariable potentiometer. This can be mounted inside the set and left alone after
once properly set.
normal manner; instead, signal strength increases for a time as the reaction setting is advanced, and then diminishes again. The trouble might be due to bad design of the set or to the detector grid coil being unsuitable, but it can often be set right by including afixed resistance of about 250 ohms between the reaction winding and the reaction condenser.
and certainly only when there is a tremendous amount of amplification prior to the detector.
It occasionally happens that parasitic oscillation occurs in the detector stage, and that this can be prevented only by disconnecting one of the reaction leads. This peculiar form of oscillation, by the way, can be recognised by the fact that operation of the reaction condenser is erratic, or that the set does not appear to go into oscillation in the
MT+
L.F. Overloading. --
The coupling component
used between the detector.
and low-frequency stage can be the cause of various kinds of distortion. If the
Fin. 99. --H.F. instability can often be overcome by connecting a resistance in paralel with the primary of the H.F. transformer, or
set is of comparatively old type, however, and employs
by using a looser coupling between the windings.
a cheap type of transformer, or even an R.C.C. unit, the user will
find that a new transformer of good, modern type will be fully
144
DISTORTION --CAUSES AND CURE
L. F TRA NSFOR MER
justified. If it is not found necessary to apply a very
high anode voltage to the
detector valve, a resistance-
fed transformer is to be pre-
ferred, for this will provide
greater amplification and
better quality than an ordin-
ary cheap transformer. On
the other hand, a really high-
grade transformer is hard to
beat.
If it is thought that the
L.F. valve is being overloaded
Fin. 100. --Overloading of the L.F. stage can be avoided by fitting avolume-control poten-
tiometer, as shown here.
a check can be made by reducing the input to it --either by including avolu me-control
potentiometer in its grid circuit, as shown in Fig. 100, or even by using the local-distance switching arrangement mentioned above. If overloading is definitely taking place the most satisfactory method is
G.8 .
G 8--
LFTRANSFORMER
OHMS
P 100 01//efS
/00 OHMS
LS.
FIG. 101. --Showing how two L.F. valves may be wired in paralel. Note the grid and anode "stopper's resistances.
to replace the valve by one of " larger " type. Thus, a small power
valve could be replaced by a super-power valve. Another method, which is not strongly recommended, is to connect another similar valve
in parallel with it, as shown in Fig. 101. None of these methods should
be used, however, until experiments have been made in connection with increasing the anode and grid-bias voltages to the existing valve;
these simple alterations will often provide acomplete cure.
10
146
CHAPTER XXII
TRACING SOU RCES OF INTERFERENCE
AGOOD deal has been written concerning the remedies to apply in order to overcome electrical interference, but the first dificulty encountered is almost invariably that of finding the cause and source of the trouble. It would be useless to fit choke-condenser filters in the mains leads if the so-called man-made static is caused by a flashing sign outside the house, just as it would be a waste of time to fit an interference-free aerial system ifthe crackles were being conveyed to the receiver through the mains-supply leads. The first step should thus be to decide the reason for the impaired reception, and although
Fi n. 102. --Showing the connections for an improvised frame aerial used with a det.-L.F. receiver.
the Post Office engineers are always very helpful in this respect when their services are requested, the service-man should endeavour to carry out alittle" detective "work on his own before asking for outside assistance.
The very first test to apply is to disconnect the aerial and the earth; if the suspected interference then disappears it is obvious that the cause is external to the set.
Pick-up or Wire-conveyed. --In the first place, it is evident that if the set is battery operated the interference must be picked up by the aerial or by the connecting leads used with it. For this reason it is a
146
TRACING SOURCES OF INTERFERENCE
good plan where a mains-operated set is used to arrange with an assistant to compare the results obtained with his set with those provided by the mains receiver in the location in which the trouble is observed. This will generally give an immediate clue to the type of interference -- mains-borne, or direct pick-up. When the interference is fed into the receiver by means of the mains leads it is evident that achoke-condenser filter is required. A certain amount of experiment may be required in order to determine the most suitable values of condensers and chokes, but the procedure will be comparatively straightforward.
If it is definitely determined that the trouble is due to pick-up, the next step should be to remove the aerial lead-in at the point where it enters the house and note whether this makes any difference. If it does, a screened down-lead might prove efective and should certainly be tried. Should this make very little diference, however, the effect of altering the position of the leads to the speaker (where it is external to the set) should be tried, whilst it should be observed whether an alteration in position of the batteries and their leads makes any difference. These points are raised because it is frequently found that some of the leads run near to a wall through which an electric conduit is taken, and if the wires are parallel to this they make ideal" interference aerials."
Bad Contacts. --If none of these changes has any pronounced effect it will often be indicated that the interference is due to a bad or intermittent contact to some piece of electrical apparatus in the house. A small motor, a vacuu m cleaner, an electric-light switch, or an electric bulb which is loose in its holder might be at the root of the trouble. If any of these is suspected it will probably be a fairly easy matter to confirm the suspicion )3y switching off the particular ite m. In this respect it should be mentioned that aswitch having contacts which are worn, and across which sparking or arcing takes place, can be the cause of a mysterious form of trouble which is inter mittent in character. In the same manner aloose electric lamp bulb can produce interference due to slight arcing between the bulb" pips" and the spring-loaded contacts in the holder. Other cases have been observed where the trouble was due to a bad contact between one of the wire coils in an electric fire and the terminal post to which it is attached.
Direction Finding. --Should it be found impossible to locate the source of trouble by trial-and-error methods it is often possible to arrange adirect system of location by the use of aframe aerial in conjunction with a battery set. A portable receiver is most convenient for the purpose, but where this is not available a simple frame aerial consisting of about 75 ft. of 26-gauge cotton-covered or enamelled wire wound round the lid of a cardboard box and connected to a battery set (preferably one having a "straight" circuit), may be used almost equally well. In the latter case the grid lead from the aerial coil should
147
PRACTICAL WIRELESS SERVICE MANUAL be disconnected and the frame wired in its place, the other end being earth connected, as shown in Fig. 102. It does not matter very much whether the tuning condenser is left in circuit or not, because the interference is very rarely confined to any particular wavelength, but is completely untuned.
When the frame aerial has been connected it should be found that the interference is most pronounced with the edge of the frame pointing in one particular direction (really there are two opposite directions because the frame responds equally to signals from the front and rear edges).
S.G FIG. 103. --Connections for an improvised frame aerial
used in conjunction with ascreen-grid set. The frame aerial should thus be rotated until the strength of the interference attains amaximu m, after which the volu me control, where fitted, should be turned down until the crackles are only just audible in apair of 'phones connected to the speaker terminals. The next step is to move the set and frame first in one, and then in the other direction of the frame, carefuly observing whether the interference increases or diminishes. It is almost sure to become more pronounced when the outfit is moved in one direction, and the frame should therefore be moved as far as possible in that direction, the frame occasionally being rotated slightly so as to keep the interference as loud as possible. It will be evident that every time the sound increases the frame has been moved nearer to the source of trouble. When the Interference is External. --If the interference is due to so me piece of apparatus inside the house it should eventually be located by progressing along the lines described. On the other hand, if it is established that the source is outside the house it will generally be necessary to call in the assistance of the Post Ofice Engineering Department. To do this, it is necessary to obtain an appropriate form from the
148
TRACING SOURCES OF INTERFERENCE
local Post Ofice, fill in the details asked for, and send it to the address given on it. In most cases an experienced engineer wil call in a few
days (without charge) and continue the search. (See Figs. 102 and
108.) The commonest sources of interference are electric signs of the
flashing type, electric motors of all kinds, ordinary motor cars, refri-
gerators, etc. Included under electric motors, of course, are a hundred-
and-one pieces of apparatus of ordinary domestic use in which the
motor is included, such as vacuu m cleaners, cofee grinders, fans, hair-
dryers and so on. From all of these items the interference can be intro-
duced into the receiver by conduction through the electric supply
mains or by radiation from the apparatus. In the latter
case, of course, it may be fed
to the receiver through the
aerial or earth leads, or even
picked up by the inter-circuit wiring or some inductive
component.
Battery or Mains Receivere. --In the case of a
receiver operated from ordin-
ary batteries, there is no possibility of the interference arriving via the mains, and thus any interference will come from the aerial or earth
leads or from induction be-
TWO o-01 mkt
0.05 .k1 450.VJ X. WORKING
750. VDC WORAIND 2,250 V.D.C. TEST.
600 V11C TEST.
FIG. 104.-- The Beling-Lee type 171
interference suppressor and circuit,
tween supply wires inside the
wals of the house. If the interfering apparatus is very
close, there may even be direct pick-up by the circuit
wiring. The users of the apparatus should take the necessary steps to avoid interference, and there are several schemes for preventing
motors and similar electrical equipment from radiating interference. When an electrical circuit is broken or interrupted a small spark will be seen and if some inductive load is included in the circuit there
will be a considerable radiation of energy when the spark occurs. A non-inductive shunt across the contacts which are broken will prevent
(in most cases completely) the formation of a spark and thus the radiation of the interference, and the shunt may take the form of a specially-wound coil or a simple condenser. In an electric
motor, for instance, the brushes will make-and-break at the co mmutator,
and if an electric motor is inspected whilst it is working the sparks will
149
PRACTICAL WIRELESS SERVICE MANUAL
be seen at the junction of brushes and commutator. Thus, in view of the
above remarks, a condenser may be joined across these two points to
prevent the interference, but it is found that better eli mination is
obtained when a condenser is joined to each brush and the junction
of the two condensers is joined to earth or to the casing of the motor.
Whilst ordinary condensers
may be employed, it is necessary
to guard against an overload
of the condenser, and where
the mains voltages are con-
nected to the motor, good .
insulation is required to avoid
the risk of anyone receiving a
serious shock on touching the
condenser. (See Figs. 104 to 109.)
Com mercial Units. --A nu m-
ber of firms make up a unit
consisting of the two necessary
condensers inside a case, with a
terminal for connection to earth
FIG. 105. --A Dubilier anti-interference unit which is obtainable in different types.
and amongst these firms may be mentioned Messrs. T.C.C., Belling-Lee, Bulgin, Dubilier, etc.
Some examples are seen in
·cx mid
Figs. 104 to 109. Where it is not possible to obtain access
SO5CAMKEPT -,-
to the actual motor (such as TO SET
in amodern domestic vacuum EARTH cleaner) the interference-re-
moving device may be includ-
ed in the mains leads to the
apparatus, and in such a case
asmaller component would be
employed, such as the Beling-
Lee Flex Lead Suppressor, shown in Fig. 110, or the MAINS
T. M.C. device shown in
Fig. 11. In some cases it may even
be found that the inclusion of this type of suppressor in
RADTIOOSET 106. --This device is employed on the input circuit to a radio receiver, and is a
Belling-Lee product.
the mains leads to a mains receiver will remove the interference,
without connecting it to the interfering apparatus, and thus, where
more than one piece of apparatus gives rise to the interference
a single suppressing device may be used continuously. If the
160
TRACING SOURCES OF INTERFERENCE trouble is only slightly reduced in intensity when aerial and earth are removed it will indicate in most cases that it is being induced, and a removal of the receiver to another part of the room should result in a further modification of the interference. In such a case it will be necessary totally to screen the receiver in order to prevent pick-up by the wiring.
If, however, no alteration in intensity results when aerial and earth are removed it may be taken that the source of the interference is the
/
Flo. 107. --A twin mains fuseholder from Bulgin.
mains supply leads and the remedies already mentioned, or to be mentioned later should be adopted.
If the interference comes from some outside source, where it is not possible to fit the interference eliminating apparatus, or where no control over the apparatus may be obtained by the listener, some form of anti-interference aerial system must be employed, and the Goltone Rejectostat (Fig. 112) aerial employed. In these forms of aerial aspecial screened lead-in wire is employed and it is intended that the aerial proper --that is, the horizontal portion --should be erected where possible clear of the area of interference. Then, to make up for the long lead from the aerial to the receiver, aspecial impedance-matching transformer is connected between the atrial and the lead-in wire, whilst asimilar transformer is then joined between the receiver and the other end of the lead-in wire. Without such transformers, the loss due to the long lead and to the capacity existing between the lead and the outside metallic (earthed) casing would be so great that only the nearest and most powerful stations would be heard.
151
PRACTICAL WIRELESS SERVICE MANUAL
So far we have only considered the interference which arises from
electrical apparatus situated either in the actual house or at least
within the near vicinity. There are, however, several other sources of
crackles or clicks which can mar the received programme, and the
commonest of these is a poor fitting connector. This may be of any
form, and does not necessarily refer to aplug or socket. It was stated
in the opening paragraphs of an earlier Chapter that when a current
-1 -
wins
FUSE
727mp):13- 1had
FUSE 1 - 1 71d° 2 V1431 -1.
was interrupted a spark took place and that this gave rise to a radiation and thereby caused a
noise in the receiver. This type of radiation is untuned --that
is to say, owing to the absence
of a resonance circuit, there is
no exact frequency upon which
the noise wil be heard. Owing
to this, it matters not what
wavelength the receiver is tuned
to, the noise will be heard as a
background. In the average
home there are many sources of
supply in which continuity of
contact has to be relied upon for
the function of some appliance or
apparatus, and it is here that an
intermittent current or supply
can give rise to the noises
already mentioned. For in-
stance, the ordinary mains plug
and socket consists of two brass
sockets into which two brass pins
are forced. To ensure a firm
Ftc. 108. --A simple but eficient suppressor made by T.C.C.
contact the usual procedure is to slot the pins and open them
slightly so that the overall diameter is slightly greater than the internal
diameter of the sockets. When the pins are pushed into the socket they
are closed together, but owing to the natural springiness of the metal
the sides make good contact with the sides of the socket and thus no
trouble should be experienced.
Worn Contaoto. --It is often found, however, that after some period
of use the natural springiness of the metal is lost and one of the pins
makes a poor contact in the socket, or only touches at one point.
Vibration, due either to passing traffic in the street, or to the weight of
a person walking across the floor, will cause the plug to shake in the socket and the contact is then made and broken erratically, with the
152
TRACING SOURCES OF INTERFERENCE result that arcing takes place and the crackles and other noises are heard through the wireless receiver. The remedy is obviously to make quite certain that the contact is firm and reliable, and in this connection the novel Clix plug which is shown (Fig. 18) may be recom mended. Instead of a slotted brass pin a unit the insulated plug head carries two cotter pins, and the wire leads are passed through the looped heads of these and when the two parts of the head are fitted together the wires are rigidly held whilst the cotter makes perfect permanent contact in the usual type of socket.
Fro. 108. --A combined condenser anti-interference unit from the Buigin range.
An ordinary electric lamp can also give rise to crackles and interference in a similar manner. The base of an ordinary electric lamp is provided with two soldered connections, the solder being usually left in a" rounded" condition, so that it may bear against the small spring pins in the lamp-holder.
It may be found in certain cases (especially with cheap or foreign lamps) that the solder is insufficient to enable the contact to be firmly made when the lamp is in its final position in the holder and this can give rise to crackles as already mentioned. The remedy is to run some more solder on the contacts, and also to inspect the spring plungers in the holder and make quite certain that they return to their" ful out" position after being depressed. (Any test at this point should, of course, only be made when the supply is switched of at the mains.)
Car Interferenee. -- When listening on the short-waves interference may easily be experienced from passing cars. The ignition syste m gives
153
PRACTICAL WI RELESS SERVICE MANUAL rise to quite a powerful radiation, and when a car radio is being employed the trouble is, of course, increased. Special interference suppressors may, however, now be obtained for the sparking plugs (which
a
C--TO FR AME OF APPLIANCE
Flo. 110. --A flex lead suppressor (together with the circuit which is employed) from
the Belling-Lee range.
NA P
Fin. 111. --This is aflex lead suppressor made by T.M.C.
provide spark from the interrupted current already referred to) and for
the magneto (Fig. 14), and these may be obtained from Messrs. BellingLee or Dubilier.
AERI AL
SOLDERED
SOLDERED AER UN
INSULATION TAPE
AERI AL REJECTOSTAT 4--EARTH
TRANSMISSION UNE TO REC0.-0 REJECTOSTAT
DE RED CONNECTED TO EARTH LEAD EVERY 18"
Flo. 112. --The aerial Rejectostat used for avoiding losses in a screened aerial
system.
154
Cr"
TRA NS MI SI ON LINE FROM
AERIAL REJECTOSTAT
CONNECTS ACROSS A g B
EARTH VIA SH I ING TO C
A
C
g
This is the remaining part of the Rejectostat equipment, and is
used to feed the receiver.
TRACING SOURCES OF INTERFERENCE
Similarly, in the car any motor-driven
apparatus must be considered as mentioned
previously if a car-radio is being used. The
wind-screen wiper, for instance, may be of the
motor-operated type and the commutator will
give rise to noises unless a suppressor is employed.
A small unit suitable for the purpose is shown in
Fig. 115 and is a Dubilier product.
H.P. Mains Interference.-- One other form of
interference may be mentioned, and this arises
from the presence of high-frequency currents in
the mains leads. An H.F. choke will, of course,
prevent this trouble, but in view of the fact that
the current passing is on the high side special
chokes must be employed designed to withstand
the high currents. Such components may be
13. --The Chic patented plug which avoids al troubles arising from crackles due to poor contact. ordinary receiver,
obtained from the various firms who have already been mentioned in this chapter, and it is possible to obtain components rated up to 300 amps., although of course, such acomponent is very expensive --costing about £21. For the the cheaper type of choke rated at -6 amps.
and costing 8s. 6d. wil be found quite suitable.
Legislation is probable, making it an offence to manufacture or use
apparatus capable of giving rise to interference with broadcast reception.
FIG. 14. --The method of preventing interference from motor-car plugs. 155
FIG. 15. --For reducing interference from windscreen wipers and similar apparatus this Dubilier unit
is ideal.
CHAPTER XXIII
TEMPORARY REPAI RS AND SUBSTITUTION
M ANY service-men are under the impression that when acomponent or valve breaks down or develops afault, asimilar item must be obtained as areplacement. In general this statement
is perfectly true, but with a little careful thought it is often possible to
utilise another component or valve until the correct parts are available.
It is, of course, unwise to make such substitutions and exchanges
when the theoretical knowledge which you possess is insu ficient to
enable aful understanding of such change to be obtained, but the follow-
ing notes will give some idea of the possibilities.
KT+
Breakdowns. --Let us deal
, t .
RESISTANCE.
first with breakdowns which might occur and prevent a
receiver from working. In
:.00
general the only .components
4
which are likely to suddenly
-
break down and prevent
, ?,
( ,
-41-.-4: --(4-:1._Otcj %;.-
reception are those across which adiference in potential
exists, or in other words, those which are carrying a
current. In the average re-
ceiver these will consist of
FIG. 16.--An H.F. choke may be replaced by a resistance in most circuits
without marring the performance.
the coils, chokes and transformers. A breakdown of a component in an anode circuit
will prevent signals and in this position we find chokes and transformers
--components which are generally found to fail when an excess current
is passed. Under normal conditions, therefore, abreakdown will indicate
that an excessive current has been passed and before carrying out any
replacement an examination should be made to ascertain the cause of
the breakdown.
When this has been located and the fault remedied anew component
will be indicated and it may so happen that asimilar item is not readily
obtainable. In the case of an H.F. choke a resistance may be included
in many cases with very little loss of eficiency, and the only general
trouble will be found in a reduced voltage on the valve (Fig. 116). In
many cases this may be remedied in avery few moments by connecting
the H.T. lead from that valve to ahigher tapping on the H.T. battery
156
TEMPORARY REPAIRS AND SUBSTITUTION or connecting it to the maximum H.T. positive lead. For normal reaction circuits the value of the substitute resistance will not be found critical and for many types of H.F. coupling also the resistance will be found to function as well as the choke except, perhaps, on the lower wavelength ranges.
Transformers and Resistances. --In the case of atransformer breakdown it may be safely taken for granted that only the primary will be found faulty, as there is no current of any magnitude flowing through
H.T+
Flu. 117. --An L.F. transformer primary winding may break down. It may be replaced by a resistance and condenser coupling as shown.
the secondary. Temporarily, the secondary could be used in the anode circuit, but such acourse is not recommended in view of the fact that it is not designed for the passage of a steady D.C. Therefore, in order to enable reception to be carried out a resistance may be placed in the anode lead, with a coupling condenser between the anode and the grid of the folowing valve, the secondary remaining in its original position but now serving as agrid leak in the form of achoke. It may even be found that results are improved by this scheme. If desired, subsequent modification would consist only of aresistance in place of the transformer secondary, thus substituting R.C. coupling for transformer coupling (Fig. 117).
157
PRACTICAL WIRELESS SERVICE MANUAL
Where a resistance has failed, it may generally be found that only
another resistance may be used in its place, although in the case of a
decoupling resistance it may generally be found that an H.F. choke will
answer the same purpose, but will permit the application of a higher
voltage to the valve. This point should therefore be borne in mind
when making this substitution.
Condensers and Valves. --A condenser of one value may generally
be replaced by a component of another value for temporary listening
purposes, although if the condenser forms part of the main tuning
circuits it is possible for ganged tuning schemes to be put out of align-
ment. A temporary expedient in such a case may be to alter the
necessary trimmers for the time being, or until acorrect value of com-
ponent may be obtained. If the faulty condenser is of a low value,
a temporary condenser may
be readily constructell by
twisting together a length of
> 4:7'c' BARE WI RE
sovisTED WIRE
INSULATED SLEEVI NG
wire, the capacity varying according to the thickness of
the wire, the insulation thickness and the length of the wire. Two alternative schemes are also shown in Fig. 18. It
may even be found that by
BARE / E
BR ASS TUSE3F
INSULATED SLEEVi NG
adopting one of these schemes in place of acondenser already
1'o. 18. --As a substitute for a broken condenser, one of the sche mes shown above
may be employed.
in use a much more critical value is found with consequent improved performance.
In the case of valves there is not the same latitude, although it is
possible to make preparations for circuit modifications at a future date.
For instance, take the case of a straight three receiver of the Detector
and two L.F. arrangement in which avalve breaks down. In obtaining
a new valve one of the S.G. type could be obtained and used in the
present detector stage. If the L.F. valve was the faulty valve achange
round should be made to enable the S.G. valve to be used in the detector
valve-holder. The necessary extra items could then be obtained at
odd times and the circuit eventually converted to an S.G. detector
Power arrangement with increased range and volu me.
Old type coils should certainly be replaced by. modern components
and no attempt should be made to obtain out-of-date coils because they
were originally used. The majority of manufacturers to-day supply a
diagram with the coil showing the connections and wiring details and
it is not a dificult matter to incorporate this in an existing circuit.
The improvement will be wel worth the expenditure involved and it
may even be said that such acourse should be adopted with all receivers
in which the coil is two years old or more.
158
CHAPTER XXIV
ADAPTING MILLIA M METERS
ALTHOUGH the famous Ohms Law has been repeatedly described and is now known by every radio experimenter, and probably nearly every listener, it is not realised by many how
the knowledge of this formula will very often save not only a lot of
worry and doubt, but even expense. The relationship between volts,
resistance and current enables certain factors to be ascertained with
very little dificulty and I have shown in earlier Chapters how the
value of a resistance may be ascertained by joining it across a voltage
MI WAMMETER
HT.+.
HI+
supply and by similar methods
have described how to test other small components.
There, is, however, another
very important application
of this small formula which
will be of great assistance to
COUPLING 000A PONE NT
users of small mains units especialy, and also to those
who are desirous of finding
out how certain parts of a
receiver are functioning.
The Effect of Load. --The
majority of mains units are
rated to deliver a certain
H.T.--
voltage at a certain current. That is to say, a unit might
FIG. 19. --A meter connected as shown by the dotted lines would generally give a false reading. The circuit may, however, be considered as shown on the right, and a milliammeter may be used to ascertain the voltage; " a "
is the valve resistance.
be stated to deliver 150 volts at 25 mA. This particular unit need not, however, be restricted to this current if for any reason a larger
drain is required. Owing to the relationship between current and
voltage, however, if any larger current drain is imposed the voltage
output will be lower than 150 volts. Similarly, if the unit is employed
with a small receiver in which the total anode current require ments
are only of the order of say 10 mA., the voltage will be in excess of
150. The actual diference between the voltage at diferent currents
depends upon what is known as the regulation of the unit and in some
modern types of instrument, especially those designed for use with
159
PRACTICAL WIRELESS SERVICE MANUAL
Q.P.P. or Class B valves, this regulation is extremely good and very
little diference in voltage exists over very wide limits of current. On
cheap and old pattern instruments, however, the diference may be very great. Let us suppose you have
CELL
such a unit joined to a receiver
which is just fully loading the
unit and you desire to ascertain
whether the voltage applied to
your valves is correct. Obviously
you would join avoltmeter across
the points where the measure-
ment was required. From what
we have just seen, however, any
excess load will vary the voltage,
and if acheap voltmeter, having
INSULATING SLEEVI NG
PRODS wi ll
a very low resistance is used for the measurement, a very considerable error will exist in the
FIG. 120. --How a millia m meter may be used for test rapnurgep.oses over a wide
reading. The Detector Stage. --It isvery
often desired to measure the volt-
age applied to the detector valve, and as this is generally only amatter of
a few milliamperes, the effect of connecting a low resistance across it
will be to give a very serious error indeed (Fig. 19). How then can
we find the actual voltage delivered
by asmall main unit, or applied to a
detector valve, or take similar read-
ings if we cannot aford to purchase
a good high-resistance voltmeter
If we obtain a good miliammeter
we can, by ascertaining the current
S.
which is flowing, obtain a fair idea
- -----0
of the output. A good milliammeter
maybe obtained much more cheaply
than agood voltmeter, and by ob-
taining a meter having a ful scale deflection of, say, 2 mA. it will be possible to utilise shunt resistances (as has already been explained in
PRO05 15 FIG. 121. --This diagra m shows how the meter may he used with a high voltage
for testing components, etc.
these pages), to obtain higher
readings. A meter of this description could, for instance, be joined in
the negative H.T. lead, with asuitable shunt to enable it to read some-
where about the total current which is used, when, fro m the above
reasoning we may obtain a very fair idea of the voltage output. If, for
instance, it is found that the total current is only 10 mA., and the unit
160
ADAPTING MILLIAMMETERS
is rated at 150 volts with 25 mA., it would be worth while to add a further resistance across the output terminals of the mains unit in order to bring up the total current drain to 25 mA. and thus give you a fair assurance of obtaining the correct 150 volts. Similarly, by including the miliammeter in the anode circuit of the detector valve, and noting the current passing, it is possible from a study of the valvemaker's characteristics to ascertain within close limits the H.T. applied to the valve. In the output stage, the combination of grid bias applied, and anode current passing, will also enable you to ascertain the voltage. Correct grid bias, with a low anode current, will indicate low H.T. voltage and vice versa.
Checking Circuits. --The same simple instrument may also be employed for the purpose of checking the eficiency of tuning circuits or of adjusting ganged circuits, simply by including it in the anode circuit and adjusting for the maximum deflection. That is to say, if atuned circuit is being tested against another known one, the local station would be tuned in and the total current reading in the anode circuit of the detector valve noted. The new coil would be then included, and tuned to the local, and the current reading compared with that obtained with the previous circuit. Similarly, for adjusting ganged circuits, each section could be connected in turn and the appropriate readings noted.
By remembering the relationship between the three diferent sources previously mentioned, it is also possible to ascertain values which in the ordinary way would perhaps be unreadable. For instance, if the meter be employed with a voltage in series and the circuit under examination be isolated and connected so that the resistance may be ascertained, upon re-connecting the circuit and including the meter in series in order to ascertain the normal current flowing, the voltage applied could easily be worked out.
There are many other uses to which the milliammeter can be put, and it should thus be the first item which the wireless listener obtains. (See Figs. 120 and 121.)
161
CHAPTER XXV
R E N O V A TI N G CA BI N E TS
THE majority of marks that make a cabinet look shabby are scratches, chips and finger marks, and such cabinets can easily be rejuvenated by the following method, which should be followed in the case of French polished and sprayed cabinets.
The materials required are :--a small bottle of French polish, a "water" stain in powder form to match the colour of the cabinet, and a fine paint brush.
For Scratched Surfaces. --To touch up scratches or chips, proceed as follows :--Place a little polish in asaucer and add just enough of the powder stain to colour the mixture, mix well and gradually add more powder until the approximate colour required is obtained. It is as wel to be alittle on the light side as the colour can always be darkened when adding a second coat.
Thoroughly clean the scratch with a cloth dipped in methylated spirits and carefully" paint" the scratch with the mixture of stain and polish, taking very great care not to get any on the polished surface of the cabinet, or the surface will be spoilt. Should the mixture get on to the cabinet, wipe it away as quickly as ever possible.
When the first coat has dried thoroughly, a second may be applied, and if the scratch has been coloured to match the rest of the cabinet by the first application of the mixture, the second "coat " should be of polish only. Otherwise, continue the treatment as outlined above with another coating of the mixture. When tackled in this way, the scratch wil soon be filed, and will scarcely be discernible.
Using Plastic Wood. --Some scratched, or chips, are too deep to be removed by this method and plastic wood should be used. This is obtainable from most ironmongers and, since it will not take a stain, the coloured variety should be used, and alittle pressed firmly into the scratch with the blade of a pen-knife. Here again, take care not to allow any of the plastic wood to creep on to the rest of the cabinet, as it contains a powerful solvent which will quickly attack the polish and give it a matt surface. Smooth of the surface of the wood as soon as it is applied, and leave to dry for at least an hour. Plastic wood contracts as it dries, so the surface when applied should be just slightly above the level of the cabinet. When hard, it should be carefully levelled down with a very fine sandpaper. The mixture of stain and polish may then be applied as before, and any irregularities in the surface of the plastic wood will be levelled up by this application. This method is so effective that the position of a scratch may be practically hidden,
162
RENOVATING CABINETS
When a cabinet is merely dul, the application of a little " oilet " or " liquid glass ", coupled with plenty of " elbow grease", should be tried. It is surprising what a few minutes of such treatment will do, but if no improvement is obtained, any of the many com mercial car polishes (cellulose type) are extremely good, not only for removing finger marks and generally cleaning up the cabinet, but also in giving an extremely high polish and removing fine scratches from most of the finishes used in cabinet manufacture. Such treatment is of no avail when the cabinet has been hand French polished.
Perhaps the worst kind of cabinet to tackle is one on which the actual polish surface has worn of, due to continual use and polishing.
FIG. 122. --Pour the polish on to the cotton wool and not on to the surface of the polishing pad.
COTTON WOOL
POUSHING PAD
FIG. -123. The polishing pad ready for use.
POPLASIDHING
Too many people make ahabit of regularly polishing their cabinet with polish, with the result that they do more harm than good and gradually
remove the surface. Rub it over occasionally by all means, but use only asoft, clean duster, with no polish.
In such acase, the only way to make a good job of the cabinet is to "strip " it, that is to remove all that remains of the existing polished surface by wel rubbing the cabinet with fine sandpaper. Always sandpaper the way of the grain, never across the grain, and make a perfectly smooth and uniform surface before attempting any repolishing.
After sandpapering, clean the cabinet with aduster and then with a rag moistened with methylated spirit to remove all grease.
This time, however, the polish cannot be applied with a brush, or a very uneven surface, showing all brush marks, would result. The following method should be followed, and although it may see m alittle tedious, it will render results equal to a new cabinet, and is wel worth
the time and patience. Polishing Hints. --Place asmall amount of cotton wool in the centre
163
PRACTICAL WIRELESS SERVICE MANUAL
of asquare of linen or fine rag, as in Fig. 122, and pour su ficient of the
staining polish (which should be made up as described on page 168) on to
the wool so as to damp it right through. Screw the rag round the wool so as to make atight pad, as in Fig. 128, with a smooth polishing surface.
Slight pressure of the fingers on the sides of the pad should cause some
of the polish to ooze through the linen.
Apply the polish in circular motions as indicated in Fig. 124, with
a gentle but firm pressure, taking care to cover every portion of the
surface with a layer of polish. If more polish is required, never pour
it on to the rag, but undo the pad and add polish to the cotton wool.
If the pad sticks to the surface being polished, apply aspot of linseed oil
METHOD OF APPLYING POLISH
to the actual polishing surface of the pad. On no account add more than one spot unless
the surface is extremely large.
This will prevent the pad
sticking as the polish hardens.
When the first application
has dried thoroughly, rub it
124. --Always apply the polish with circular movements, and do not stop in the
middle of a piece of work,
over very lightly with extrafine sandpaper, working the way of the grain as before,
and then apply another layer of polish, but using, this time, a slightly
drier pad, and working in very small circles. Work up and down the sur-
face in these small circles and, at the end of each line, draw the pad very
lightly across the newly-polished surface in the direction of the grain.
If it is found that the surface is patchy when dry, i.e., polished in
some places, but with dul patches here and there, the surface must be
lightly sandpapered once more and the above procedure repeated again,
until, finally, ahighly polished surface is obtained. Practice, of course,
makes a great deal of diference to the ease with which a cabinet may be completely re-polished.
Finally, there are one or two hints that may save time and patience,
viz., never attempt polishing except in adry atmosphere; always see
that the pad is clean and smooth and, above all, never go over apolished surface until it has become quite hard. It is not the amount of polish,
nor the pressure, that produces a good surface, simply the continued rubbing. Speed does not matter, but rather an even steady circular
movement with even pressure.
Cabinet Scratch Remover. --A useful scratch remover has been produced by Messrs. Holiday & Hemmerdinger. This is in the form of
a tube with a small felt "wick " at·the end, and this is used as a brush
with which the liquid contained in the tube may be applied, and at the
other end of the tube is astain for application when the colouring of the
cabinet has been removed. The price is 2s., and on test the apparatus
works most efectively on cabinets of all types. 164
CHAPTER XXVI
SERVICING WITH THE CATHODE-RAY TUBE
FOR service-bench testing the cathode-ray oscillograph is rapidly coming into its own. Many service establishments have cathode-ray apparatus, but there are still many which have yet to take up cathode-ray testing technique. It can be prophesied, however, that in the natural course of the evolution which is going on it will only be a matter of time before the cathode-ray oscilograph becomes as commonplace as any other of the testing instru ments which are now regarded as essential for speedy and eficient servicing.
Of all the indicating instruments that hu man ingenuity has devised the cathode-ray oscillograph is probably the most versatile, although many of its applications necessitate the use of apparatus additional to the oscillograph itself. In some cases elaborate set-ups are required.
There must be many of our readers who are interested in the radio servicing possibilities of the oscillbgraph when used, at the most, with an oscillator of suitable type, and this Chapter has been written for their especial benefit.
Electron Beam Deflection. --Various types of C.R. tubes are employed in servicing oscilographs; some are of the gas-focused type, others of the high-vacuu m, electron lens-focused type. There are differences of screen sizes, diferences in the visual and photographic properties of the screen traces, etc., but a feature that they have in common is the provision of two sets of deflector plates (in the simplest case, two pairs of plates) mutually at right angles and so arranged that the beam of electrons shot from the gun (anode) on to the fluorescent screen passes through both sets of plates. The electric field produced by the application of a potential diference to one set of plates deflects the electron beam in adirection perpendicular to the surfaces of these plates and to an extent proportional to the value of the applied voltage. Certain oscilographs also contain coils external to the tube asse mbly but capable of causing deflection of the beam by the magnetic field set up by currents in the coils.
It is normally arranged that the deflecting force of one set of plates tends to move the light spot horizontally across the screen and these particular plates are invariably referred to as the X plates. The other set of plates tend to move the light spot vertically, and are called the Y plates (see Fig. 125).
If deflecting voltages are applied to both sets of plates simultaneously the light spot will be moved in adirection somewhere between
165
PRACTICAL WIRELESS SERVICE MANUAL
Yaws
horizontal and vertical, de-
pendent upon the vector re-
:
sultant of the two deflecting
forces.
VI
If constant deflection voltages are applied the light
X ax IS spot will take up a position
X 1
1X 2
dependent upon the deflection
resultant but will thereafter
I Y2
remain steady so long as the
plate potentials keep constant.
If there is any variation of
voltage at the deflecting plates
the light spot will move in
125. --Plan view of the deflector plate arrangement to give horizontal, vertical, or
resultant-deflection.
sympathy and a very slow variation of voltage will be
shown by a slowly moving
spot (definitely visible as a spot). Fast voltage variations, however, will, owing to the persis-
a,
tence of vision, cause the movement of the
light spot to show up as aline trace.
X Deflection as a Function of Time. --In
radio we are particularly interested in voltage
wave forms. This is another way of saying that
we are particularly interested in the exact
manner in which the voltage varies with respect
to time. If we wish a" picture " of a voltage
wave-form to be visible on the screen it will be
of no use applying the voltage either to the X
plates alone, or to the Y plates alone, or to both
together for that matter. Suppose we did apply
an alternating voltage to the X plates. The
light spot will certainly move in strict accor-
dance with the voltage variations, but as it wil
run backwards and forwards along a straight
line the trace of this line will be the only visible result (see Fig. 126).
What is required is that the X deflection shall cause movement of the light spot across
the screen independently of any variation of the voltage under test; preferably the movement of the spot with reference to the X axis should be at uniform speed, in which case the position of the spot with reference to the X axis wil be a function of time. Then if
126. --Showing the efect of applying periodic deflecting voltages, without time base :--
(a) Trace obtained when the voltage is applied to
the X plates only. (b) Trace obtained when the voltage is applied to
the Y plates only. 41 Trace obtained when the voltage is applied to both X and Y plates
simultaneously.
166
SERVICING WITH THE CATHODE-RAY TUBE
the voltage under test is applied to the Y plates the position of the
spot with reference to the Y axis will be a function of this voltage. In
brief, horizontal movement represents time, vertical movement
represents voltage variation, and the excursion of the spot over the
screen will actually "graph "
the waveform of the voltage.
As to how many cycles wil
show up will depend upon
the ratio of the voltage
frequency to the X sweep
127. --Saw-tooth waveform of voltage for X deflection as afunction of time.
frequency, but we will return to this point presently.
In radio service-testing we are not normally concerned with tran-
sient waveforms and, therefore, do not want the X deflection to give
just a single sweep across the screen. What is desirable is that a hori-
zontal traverse shall occur at a uniform speed (usually from left to
right), that the spot shall then fly back rapidly to the starting point
and immediately start another left to right traverse at the same speed
as before, and so on. Remembering that X deflection depends upon the voltage applied
to the X plates we can now
see what kind of deflecting
I
art
voltage is required. For the left-to-right traverse a
BIAS cf.,
voltage increasing in a
linear manner will be
necessary, and for the
rapid right-to-left fly back
asudden drop of this volt-
age to the starting value
wiolrl dbse, rewqeu irweda.n tInaont heXr
H
deflecting voltage of saw-
FIG. 128. --Simple circuit ilustrating the principle of the time base.
tooth waveform, as shown
graphically in Fig. 127, and a" time base" circuit to produce such a
voltage wil be an essential part of the oscillograph. Time Base Systems. --Quite a lot can be written about time base
systems, but we will content ourselves here with the basic principle.
Fig. 128 is a greatly simplified sketch of a time base circuit and will serve to ilustrate the principle. A condenser, C, is connected in
series with a resistance, R, across an H.T./D.C. source. Upon first
switching on, the condenser will charge up and the voltage across it will rise at a rate governed by the values of both C and R. If it were not for the presence, across C, of the gas-filled triode V, the condenser
would charge up to the voltage of the D.C. supply and further action
167
PRACTICAL WIRELESS SERVICE MANUAL
would cease. As matters stand, however, the condenser will charge up
to the break-down voltage of V (the voltage at which the gas ionizes).
The valve becomes conductive and the condenser will discharge through
it, the condenser voltage dropping very rapidly to the extinguishing
voltage of V. Then C will charge up again until the triode once more
permits discharge, and so on.
There is anatural tendency with the simple arrange ment of Fig. 128
for the condenser voltage, during the charging period, to rise expo-
nentially, but it is very desirable that the voltage rise shall be linear.
If the striking voltage of the
triode is kept wel below the
H.T. voltage the voltage rise
will then be approximately
linear, but another way of
tackling the problem is to
charge the condenser through
a valve, generally a diode or
a pentode, worked at satur-
ation point. With suitable
operating conditions the con-
denser voltage variation can
be made to be a close approx-
imation to the saw-tooth
waveform, and can be utilised
to give us the particular X
deflection discussed above.
Fics. 129, 130, and 131. --Screen tracPs of an alternating voltage waveform for diferent ratios of A.C. frequency to the time base
Flexibility of control is an important requirement with a CRO and it is very desirable
"sweep" frequency. The traces are appropriate to ratios of 2/1, 6/4 and 1/1
respectively.
that the speed of the left-toright traverse shall be adjust-
able, according to needs of any
particular test that is being made. With the simple arrangement of
Fig. 128, variation of C or R wil give speed control. As the light spot
keeps repeating its left-to-right traverse we can speak of the " sweep
frequency." Obviously, the speed of the traverse, and the sweep
frequency are closely related.
It is to be understood that the fly back (right-to-left) sweep of the
spot should be so fast as to render the return stroke invisible (or only
faintly discernible). It is to be mentioned, however, that certain
oscilographs have arrangements for modulating out the beam during
the fly back sweeps.
A Waveform "Picture." --Any person handling CRO testing gear for the first time will be well advised to make some tests on an A.C.
voltage derived from the mains (suitably stepped down). Such tests
168
SERVICING WITH THE CATHODE-RAY TUBE
form useful preliminary practice in handling the apparatus and getting to know the scope of the controls of the oscilograph.
Suppose the time base voltage is in operation on the X plates at a frequency of 25 sweeps per second and that the A.C. test voltage is supplied to the Y plates at a frequency of 50 cycles per second. During each visible traverse of the spot it will be moved from left to right by the X deflection and will also be moved up and down by the voltage on the Y plates, the vertical deflection being in strict accordance with the Y voltage. The net result will be that the line trace made by the spot will mark out the waveform of the voltage on the Y plates and with the frequency values specified above two cycles will show up on the screen (see Fig. 129). If the time base is speeded up to 50 sweeps per second 1 cycle only will appear (see Fig. 181).
In both these cases the ratio of the " work " frequency (as we will now call the frequency of the voltage under test) to the sweep frequency
is a whole nu mber. The ratio of work to sweep frequency is 2/1 for Fig. 181, and 1/1 for Fig. 129. It is interesting to consider what will happen for work/sweep frequency ratios which are not whole nu mbers. If, starting with the conditions appropriate to Fig. 181 the time base is slowly speeded up it will be found that before the simple, single cycle " picture " of Fig. 181 appears the trace on the screen will pass through a succession of interlace patterns. An interlace pattern will appear whenever conditions are such that the light spot finishes the end of one sweep at adiferent point in the A.C. cycle to that at which it started the sweep. Then, naturally it wil trace out, during the next sweep, a curve which appears on the screen in out-of-phase relationship to the first curve.
An example is shown in Fig. 130. Such an interlace pattern is not as crazy as it might appear at first sight. As a matter of fact the ratio of work to sweep frequency can be accurately determined by inspection of the pattern. The rule for determining the work/sweep frequency is simple. The ratio of work to sweep frequency is equal to the ratio of the number of peak amplitude points to the number of curves. In Fig. 180 there are 5 peak amplitude points (A and B together count as one) and 4 curves. Hence, the ratio of work to sweep frequency is 5/4.
Frequency Calibration. --From the foregoing it is easy to see that if the work frequency is known, the sweep frequency can be very readily ascertained. Conversely, if the sweep frequency is known the work frequency can be determined. With the CRO, therefore, it becomes a simple matter to determine the frequency of any L.F. voltage applied to the Y plates.
In normal radio fault tracing, or ganging work, the need to do this is not likely to arise, but since most service workshops contain audiooscillators (even if they are only rough and ready " howlers ") it is
169
PRACTICAL WIRELESS SERVICE MANUAL
worth noting that the CRO provides a delightfully simple means of
calibrating an audio-oscillator in terms of cycles per second. If the audio-oscillator's output is made to operate on the Y plates,
and the ratio of work to sweep frequency determined, as described above, then, provided that the sweep frequency is known, it is a matter of simple calculation to determine the frequency of the Y voltage. The fact that the sweep frequency must be known, and known accurately, presents no dificulty, because this can be ascertained beforehand with the aid of the A.C. mains.
Locking the "Picture." --Any irregularities, or drift, in the operation of the time base are most undesirable in frequency calibration work, or when awaveform is being closely examined, and it is therefore important to have provision for synchronising.
In the case of a gas-filled triode the voltage at which the discharge occurs is very dependent upon the grid potential, and by applying an alternating voltage to the grid it becomes possible to synchronise the time base, and this A.C. voltage. It is customary, with servicing oscillographs to have switching, or terminal connections, enabling the time base to be synchronised either with the mains voltage or with any particular test voltage that is operating on the Y plates.
When the time base and the voltage on the Y plates are synchronised it is possible to lock the screen trace so that it does not wander, and thus a steady " picture " is obtained.
In frequency calibration work the time base should, of course, be kept synchronised with the A.C. mains throughout the tests since the mains frequency is the frequency standard upon which the calibration is based.
Obtaining H.F. Response Curves. --For radio service testing it is absolutely essential to have, in addition to the oscillograph itself, an accurately calibrated oscillator providing not only the normal amplitude modulated H.F. output but also, when required, an H.F. output that is " wobbled " by the time base of the oscilograph. The expressive word " wobbulation "has been created to describe avariation of the oscilator frequency over a band of frequency values and in the manner described below.
For an H.F. response curve to be shown on the screen of the CRO we want, first, the frequency of the testing oscillator to vary in a linear manner during each sweep of the light spot. With the light spot at the commencement of a sweep the oscillator frequency must be so many Kcs. (according to the band-width covered) below the frequency corresponding to the tuning setting of the oscilator. Then, as the light spot moves across the screen, the oscillator frequency must come up to the tuning setting value, rise above it and, at the end of the sweep, be as many Kcs. above the tuning setting value as it was below at the commencement of the sweep. The next sweep must give arepetition of the foregoing, and so on.
170
SERVICING WITH THE CATHODE-RAY TUBE
Wobble. --Since the X deflecting voltage, from the time base, con-
trols the horizontal traverse of the light spot, it should be obvious that, in some way, the X deflecting voltage must be made to bring about the required frequency variation (wobbling) of the oscillator. There are various ways in which this can be brought about. One commonly used method takes advantage of the fact that the input capacity of a triode is dependent upon the working amplification factor of the valve, this
effect occurring as a result of the anode to grid feed back through the inter-electrode capacity. It is a simple matter to arrange that the input capacity of a triode shall form part of the capacity of the oscillator's tuned circuit. For" wobble" of the oscilator output the input
capacity of the triode must vary with the sweep of the oscillograph. The input capacity changes with the efective amplification of the valve and this amplification depends upon grid bias, so it should be obvious that some connection between Voltage and tuning can be secured. Very careful design is, of course, necessary to secure the particular kind of frequency that is required, but as far as we are concerned at the moment it will be suficient for us to appreciate that the "wobbling " of the
oscilator H.F. output can be brought about by feeding the grid of the triode (or other type of valve connected as atriode) from the time base
circuit. Constant Amplitude. --The ordinary amplitude modulation of the
oscillator is cut out of action while the oscillator is being " wobbled " and with a good outfit the output of the oscillator will be constant in
amplitude but varying in frequency in the manner described. It is important to understand that the X deflection of the oscillo-
graph must now be looked upon not so much as being afunction of time as a function of frequency. The X axis of the screen must, in other words, be regarded as a frequency base.
For obtaining the " picture " of an H.F. response curve it wil be necessary that-a direct voltage act on the Y plates and that this voltage
shall vary with the amplitude of oscillations in the circuits under test. As regards receiver testing the receiver itself contains the necessary item for the production of the direct voltage. We refer to the receiver's detector. In radio service testing, therefore, the Y deflection is operated
by the rectified output of the detector. The trace on the screen wil be a graph of the detector output volts against frequency, over the range of H.F. values covered by the " wobbulation."
The horizontal and vertical scales are to be regarded as linear scales,
by the way --not logarithmic. Fig. 182 shows apossible result that would be obtained from one or
more H.F. circuits, plus the detector, in the case where the receiver has no band-pass couplings. Sometimes the curve will be inverted, as in Fig. 188. As to which way the curve will extend, up or down, on the
screen will depend, first, upon the type of detector and the point from
171
PRACTICAL WIRELESS SERVICE MANUAL
which the Y voltage is taken, e.g., aY input taken from agrid detector
anode will give inversion with respect to the trace obtained if the Y input is taken fro m a diode anode. The question of inversion will also depend upon the use, or otherwise, of any stages of R.C.C. amplification
between the receiver's detector and the Y plates, and upon the nu mber of stages, if an amplifier is M use. The matter is not one of any very great concern to the service engineer because whatever the vertical sense of the " picture " on the screen it is of equal utility.
The service engineer is not so much concerned with absolute measurements as he is with comparative measure ments. It is true that where such values as valve voltages and currents, component resistances, etc., are concerned, straight measurements form an ordinary testing procedure, but where such factors as receiver sensitivity, selectivity and
quality of reproduction are concerned, the service engineer
works in an atmosphere of
comparisons. ·
CRO apparatus enables
the service engineer to see,
Flos. 132 and 133. --H.F. response curve inversion.
instantly and exactly, what happens as a result of many possible component changes
or adjustments, and re moves a great .deal of the uncertainty that
inevitably overhangs the work of anybody who has to service receivers
without the aid of the cathode-ray. The very considerable removal
of uncertainty is the outstanding feature of cathode-ray testing technique, and labels the CRO as essentially a time saver. To appreciate this time saving ele ment of cathode-ray testing, one has only to consider that very common example of aservice engineer trying to improve the selectivity of a receiver. Whatever adjustment or component repair that he may make, it is necessary for hi m to check up to find the extent
of the improvement, if any. How will he do this ? Suppose we say, by areception test. This may see m avery proper way to make acheck,
but it is going to take time, even if it is only a matter of minutes. Minutes are precious in service work, and it may wel happen that a
succession of such reception checks are required, in which case the time involved will mount up considerably. With CRO apparatus, however,
an H.F. response curve check will give all the information that is wanted,
and the occurrence of any change in selectivity will be instantly apparent, and without the least uncertainty.
CRO apparatus for service testing consists essentially of two instruments, the oscillograph and the oscillator, the latter being of the special type previously referred to. The oscillograph will contain a time base
and one, or perhaps two, R.C.C. amplifiers, the latter being designed for linear amplification over a wide range of frequencies. For normal
172
SERVICING WITH THE CATHODE-RAY TUBE
service test requirements one amplifier will usually be found to be su ficient, this being used to amplify the Y deflecting voltage in the cases where insuficient deflection would be obtained without amplification.
As regards the controls on the oscilograph, that are to be considered as most usual the& are:
Sweep frequency control (generally coarse and fine adjust ments are provided).
Amplifier gain control. Synchronising control. Focusing control. Sweep amplitude control.
Shift control (It is very convenient to have means of shifting the screen "picture " up or down, particularly when H.F. response curves are being obtained).
It is to be understood, of course, that the time base can be switched in or out, as required.
Most receiver servicing tests are carried out with the time base in action, the Y deflection being controled by signals picked up at various alternative points in the receiver. It is to be mentioned that one side of both the X and Y deflecting systems are normally "tied down" in potential, and that the deflecting voltages are applied between the "free" X plate and E, or the "free" Y plate and E. Thus, with the time base switched on for X deflection, the Y deflecting voltage can be picked up from the receiver with the aid of one test prod. The amount of information that can be gathered by arun round with this one test prod is usually a source of surprise to people not previously familiar with cathode-ray testing technique.
The ORO as Output Meter. -- When straightforward peak signal trim ming adjustments are being made the oscilator can be used on amplitude modulation, and the CRO employed as an output meter.
Assuming that receiver, oscillator and CRO are all earthed, all that will be necessary will be to take the Y lead to the anode of the output valve. With push-pull, or Class "B" output, the Y lead can be taken to either of the two anodes. It wil usually be desirable to have the CRO'S amplifier in use between receiver and Y, and the gain should be adjusted to give maximum sensitivity of indication (consistent with the trace not running of the screen vertically).
If the time base is switched on, the trace wil be that of an A.C. waveform (due to the audio frequency modulation of the oscillator), the Y deflection being controlled by the potential variations at the output valve anode. Trimming adjustments wil have the effect of causing the screen " picture " to expand or contract vertically, and a peak adjustment is indicated by maximu m vertical distance between positive and negative peaks. Alternatively, the time base could be switched out, in which case the Y deflection will produce a simple
173
PRACTICAL WIRELESS SERVICE MANUAL
vertical straight line trace, and trimming may be adjusted to give the
maximu m length of line. Ganging. --As an aid towards the re-ganging of receivers in general,
and of modern type, the CRO and associated oscillator, must be regarded as virtually a necessity if speed, with certainty, is to charac-
terise the work. In the simple case of a receiver having no pretensions to band-pass
characteristics, and where all trimmer settings wil be of the peak type, the CRO may usefully be employed as an output meter, as described above. Perhaps in connection with such a simple case, one could say that simpler gear than CRO apparatus would be quite satisfactory, but it must be remembered that the CRO provides the means of checking up on H.F. response curves. This is a valuable attribute, even where a "straight three" receiver is concerned.
It is with the receiver containing band-pass couplings that the CRO really shines as atrouble remover, and atime saver.
In such a case a considerable amount of the adjusting will be done with reference to H.F. response curves. The procedure is easy to follow, and such precautions are as necessary are not at all complicated. We will deal with the latter first.
The A.V.C. should be kept cut out while all the trimming is being done. (Disconnect A.V.C. feed lines from A.V.C. detector and earth them to chassis). It is important to realise that the action of A.V.C. can lead to false response curves appearing on the screen. The reason for this is that although the trace on the screen represents a graph of detector output volts against frequency, the conditions under which any particular response curve is produced are artificial, and difer in one important respect from the conditions of actual radio reception. In actual radio reception carrier and sideband frequencies are present simultaneously, but with a "wobbled" H.F. input the frequencies covered by the base of the response curve are passed through by the oscillator in succession, and at acertain rate. Referring to Fig. 184 the oscilator will attain the frequency Fl before it reaches the frequency F2. If the rise of H.F. amplitude exceeds the A.V.C. delay voltage the A.V.C. diode will commence to rectify. The A.V.C. feed line to the controlled valves has, however, acertain time constant, and in view of the consequent" sluggishness" of the A.V.C. bias variation, it should be easy to appreciate that although the H.F. response of the receiver under reception conditions might correspond to asymmetrical curve such as that of
Fig. 181, yet, under" wobbulation "conditions, the trace shown on the screen could be rendered lop-sided by the effect of A.V.C.
The idea of keeping the test signal wel down in amplitude, so that the delay voltage keeps the A.V.C. out of action, may suggest itself to the reader, as an alternative to cutting out the A.V.C. by circuit modification, but while this is satisfactory enough for quick check test
174
SERVICING WITH THE CATHODE-RAY TUBE
purposes it is not to be advised when actual ganging work is being done. When making trimmer adjustments with an H.F. response curve as guide it is desirable to have a reasonably big " picture " on the screen, and this makes avery low amplitude of input unsuitable. At the same time care must be taken that valve overloading is not permitted as this, again, would give distortion of the trace. Using the CRO amplifier in front of the Y plate with the gain well up will make it easier to get a good size of trace, without overloading of the receiver valves.
Another precaution concerns the frequency at which the time base is run while H.F. response curves are being taken. This should not be too high and, as a general rule, 25 sweeps per second can be considered as satisfactory.
The procedure for ganging follows the usual practice as regards the sequence of the adjustments. With a superhet, for instance, the I.F. trimming should be done first.
When adjusting band-pass I.F. stages for satisfactory response the Y amplifier lead should be taken from the detector. If this is adiode
Fl F2
FIG. 134. --Ilustrating the efect of A.V.C. on the
tube,
FIGS. 135 and 133. --On the left is seen the efect of heavy damping on an I.F. circuit, and on the right how it appears when the damping
is removed.
the high potential end of the diode load resistance will be the proper test point.
Where grid detectors are concerned one most frequently finds a resistance anode load in modern receivers, but should the case be met where L.F. transformer coupling is used between the detector and the succeeding valve the anode circuit should be temporarily modified to have a resistance load while an H.F. response curve is being taken; otherwise the reactance component of the primary impedance will tend to produce a false shape of curve. Use the detector anode as a test point.
If a receiver contains three I.F. transformers the I.F. test signal (" wobbled ") should be applied to the grid of the first I.F. valve, and the combined response curve of Nos. 2and 3transformers got right first.
Then the test oscillator input can be shifted back to the frequency changer grid, and No. 1transformer adjusted to give satisfactory response of the I.F. section of the receiver, as a whole. During I.F.
175
PRACTICAL WIRELESS SERVICE MANUAL
aligning work the receiver's oscillator should, of course, be kept out of
action. When band-pass circuit adjustments are tackled with the aid of
an output meter indication only, it is particularly easy to land up with a pseudo band-pass response that is, in reality, a double peaked curve obtained by mis-tuning. There is little risk of doing this with a CRO response curve indication if a careful eye is kept on the changes of response curve shape with each trimmer adjustment. The CRO user scores heavily over his less fortunate technical brethren in the respect that the former is able to see instantly what efect any particular bandpass trimmer adjustment is having.
After the I.F. stages of a superhet have been correctly aligned the oscillator trimming and padding can be attended to, using the CRO as output meter. Then the signal frequency circuits should be aligned, and if there are any band-pass couplings involved, the adjustments should be done against a receiver over-all H.F. response curve, using a " wobbled " test signal. In any case, a final check should be made of the over-all H.F. response curve.
The fact that H.F. response traces can be obtained makes the CRO an extremely useful device as an aid to the location of faults. As many of our readers will be able to testify, the locating of a fault frequently takes far more time than the actual rectifying of the fault, once it has been found. Particularly is this liable to be the case when the fault is associated with an H.F. circuit and afects the selectivity or sensitivity of the receiver.
Considering the H.F. section of a receiver it will be appreciated that no fault can develop afecting selectivity, or sensitivity, without in some way "making its mark" on the H.F. response curve.
Figs. 185 and 186 ilustrate a case in point. A superhet gave weak reception. Results were so poor on all wavebands that it became a matter of immediate interest to find out what the I.F. stages had to say for themselves. A response curve check on the I.F. section of the receiver gave the result of Fig. 185. This curve showed definitely that there was trouble in the I.F. section; moreover, its shape and size suggested the presence of excessive H.F. damping. Fig. 136 shows the efect of clearing the fault (a resistance leak through an I.F. trimmer).
Time in locating intermittent H.F. faults can very often be saved by obtaining an H.F. response curve and watching to see exactly what happens to it when the fault occurs. To quote an example: A superhet developed an intermittent signal fluctuation on one waveband. A " wobbled " test signal was applied, and a watch kept upon the over-all
H.F. response curve. It was observed that when the signal fluctuation occurred it was associated with asideways jump of the response curve. A sideways movement of an H.F. response curve indicates one of two possible happenings, either a change of applied signal tuning, or a
176
SERVICING WITH THE CATHODE-RAY TUBE
change of receiver oscillator frequency. In the case under consideration, the first possibility could be ruled out so itbecame obvious that the fault, whatever it might be, was causing the receiver oscillator frequency to shift. This information made the tracing of the fault acomparatively simple matter, and a certain padding condenser was very soon on the scrap heap.
In the case of a dead receiver, with no signal coming through, the common procedure of stage jumping can often be superseded by the
Fm. 137 (toP). --" Looking " at the signal at the anode of the output valve, and in
138 (bottom) at the grid of the output valve.
process of searching for the" lost "signal with the Y amplifier test lead
of the CRO, with the time base switched on. Within limitations it is even possible to probe around the H.F.
section of areceiver, and to detect the presence of H.F. oscillations. The actual appearance of the screen " picture " produced by modulated high frequency will depend upon the time base frequency, and the type of oscilograph. With the usual servicing type of oscillograph, and a comparatively low time base frequency, the H.F. trace will appear as a
blur of light shaped within an outline corresponding to the audio modulation of the H.F. We have used the word limitations in connection with this method of H.F. circuit testing. Obviously, the amplification
provided by the Y amplifier will be the deciding factor as to how small an amplitude of H.F. wil be the minimum that wil show any signs on
the CRO screen. Figs. 187 to 144) reproduce the tracking of a modulated test signal,
starting from the anode of the output valve of asuperhet, and working
12
177
PRACTICAL WIRELESS SERVICE MANUAL forward for a certain distance through the receiver. Fig. 187 was obtained from the anode of the output valve, and Fig. 188 from the grid of this valve. For Fig. 139 the Y amplifier lead was taken to the grid of the double diode triode which preceded the output valve. The amplitudes of the traces of Figs. 187 and 188 should not be compared with those of Figs. 189 and 140. The gain of the Y amplifier was kept well down for the traces of Figs. 187 and 138 but was brought up for the remainder of the tests.
/VVVv W A
\AI WAIV
Fin. 139 (to). --The signal at the grid of a double-diode-triode. FIG. 140 (bottom.) -- The signal at the detector diode anode. The decrease of amplitude of Fig. 140, with respect to Fig. 139, is due to loss in the resistance-capacity network between diode
and grid.
Performance over a Waveband. --If an over-all H.F. response curve of a receiver is taken on a" wobbled " test signal at one end of a waveband, and the receiver tuning control is turned, the curve on the screen wil shift along towards the edge of the screen (and disappear if the receiver tuning is altered too much). By a corresponding change of test oscillator tuning the " picture " can be puled back in position. It should be obvious that by appropriate changes of both receiver and test oscilator tuning the H.F. sensitivity and selectivity of the receiver can be studied at every point over the whole waveband. The test oscillator output amplitude should be constant.
Receiver Co mparisons. --By arranging aconvenient form of changeover switching for the test oscillator output, and the Y amplifier input leads, the relative performances of two receivers can be very readily deter mined. The use of over-all H.F. response curve indications will give
178
SERVICING WITH THE CATHODE-RAY TUBE
all the information that is required as regards comparisons between the
H.F. sensitivity and selectivity of the two reFeivers. To allow for overall sensitivity, the CRO could be used as an output
meter. If it becomes necessary to compare L.F. gains only, then an audio test voltage could be applied to the appropriate points of the two
receivers.
It is hoped that readers who have not yet used CRO apparatus will
have gathered some idea from the foregoing of the versatility of the CRO
from the servicing
point of view, even
when it is associated
Flo. 141.
T"hetel-tCalReO" aswhean fauusletsd as an output meter. Ftc. 141. --A signal trace taken from No. 1anode of a Class B output valve. All wel. Ric. 142. --aanncoWedheaintwitNhaho.ppae2nhisganhoatderesNio.scitr--1
with no more than a "wobbulator oscillator."
There are many test applications that must suggest themselves to the reader
FIG. 142.
cefuiet.ct at FNoI.G. 1a14n3o.de--Tofhea break in No. 2 anode circuit. These curves
who the of
understands principles the C R O.
reprateisoenns,t npoottencutirralent.vari-
Obvious possibilities associated with the
Flo. 143.
waveform obtained on an output meter
test will co me to mind. Figs. 141,142 and 143 are given just to emphasise
the fact that even when the CRO is acting as an output meter it is still
a fault " tell-tale." The reader will no doubt consider the idea of a
study of output waveform giving aclue as to the type of any distortion
that may be caused by afault, although it is to be re me mbered that the
ear of an experienced man will take some beating when it comes to a
matter of drawing a quick distinction between one form of distortion
and another.
179
CHAPTER XXVII
TRACING AN D ELI MI NATING HU M
0F all the possible defects in receiver performance, none is more annoying than hum, and although hum emanates from one source, the possible immediate causes are so nu merous that tracking the seat of the trouble in any particular set is far from asimple task, particularly as it is likely that several factors will be found to be contributing to render the hu m-level higher than isdesirable or necessary.
Hum, of course, is always derived in the first place from the electricity mains --even in abattery-operated receiver --but the immediate causes are many and varied and can be briefly classified under four main heads: first, faulty receiver design and layout; second, maladjustment of circuits or components; third, faulty components or internal wiring; and fourth, external causes. It is obvious that if the design and layout of any set and its attendant wiring is such that undesirable coupling between signal circuits and the mains is avoided, and, in the case of a mains set or receiver using abattery eliminator, if adequate smoothing and decoupling circuits have been provided and are in good order, the final hum level should be very low. Any tests, therefore, for discovering the seat of excessive hu m must be directed to checking al the above points in logical sequence.
Looking for Clues. --In the descriptions of the various tests to be applied, common errors in design and layout will be noted and, of course, the tests are calculated to reveal such errors as wel as actual electrical and mechanical defects. However, when examining areceiver with the object of organising a hum hunt, it is as wel first to make a preliminary observation of the smoothing circuit and of the general layout of the components and wiring in order to gauge the general standard of the receiver, and possibly to discover a clue as to likely trouble spots.
The next point is to examine the adjustment of any device included for reducing hu m, such as apotentiometer across the filament winding of the mains transformer, the slider of which determines the centre-tap from which the cathode line of the set is taken.
If these obvious tests fail to effect a cure, it is next necessary to discover whether the trouble is due to defects within the receiver itself or to external causes, and the possibility of the latter should be eli minated first. The route of the aerial and earth leads should be traced back to discover whether either of these wires passes close to or runs parallel with electric light wiring --a very fruitful cause of hu m. It should be
180
TRACING AND -ELIMINATING HUM
mentioned here that hum from this cause can occur in battery sets as
wel as mains sets, and if the receiver under examination is battery-
driven, coupling between aerial and/or earth leads and the electric light circuits should be at once suspected. This type of trouble is most likely to occur if the aerial and earth leads are long or their route is straggly;
if the aerial itself is of the indoor type, fixed to the picture rail, under
floor-boards or in the loft; or if a considerable amount of temporary wiring or flexible leads for lamps and domestic appliances is employed.
Another point which should be watched is the flexible wire connecting the receiver or battery eliminator to the mains. This should be kept
as far away as possible from the aerial and earth wires. A Tunable Effeet. --A very large proportion of the hu m due to
external agencies in mains-
operated sets if of the type known as "modulation hu m"
or "tunable hu m," the latter
name referring to the fact that its intensity is only serious when the set is tuned to certain definite wave-
lengths. It is due to the presence, in the mains supply,
FIG. 144. --A condenser between earth and one side of the primary will prove very useful as a hum
re mover.
145. --Two condensers connected across the primary winding, and the centre tap joined to
earth.
of certain radio frequencies which become modulated by the 50-cycle mains frequency.
If the set is tuned to the radio frequencies present in the mains supply, only quite
asmall amount of coupling between the mains and the radio-frequency circuits of the receiver is needed to result in amplification and rectification
of the hu m-modulated, mains-borne radio-frequency signals and the reproduction of avery considerable amount of hu m in the loudspeaker.
If tunable hum isdiscovered, one or another of the following sche mes should effect a cure. A fixed condenser of from .0008 to -01 mfd. capacity may be connected either across the mains input to the receiver
or from one side of the mains input to earth (see Figs. 144 and 145).
By reversing the mains plug the correct side can be ascertained. The condensers employed should be of high quality, with mica insulation,
and preferably of 1,000 volts test rating in order to avoid the slightest
risk of ashort circuit across the mains. When the hum is tunable, that is to say, only occurs while tuned
to a station, usually a powerful " local," it can be cured by connecting a-01 mfd. fixed condenser on each side of the rectifying valve filament,
and the secondary " outers " (see Fig. 146). In the case of D C. mains, which are often very much worse than A.C.
181
PRACTICAL WIRELESS SERVICE MANUAL
mains in respect of modulation hu m, it may be necessary to include a high-frequency choke in each mains lead to the set, each choke being by-passed to earth by a condenser of, say, -01 mfd. should this be found necessary. It is very important that the insulation of these
146. --Two "bufer " condensers may be joined to the anodes of a ful-wave rectifying valve to remove modulation hum.
chokes and condensers be of ahigh order to withstand the mains voltage and, of course, the H.F. chokes must be capable of carrying the full
load current of the receiver.
Inside the Set --Next for consideration are those causes of hu m which are to be found in the receiver itself --in most cases definite faults in individual components or in portions of the wiring. In nearly every
case the trouble will be found either in the mains circuits, that is to say,
the power pack or battery eliminator, or in the low-frequency section of the receiver, and these
two portions of the set should be tested in sequence, working
backwards from the mains
transformer. Sometimes hu m assu mesA.C. MAINS
serious proportions simply
because the power unit,
designed as a ful-wave rectifier, is operating actually
only as half-wave rectifier, with the result that the normal smoothing, adequate
148a. --H.F. chokes (of correct design) may be advantageously included
in each main lead.
for full-wave rectification, is now insuficient. Various faults could
result in one half of the rectifier section becoming inoperative --for
example, adisconnection in one half of the H.T. secondary winding, or a bad or broken connection at one of the rectifier anodes, either in the
valve or on the holder or in its wiring (see Fig. 147). Another cause of
182
TRACING AND ELIMINATING HUM
excessive hum is an incorrect centre-tap on the H.T. secondary of the
mains transformer. Any of these faults can be discovered by measuring
the voltage between H.T. negative and each of the rectifier anodes with
an A.C. voltmeter. The two readings should, of course, be practically
identical. It should also be
noted that abnormal hu m
·
may be due to the absence of an earth connection to the
core of the transformer or of
the smoothing choke, or to
this connection making poor
contact.
The next section of the
receiver which may be the
seat of hu m is the high-
tension smoothing circuit. A
147. --Any disconnection or poor contact between A and B or A and C will produce hum as the mains unit will be working as a half. wave rectifier. Test voltages between A and B and A and C with an A.C. voltmeter. Readings
should be the same.
complete or even partial short circuit in the smoothing choke
may be the trouble and can usually be traced by measuring the D.C. resistance of
the choke. This will immediately reveal a dead short (indicated by
practically zero resistance) but unless the original resistance is known
it is not always easy to discover whether a partial short exists, and
substitution by achoke known to be intact is the best test.
Condenser Tests. --Faults in the smoothing condensers can usually
be discovered best by substitution, and for this purpose any 4 mfd.
T+Y A condenser of suitable voltage rating may be connected first in parallel
with each smoothing con-
denser in turn. A marked
x
decrease in the amount of
hu m suggests an open circuit TO SET
or low capacity in the con-
T72
STIMCIRECTIFIETR.O
denser under test. If this HT. <
test, applied in turn to each smoothing condenser, does
FIG. 148. --Short or partial short between A and B wil result in hum. Also adisconnection
not reveal the fault, it is possible that there is a leak-
or cushrorretntcircatuitX inwillC, bore Cgr2.eateInr ththeanlattater Y.case
agein one of the condensers, and each should be removed in turn and sub-
stituted by a known good condenser (see Fig. 148). A leaky condenser
can also be detected by measuring the total rectified current in the lead
between the rectifier cathode and the first smoothing condenser, and
comparing it with the total H.T. current in the lead between the final
smoothing choke and the receiver. An individual test on a condenser can be applied easily by
183
PRACTICAL WIRELESS SERVICE MANUAL
connecting an H.T. source momentarily to its terminals. After removing the H.T. source, the terminals of the condenser may be shorted by the blade of the screwdriver, when a substantial spark should occur, indicating that there is no leak in the condenser. A quantitative test on the
capacity of a condenser is not so easy to arrange without special apparatus, but if an A.C. ammeter or miliammeter is available the following rough
(7)
C2
·
VOLT.A..A.E..TER
tests may be applied: a 4 mfd. cmoantdeeiynse.2r5 will paaass.:s 200 volts
,, .....
50 cycles. Tests with other
CF2IG.wh1e4n9.C,--Aisroknuogwhn.testMefoarsurthee
capacity of volts across
C, (V,) and across C, (V,). Then C,
V1 XCI.
voltages, frequencies and capacities can be made, working on the formula that the current
through a condenser is equal to
6.3 multiplied by the frequency, multiplied by the capacity and
multiplied by the voltage. Another test is to connect the condenser
under examination in series with another of known capacity, and the whole across an A.C. supply of known voltage, measuring then the voltage across each condenser, as indicated in Fig. 149. The capacity of the unknown condenser can be estimated roughly from the proportion 'of the total voltage appearing across it, or by the ratio of one value to the other.
If the smoothing circuit appears to be in order so far as the quality of the components is concerned, it may be worth while to ascertain whether asuficiently low hu m level can be obtained by increasing the capacity of the smoothing condensers. In many old sets condensers of only 2 mfd. or 4 mfd. were used, but to-day 4mfd. and 8mfd.
are common values. Increased capacity up
to a total of 16 mfd. for
each condenser may result in improvement
FIG. 150. --Additional smoothing may be provided for the H.F. and detector stages as shown here.
in respect of hum, but
it is of no practical value to use greater capacities.
Other -Possible Sources. --Here the trouble is most likely to be due to a disconnection in agrid bias by-pass condenser, or a grid decoupling
condenser, although there are other parts which should be given atten-
tion. The best method is to test stage by stage from the output end
184
TRACING AND ELIMINATING HUM
of the receiver, by silencing all the previous stages. Commence operations by connecting grid of the output valve to chassis via a condenser of frpm .01 to 1.0 mfd. capacity. This wil ensure that no signal from any other part of the circuit wil reach the valve. If the hu m persists it probably arises from abreakdown in the bias by-pass condenser, or, of course, it may be due to insuficiently smoothed supply to the field of an energised moving-coil speaker. If, however, the hu m ceases when the output valve grid is shorted to chassis in this way, the source of the hu m must be sought further back in the set, by disconnecting the grid short-circuiting condenser and re-connecting it between the grid of the previous valve and chassis. If the hum continues, afaulty bias by-pass
151. --The coupling condenser is in series with the choke and leak across
H.T.
FIG. 152. -- Where a tuned-grid coupling is employed this also is across H.T.
as shown here.
or decoupling condenser should be looked for, or a faulty anode decoupling condenser, or even a defective grid leak. Gradually, by work-
ing back in this way, the trouble will be isolated to one stage, and all likely spots in that region can be examined. In the detector stage, for
example, an open circuit in agrid leak, direct pick-up from the heater circuit by grid leaks or diode detector wiring, can be suspected. If the
hum is more noticeable on gramophone reproduction the layout of the pick-up leads should be examined to see whether there is undue risk of picking up from mains leads, and screened pick-up leads provided with the screening properly earthed.
In this connection all screened leads should be carefuly examined at an early stage of the proceedings as a broken or poor earth connection to the screening is a prolific cause of hum. Other likely causes are a
high resistance earth, a high resistance joint anywhere in the wiring,
and bad earthing connections to condenser cases. Another point which should be looked to is the condition of all
indirectly-heated valves. Poor insulation between the heater and
185
PRACTICAL WIRELESS SERVICE MANUAL
cathode may appear after a valve has been in service for a long period
and wil inevitably result in excessive hu m. Hu m and the S.G. Stage. --It is true to say that mains hu m can be
so reduced that it is definitely impossible to tell when the set is on and
IE
H.TFin. 153. --A standard tuned-anode circuit. causes, and there are means whereby the eficiency of the mains smoothing apparatus can be raised to a high state of eficiency at a cost wel below that which would be necessary if smoothing were
when it is switched off; such a state of perfection is dificult and costly to obtain, but a little attention wil render a receiver silent to the extent that the ear must be placed against the loudspeaker to detect any trace of hu m. The elimination of hu m is not, as has already been explained, solely concerned with smoothing in the power pack; there are other
H.T.+
accomplished by the sheer size of chokes and condensers.
The main smoothing choke must, necessarily,
possess a moderate in-
U
H.T.-
ductance, as it will have to carry considerable current, and a choke
FIG. 154. --How the smoothing may be increased by including an L.F. choke.
having a high inductance and current-carrying capacity would be too
large for the average set. It is usual, therefore, to have a choke in the
high-tension lead that is common to all valves and then to use another
choke of high-inductance in the lead that carries the high-tension current
to all the valves other than the power valve, and in this manner a most
eficient smoothing unit can be obtained. This arrangement is shown
at Fig. 150.
136
TRACING AND ELIMINATING HUM
One must not overlook the fact that smoothing is more important in
the earlier stages, due to the fact that the following stages amplify it,
and for this reason the screen-grid stage, or stages, should be most
carefully designed to prevent even the least trace of hu m creeping in.
Returning to the subject of mains hu m in the screen-grid stage, let
us turn our attention to tuned anode coupling which is often a serious
cause of this trouble. A glance at Fig. 151 will show that the detector grid is connected to a point midway between a grid leak and condenser
which are in turn connected between cathode and the high-tension supply, when there is sure to be atrace of mains ripple. If the condenser
were large, say, 1 mfd.,
HT+ the grid would have im-
posed upon it all the
ripple in the high tension
supply, since the A.C.
voltage present (in the
form of a ripple), would all be across the grid leak
(the impedance of such a
large condenser being
small). In Fig. 151, how-
ever, the grid condenser
is small, so the grid will
be subjected to a some-
what lower voltage, but
still be quite large enough
155. --The tuned-grid circuit which overcomes the difficulties mentioned above.
to cause trouble. If reference is made to Fig. 152
it wil be seen that the
detector valve is connected
across the tuned-grid coil, which is in series with the coupling condenser
" C." The impedance of the tuned grid coil at the mains frequency
wil only be aminute fraction of that of" C," consequently only anegligible portion of the A.C. ripple will be across the grid and cathode of
the valve and no mains hum will be passed on to the detector fro m this source. The foregoing remarks clearly show that a bad hu m might in
certain circu mstances be cured by changing the method of H.F. coup-
ling, aprocedure that is far from being as well known as itdeserves to be.
An Unusual Defect. --Before going on to the question of modulation
hu m there is asmall defect possible in the valve that may cause aform of mains hu m that cannot be cured in any way save by replacement. This
trouble is defective insulation between the cathode and the heater
wire which it surrounds bringing cathode and heater into direct
metallic connection, and this makes the valve behave as though it
were not indirectly heated.
187
PRACTICAL WIRELESS SERVICE MANUAL
Modulation hu m is not so common as it used to be owing to the mode m mains transformer having a shield between pri mary and secondary, interposed for the sole purpose of stopping H.F. currents from flowing into the mains, which is the usual cause of this parasitic hum. As already explained, this type of interference only appears when acarrier wave is present, since it takes the form of amodulation of the low frequency on the carrier in just the same way that alow note would be imposed upon the carrier wave at the broadcasting station. It should be remembered that 50 cycles, the usual electric mains periodicity, is an audible frequency and is just over two octaves below middle C on the piano. If a loudspeaker were connected to the mains direct, through a suitable resistance, it would hu m in just the same manner as if areceiver were used.
Another cause of modulation hu m is A.C. ripple in the screen-grid lead which can again be cured by extra smoothing, although there are exceptions. A case was recently investigated where the A.C. ripple entered the S.G. valve by way of the screen condenser from stray currents flowing in the chassis, a truly curious happening.
Faint Hum Unavoidable. --The most carefuly designed set will often give a faint trace of hum, after it has been attacked at all points, but even this can be cleaned up by amoving-coil having ahu m-bucking coil, a device that feeds alittle A.C. into the speech coil in opposite phase to the hum, so that the latter is exactly cancelled out.
Hum and the Tuned-Anode Coupling. --The tuned-anode coupling, or some modification of it, is still in use in many commercial and constructor-built receivers; furthermore many of the popular kit receivers use tuned-anode couplings, and there must be tens of thousands of these still in use. The advantages or disadvantages of tuned-anode couplings from apoint of view of stage gain and selectivity is another story, and it is the intention in this chapter to discuss it from the aspect of introducing mains hum. The use of the tuned-anode coupling is probably the very last circuit detail that the constructor would blame for mains hum.
If the smoothing arrangements in the power pack are perfect, or very nearly so, all well and good, but if, as is usually the case, they are short of this ideal to a greater or lesser extent, the tuned-anode coupling will produce asurprising hum level compared to that obtainable with any other coupling. Fig. 158 shows a skeleton diagram of a tuned-anode H.F. stage folowed by the conventional detector. Now any unsmoothed ripple will be in the form of avoltage ripple across the points A-B. The impedance of the tuned-anode coil T will be negligible to mains frequency, so the unsmoothed ripple in the H.T. voltage is in effect across the points D-B. The ripple voltage is therefore across D-B, and aproportion of it across the grid cathode of the detector valve G-B., in proportion to the resistance of the grid leak R and the impedance of
188
TRACING AND ELIMINATING HUM
condenser C. This considerable proportion of the mains ripple being fed into the detector valve will be duly amplified by it and the succeeding L.F. stage or stages. The detector valve transformer coupled to the power valve might easily give an amplification of 200 or more times; thus it will be seen how necessary it is to keep hum out of the detector grid.
Tuned-Grid 18 Sate. --Except by gross carelessness mains hu m is not, of course, introduced into the grid of the H.F. valve, as the grid circuit has no connection whatsoever with positive H.T. Where hu m is present in a set using a tuned-anode coupling, it is a simple matter to note the amount of the prevailing hum caused by ripple getting through the tuned-anode coil by merely shorting the detector grid to cathode (or in the case of abattery set with eliminator to L.T. +). If this simple test shows that considerable hum is emanating from this source, two courses are open. One is to increase the smoothing in the ordinary way by inserting achoke and condenser as shown at Fig. 154, the reason for so positioning the choke being that the small anode current taken by the screen-grid valve permits a choke of fine gauge wire to be used, with consequently greater inductance for agiven size or cost. The other cure is to change the coupling to tuned-grid. The reason why this modification eradicates the hu m is particularly interesting, and an explanation is thought worth while. Fig. 155 shows the skeleton circuit of a tuned-grid stage followed by the conventional detector. As in Fig. 158 mains hum would appear as an A.C. ripple voltage across A-B, and in consequence will be across the choke K, the condenser C-1, and the tuned grid coil T, the three being considered in series. Now at mains ripple frequency the impedance of T is negligible compared with the impedance of CI and K consequently for practical purposes all ripple voltage will be across CI and K, the grid cathode circuit of the detector valve being tapped across the coil T, which, as mentioned above, has a negligible mains hu m voltage developed across it, thus no appreciable hum is introduced into the detector grid, with a very considerable decrease in the overall mains hum of the receiver.
Hum in the Radiogram. --The average home-built radiogram has a disappointingly high hum level, whether built as acomplete radiogram in the first place, or whether built by uniting apick-up and gramophone motor with an existing chassis.
It should not be overlooked that where mains hum is exactly the same on gramophone and radio, it wil appear that the mains hu m is worse on the gramophone, due to the soft passages met with on gramophone records, which, dropping down to a volume level approaching that of the hum, naturally tend to accentuate the presence of the latter. Where the smoothing arrangements are such that the hum level, while being the same on radio and gramophone, isobjectionable, the usual steps must be taken to increase the eficiency of the smoothing arrangement,
189
PRACTICAL WIRELESS SERVICE MANUAL
a subject which has already received so much attention in these colu mns that it would be redundant to include it here. On the other hand, when a noticeable increase in hum is apparent when switching to gramophone, very simple steps can be taken to overcome the trouble. In the first place, it is impossible to over-emphasise the necessity of earthing the pick-up arm, and for screening the pick-up leads and properly earthing such screening. Most pick-ups are provided with an earth terminal, but unfortunately in several makes this earth terminal is not in proper metallic connection with the whole arm. This state of afairs arises in those designs where the tone-arm is in two pieces, earthing of one piece relying entirely on contact through some swivel or rocker which, while mechanically sound, is electrically indiferent. The remedy is, of course, to earth the odd pieces together, so that the earth terminal provided is in metallic contact with the whole structure. There are usually quite a few screws in various parts of a pick-up terminal, and it is not dificult to link them together with flexible wire. Many metal pick-up arms are finished in black or brown" lacquer," the conductivity of which is questionable, and care should be taken to see that earthing is accomplished to pure metal, the lacquer or other synthetic finish having been carefully scraped away for the purpose.
Screening. --For screening pick-up leads almost any screen wire is suitable, provided that it is reasonably dense, but it is necessary to take
more care in earthing the screening than would be imagined necessary. In the absence of a metal band going right round the screen tubing, it is
suggested that if an inch of the screening be unplaited and twisted together, and the earthing wire afixed by soldering, a decent contact to each strand of the screen will be ensured.
The very position of the gramophone motor and pick-up causes the leads to these two components to be more or less parallel, and for this reason it is desirable, and is, in fact, general commercial practice to screen the mains leading to the gramophone motor for about two feet of their length, starting, of course, close up to the motor. The electric motor frame wil certainly have an earthing terminal on it, which should be duly earthed, and if the motor board is metal this should also be earthed. It is not good enough to rely on the fact that the motor frame
is bolted to the motor board, as, in time, oil may find its way under the bolts when they become slackened by vibration, resulting in absence of electrical continuity.
It is nut satisfactory to take a single wire connected in turn at the points above mentioned, and finishing at the earth terminal. The most eficient and convenient arrangement being one wire earthing the motor and motor board, and aseparate wire earthing the tone-arm and pick-up leads. Reversal of the pick-up leads should be tried, as in certain types of pick-up, one particular way round will tend to induce less hum.
190
CHAPTER XXVIII
SI MPLE TESTS FOR CO MPONENTS
THIS chapter is intended particularly for home constructors, who will not possess extensive testing equipment, and must therefore rely upon simple apparatus to locate faults. During recent years receivers have gradually become more complicated, in order to cope with the necessity for increased selectivity. There has also been atendency to use one or more dual-purpose valves in the majority of receivers. It is not surprising, therefore, that partial breakdowns often occur in modern sets, even though the standard of reliability of most components has steadily been improving. Fortunately, however, it is possible to trace the majority of faults by means of a combined meter of the volts-miliamperes type, as the measurement of the voltage
41L1/A MMETE R no. 156. --Showing method of testing transformer continuity.
and current at various vital points in the receiver will enable the owner to localise the defective component. The home-constructor has adefinite advantage over the commercial set owner when abreakdown occurs, of course, as he knows the receiver circuit arrangement and the purpose of each component.
When building a receiver it is advisable roughly to test the components before assembly, for although these are carefully tested by the
191
PRACTICAL WIRELESS SERVICE MANUAL
manufacturer, damage often occurs in transit. This is particularly so in the case of inductance components, such as coils, transformers, and chokes, as the very fine wire used in these is easily damaged. As a general rule it is necessary only to test these co mponents for continuity, as in nine cases out of ten the defect which occurs is a broken wire in one of the windings.
Transformers and Coils. --An easy method of testing transformer continuity is shown in Fig. 156. As indicated, the miliammeter should be connected in series with the transformer winding and asmall battery. A cheap meter is suitable for this test, but readers who do not already possess ameter are advised to obtain amoving-coil instru ment. These meters are more expensive than the moving-iron type, but are more reliable for general test work, and are therefore agood investment. The majority of L.F. transformers have a primary winding (P. to H.T.) resistance of approximately 500 ohms and a secondary winding (G to GB) resistance of approximately 1,500 ohms. If amilliammeter having a ful-scale deflection of 10 mA. is used, a 44-volt grid-bias battery will therefore be suitable for testing purposes. A definite kick of the meter needle when the battery plug is inserted will indicate that the winding is not broken. If it is desired to measure the actual resistance of the winding, Ohm's Law must be applied. For example, if a battery of 44 volts is used, and the meter registers 9 mA., it will indicate that the resistance of the winding is 500 ohms. Likewise areading of 8mA. with the same battery in use will indicate awinding resistance of 1,500 ohms. When making the preliminary test it will be advisable to use a lower voltage, say 14 volts, in order to avoid damage to the meter should the transformer winding resistance be very low. If an abnormally high current reading is obtained it will indicate that there are shorted turns in the transformer winding --this, by the way, is not an unco mmon defect. Low-frequency chokes can be checked in the same way as transformers, but in the case of output chokes the resistance is sometimes as low as 200 ohms, and therefore care must be taken to use a low voltage for testing purposes.
Breaks in coil and high-frequency choke windings are often experienced, due in most cases to the wire attached to the terminal tags being twisted too tightly. As the resistance of coil windings is very low, however --between approxi mately 1and 25 ohms in the case of mediu mand long-wave windings --the method adopted for transformer testing cannot be used, as asuficiently low voltage cannot be obtained. To test coil continuity it is therefore necessary to connect a resistance in series with the milliammeter, as shown for the condenser test in Fig. 157. If a 10 mA. meter and a 44-volt battery are used, a one-watt resistance having avalue of 1,000 ohms wil be suitable. The resistance of the coil is negligible as co mpared with that of the extra resistance, and therefore a reading of approxi mately 44 mA. should be obtained. The exact
1N
SIMPLE TESTS FOR COMPONENTS
resistance of the winding cannot be measured by this simple means, of course, but the measurement of the D.C. resistance of acoil is not very important as the inductance is not entirely governed by the resistance. High-frequency chokes may be checked for continuity in exactly the same manner as coils, and as in the case of the coil winding the actual resistance cannot be measured accurately by amiliammeter. As mentioned before, however, the usual defect in coils and chokes is a broken winding and therefore acontinuity test wil sufice.
Condenser Tests. --Variable condensers work on the principle of one set of vanes moving in or out of a set of fixed vanes, with the air (or
C. wYCIENSER
GB S4rre-RY gesi.rrApere
0/L4/A M METER FIG. 157. --Testing acondenser for short circuits.
solid insulating material) in between the vanes acting as a dielectric.
It is therefore essential that the vanes do not come in contact with each other. Owing to the close proximity of the fixed and moving vanes in
the modern midget condensers it often happens that they touch each other at certain points during rotation. It is therefore advisable to
test these components before assembling them. This can be done as
shown in Fig. 157, using amiliammeter, a1,000 ohms resistance, and a 4i-volt battery. If the condenser is in order areading will not be regis-
tered, but ifthere is ashort between the fixed and moving plates at any
point areading of 4i mA. wil be obtained. It is sometimes found, however, that fixed condensers are defective.
These components are given an insulation test by the manufacturers, and are then given aworking voltage rating of approximately half the
test voltage. It is very important that the condenser should not be
connected in acircuit where the voltage across its terminals exceeds the
13
193
PRACTICAL WIRELESS SERVICE MANUAL
working voltage specified by the manufacturers, and it is desirable in
the interests of safety to choose a component having a slightly higher rating than the actual circuit voltage. Condensers having a capacity
over .1 mfd. can be tested for leakage by the method shown in Fig. 158.
A voltage approximately equal to the working voltage of the condenser
should be connected momentarily across the condenser terminals. A
leakage period of approximately three minutes should then be allowed
before the screwdriver test is made. It wil be noted that the screw-
driver is rested on one terminal of the condenser, the end being then
brought close to the other condenser terminal. If the component is in
order a spark will occur, but if a spark does not take place a defective
ArJ-7kode
04
condenser will be indicated. When conduct-
ing this test care should
be taken to keep the
hand clear of the metal
portion of the screw-
JektriV-ZA WER
driver, otherwise an uncomfortable shock
may be experienced,
and it is also emphasised
a wortirEa
that if an electrolytic
FIG. 158.-- A method of testing high-capacity
condensers.
condenser is under test, the test voltage leads
must be connected the right way round otherwise the component
may be damaged. It is also possible to test condensers by means
of a neon lamp and a source of voltage, the condenser being con-
nected in series with the neon lamp and the D.C. supply leads. If the
neon glows continuously it will indicate that the condenser is useless,
but if the discharge flashes occur at intervals of one minute or more the
component may be considered satisfactory.
To test low-capacity condensers of -001 mfd. or less it is advisable
to use a pair of 'phones and a G.B. battery. The 'phones should be connected in series with the battery and the condenser to be tested.
If aslight click is heard in the 'phones when the battery plug is inserted
it will indicate that there is no break inside the condenser, but a very
loud click will indicate a short circuit. If a slight click is also heard in
the 'phones when the 'phone leads are connected across the condenser
after the latter has been disconnected from the battery for two or three
minutes, it will indicate that the condenser is capable of holding its
charge. The simple tests outlined above will not enable the service-man
to decide the exact capacity of a condenser, of course. A capacity
bridge is necessary for this purpose, but as the instrument is rather ex-
pensive it is not always considered worth while for the average service-
man. We would insist, however, that such an instru ment be purchased.
194
SIMPLE TESTS FOR COMPONENTS
Testing Resistances. --It is not often that afaulty resistance is found in akit of parts, but it is as wel to give these components atest before commencing constructional work. A miliammeter should be used for this purpose in conjunction with a battery, as shown in Fig. 159. Assuming that the value of the resistance is not even roughly known, alow voltage should be used for the preliminary test. By applying Ohm's Law the exact value of the component to be tested may be decided. For example, if a voltage of If volts is used and a reading of 10 mA. is obtained it will indicate that the resistance has a value of 150 oh ms. Similarly, if a6mA. reading is obtained with a60-volt battery in use, the resistance value will be 10,000 ohms.
4AL/.444METER
159. --Checking the value of afixed resistance.
Valve Tests. --It is very seldom that anew valve becomes defective before reaching the constructor, but as it is very important that valves be in perfect order, ameans of testing them is very desirable. The simple method of checking filament continuity by means of aflashlamp bulb and battery, as often used by radio dealers, is quite inadequate, as avalve may register filament continuity and yet be useless.
The valve depends for its action on the fact that aheated wire shoots off electrons into space. In the case of the battery valve this wire constitutes the filament (the cathode in the case of the indirectly-heated valve).
This filament is heated by means of the L.T. accu mulator, and free electrons are shot of into the valve vacuum. The filament issurrounded by a metal plate, known as the anode, to which a high positive voltage is applied. This plate attracts the electrons from the filament, thereby completing the plate-filament circuit, and allowing current to flow through the valve from H.T. + to H.T. -- . It is apparent, therefore, that the valve can only operate when the filament is capable of emitting electrons. After avalve has been in use alengthy period the number of electrons shot of from the filament gradualy falls, and then the valve is said to be losing its emission. In order to test the condition of a valve it is therefore necessary to measure the current passing through the
195
PRACTICAL WIRELESS SERVICE MANUAL plate-filament circuit when a certain voltage is being applied to the plate and filament. This may be effected by means of arailiammeter, and service-men are advised to build the cheap tester shown in Fig. 160. The valve to be tested should be inserted in the holder, and the requisite L.T., G.B., and H.T. voltages connected across the respective terminals on the tester. The reading on the milliammeter should then be co mpared with the figures given on the valve manufacturer's leaflet.
Fla. 180.--Wiring diagram of asimple valve tester When testing triodes, terminal H.T.1 should be left disconnected, this being only brought into use when testing S.G. or H.F. pentode valves. Lead marked No. 1should then be transferred to the cap terminal, and H.T.1 terminal connected to the 60 or 80-volt socket of the H.T. battery. When testing five-pin directly-heated pentodes, the centre socket of the holder (marked C) should be connected to H.T.2 terminal. If an indirectly-heated valve is to be tested, the lead shown connected to H.T. --, G.B. +, and L.T. -- should be transferred to the unused heater terminal, and alead should be connected between the centre pin of the valve-holder and the H.T. -- terminal. A large 4-volt accumulator (or the 4volt 1amp. winding of amains transformer), connected across the heater terminals may then be used for L.T. supply.
196
SIMPLE TESTS FOR COMPONENTS
After the components have been tested in accordance with the instructions given the assembly work may be completed and the receiver prepared for its preliminary test. In the first place, the H.T., L.T., and G.B. batteries should be given arough voltage test, but it is emphasised that the actual voltage under working conditions can only be ascertained whilst the batteries are supplying the normal current require-
ments of the set. Nevertheless, it is advisable to give the batteries the suggested preliminary check as this provides arough indication of the condition of the cells. If the H.T. battery registers less than 75 per cent of its rated maximum voltage, it should not be used, as apart from the fact that it will be ineficient as avoltage supplier, its internal resistance may be so high that it will cause instability in the receiver. This type of instability generally takes the form of a motor-boating noise. The L.T. cel should register two volts, and if the voltage is lower than this when the cell is not supplying current the necessity for a re-charge is indicated. It is emphasised, however, that the condition of a cell is better ascertained by means of ahydrometer than by avoltmeter. When the accumulator is fuly charged the specific gravity as registered by the hydrometer should be approximately 1,210, and should not be allowed to drop below 1,170. The voltage should remain practically steady at 2 volts between these gravity limits.
Voltage Tests. --If the batteries are found to be in order, the battery
leads from the receiver may be connected up and the voltage at the valve holder pins checked before the valves have been inserted. This will give the true voltage at these points unless there is a high resistance in the
circuit between the valve-holder pin and the battery. In simple receivers where no decoupling resistances are used, a true voltage indication will therefore be obtained by this method, but in modern multivalve sets decoupling resistances are generally connected in the anode circuits and therefore the readings obtained will be slightly erroneous,
even if areliable meter is used. This is an opportune moment to point out that it is very desirable to use a meter having a high "ohms per
volt" constant --preferably 500 ohms per volt, or higher --as the error occurring when testing voltages in circuits possessing a high resistance is much lower when the resistance of the meter is high.
If areading is not obtained when the voltmeter leads are prodded across the valve-holder filament pins, the receiver filament wires, the on-of switch, and the L.T. leads to the accumulator should be suspected. It is pointed out that lack of voltage may be due to a short circuit as wel as to abreak in the leads or at the switch. If the leads are in order acheck for ashort-circuit should therefore be made. This is often due to the L.T.-F terminal of the on-of switch making contact with ametal or metallised chassis, or to twisted L.T. leads being frayed. The H.T. voltage should be measured by prodding the + and -- leads of the meter across the anode pin of the valve-holder and H.T. -- respectively; it
197
PRACTICAL WIRELESS SERVICE MANUAL
should be noted that the anode of the British type S.G. or H.F. pentode
valve is connected to the top cap terminal, the normal anode pin of the
valve being joined to the screening grid. If a reading is not obtained
at the anode pin, the circuit between the pin and the
lead should
be checked. If the wiring between these two points is in order, the
components (H.F. choke, H.F. coil primary winding, L.F. transformer
primary, or anode resistance) should be tested for a break; it is also
pointed out that as in the case of the L.T. circuit, lack of voltage may
be due to ashort across the circuit. This generally occurs in acondenser,
and therefore if a break cannot be located, the decoupling and by-pass
condensers connected across the circuit should be tested.
r-
LEAD 'Teem .fer
111:49Arrzof Flo. 161.--Checking total current consumption.
Current Tests. --After approximately correct voltage readings have been obtained at the valve-holder anode and screen pins, the valves may be inserted and the current consumption checked. The total consumption may be measured by connecting the milliammeter between the H.T. -- socket of the battery and the H.T. -- lead from the receiver, as shown in Fig. 181. When connected in this position the meter will register the total valve consumption plus the current taken by screen potentiometers connected between H.T.1- and H.T. --, and the current lost due to leakage across these two points. The individual valve consumption may roughly be checked by removing each valve in turn, commencing with the output valve; it should be noted that this test will only prove accurate if the batteries are in good condition. The current drop occurring as each valve is removed in turn will indicate the current consumption of the removed valve, of course. The reading registered after all the valves have been removed will indicate the current taken by the potentiometers, plus leakage. The actual current that should be taken by these potentiometers may be calculated by applying Ohm's Law --current equals voltage divided by resistance. If
198
SIMPLE TESTS FOR COMPONENTS
the current registered by the
meter exceeds that which should
be taken by the potentiometers,
a leakage across the H.T.
battery wil be indicated and
therefore the aforementioned
VALVE
condenser tests must be conducted and the wiring also care-
/ifs/00E
fully re-checked. It is also
possible to test the current
consumption of the valves by
con -meting a miliammeter be-
tween the valve 'anode and the
anode lead, as shown in Figs.
162 and 168. This, by the way,
To
is the more accurate method of
the two, but is the more dificult
Fm. 162. --Shoawniondge avolmteatgheo.d of checking
to conduct. Clan B and Q.P.P. Stages. --
When a pentode or a triode
valve is used in the output stage, the current consumption should
remain constant, irrespective of the volume of the received station.
When aQ.P.P. or aClass B output stage is used, however, the current
consu mption varies with the volu me, and therefore the
M/LLIAMMETER
quiescent consumption of the
valves should be measured
when no signals are being
received. The consumption
is of course much higher
when a strong signal is being
received and, therefore, the
actual consumption may be
decided approximately by taking the mean of the
maximum peak current and
EX/STING VALVE CAP LEAD
the quiewent current. Bum mary. --Low voltage
reading ;may be due to run-
S .OR liF Pen/ TODE VALVE
down II.T. battery or to a
high res stance in the anode
or scrt :n circuits. High
voltage readings, on the
other ha lid, will be due to the anode nsistances having a
Fin. 163. --Checking the current consumption of an S.G. valve.
199
PRACTICAL WIRELESS SERVICE MANUAL
value below that specified. High current consumption will indicate
that the bias voltage is too low, that there is a leakage across the H.T. circuit, or that excessive voltage is being applied to the valve anodes or screens. Low current consumption indicates the use of a run-down H.T. battery or to the application of excessive G.B. voltage.
It is emphasised that the foregoing instructions apply only to sets supplied from H.T. batteries. If an eliminator is used the tests which will be suggested for mains receivers in the next paragraph should be made.
Mains Sets. --In preceding pages it was pointed out that the tests described were not, in all cases, applicable to mains-operated receivers. It was suggested that voltage tests could be made before inserting the valves in their respective sockets, and that the anode current passed could be measured by removing each valve in turn. These tests must not be applied to mains sets, as removal of one or more of the valves while the mains unit is on may cause damage to condensers connected
across the output circuit of the rectifier, or to the rectifier itself. This rule also applies to receivers employing battery type valves supplied from an eliminator. In receivers of this type it is therefore advisable to fit afuse in the input circuit so that components will be safeguarded if there happens to be a short circuit across the receiver wiring. This fuse should have a current rating slightly higher than the normal current consumption of the set, so that the current surge which occurs when switching on can safely be handled.
Voltage Tests on A.C. Sets. --As in the case of the battery type of set voltage tests should be conducted first. If the receiver is supplied from an A.C. source of supply an A.C. and aD.C. meter should be used in order to make athorough check of the voltages. Most modern valves of the A.C. type require a heater voltage of 4 volts; this may be measured by prodding an A.C. voltmeter across the heater terminals of the valve-holders. It is very desirable to keep the heater voltage within 5per cent of the specified value if good results are to be obtained. Should the voltage not fall within the 4-volt limit the mains transformer should be inspected. It will have been noticed that transformers have
voltage and current ratings marked on their terminal strips --for example, 4 volts at 5 amps. It is necessary to connect the correct load across the heater winding in order to obtain the correct output voltage. In the case quoted above, five 4-volt 1amp. valves may be fed from the 5-amp. winding. If only one or two valves were used, the load would be
too low and, in most cases, the voltage applied to the heater circuit would be excessive. In such cases a dissipating resistance should be connected across the winding (calculated by applying Oh m's Law) in
order fuly to load the winding. Another cause of incorrect heater voltage is the use of the wrong input socket on the mains transformer. If the voltage of the mains does not exactly coincide with any of the
200
SIMPLE TESTS FOR COMPONENTS markings on the transformer, a socket having the next higher voltage marking should be used: for example, if the supply is 235 volts and the transformer is marked 200, 220, and 240, the 240-volt socket should be used. The use of the 220 socket in this case would cause excessive voltage to be applied to the heater circuit. Lack of voltage across the valve heater pins may be due to a break in the leads between the heater winding of the transformer and the valve pins, a break in the heater winding, or a break in the primary winding of the transformer it is assu med, of course, that the input fuse is in order.
76 Ori-ie,e 17:4` yes
D eoPP/iva
REJ7JTANCZ
.4,161f Erie
184. --Measuring the heater current in A.C./D.C. receiver.
The A.C. meter may then be used to measure the voltage across the H.T. secondary winding of the transformer, in order to ascertain whether the correct voltage is being applied to the rectifier. Lack of voltage at this point may be due to a break in the leads from the transformer, or to abreak in the pri mary or the secondary winding of the transformer. A dead short circuit across the winding would also cause lack of voltage, but this fault does not often occur. Low voltage at this point may be due to shorted turns in the transformer secondary, and high voltage may be due to shorted turns in the primary winding. After ascertaining that the A.C. voltages are in order, the D.C. meter may be used to test the voltages on the anodes and the screens of the valves. It is essential for the meter to have ahigh internal resistance if accurate readings are to be obtained, especially in the case of valve screens. If the receiver chassis cannot conveniently be re moved, the voltages may be measured from the top of the chassis by slightly lifting the valves so that the meter prod may be touched on the requisite pin. When this is done care must be taken not to lift the valve too far so as to break contact in the
201
PRACTICAL WIRELESS SERVICE MANUAL
mr-/- To
Xlziee.4ivoziEs.
165. --Measuring the total H.T. current consumption.
be due to low anode or screen resistances, excessive smoothed voltage, or high bias resistance. Lack of voltage will indicate a break in the anode or screen circuit due to a defective component (e.g., transformer primary, resistance, or choke) or wire.
Current Tests. --If the voltages are in order, it is generally found that the current consumed by the valves is also of the correct value, as voltage and current is to a great extent interdependent in mains receivers. The total current consumption may best be measured by connecting a milliammeter in the output circuit of the rectifier, and the
202
holder sockets, and it must also carefully be ascertained that the meter positive prod does not touch the chassis. After the positive prod has been placed in contact with the point at which avoltage measurement is required, the negative prod should be connected to the metal chassis. This wil give the anode voltage plus the bias voltage if contact is being made to the anode pin of the valve, or the screen voltage if the positive prod of the meter is in contact with the valve screen pin. Low voltage at these points may be due to high anode or screen circuit resistances, to low smoothed voltage being supplied from the rectifier, or to the use of a low value bias resistance. High voltage, on the other hand, may
166.--Showing method of testing voltage of ELF. pentode.
SIMPLE TESTS FOR COMPONENTS
individual consu mption of the valves may be checked by breaking the anode lead and connecting amilliammeter to the free ends; the mains supply must, of course, be switched off before the break is made. High current consumption wil indicate that the applied voltage is excessive; that the bias resistances have a low value; or to a short or partial short across the circuit; partial shorts are generally due to leakage in condensers. Low consu mption may be due to the use of high
2MFo 71-srave avuotwsze
1/AL vENo wela
A w a,c0RAIER:
167. --Localising hum by grid short-circuit method.
bias resistances, or low input voltage. In receivers of the A.C. /D.C.
type the heater current should also be measured. This may be done in the manner shown in Fig. 164, where an ammeter isshown connected
between the negative end of the main dropping resistance and the heater pin of the first valve in the series sequence. It is emphasised at this point that valve heaters are connected in series in A.C. /D.C.
and D.C. receivers, and therefore the removal of avalve or the use of a valve with a broken heater circuit will produce a break in the whole heater circuit. (See also Figs. 165 and 166).
203
CHAPTER XXIX
ADJUSTING AND TESTING COILS
THIS information is taken from our companion Wireless Coils, Chokes, and Transformers.
handbook,
Mode m coils are so eficient and generally free from faults
that a chapter under the above heading might scarcely seem to be
justified. But the queries received by me from alarge nu mber of readers
·indicate that, as arule, coils are by no means well understood, and that
the average novice in home construction is at a loss to know how to
carry out proper tests when the coils come under suspicion. Let it be pointed out right away that a large number of the minor dificulties which do present themselves in connection with tuners and coil assemblies are due to unsympathetic treatment of them. As an example of
this it should be explained that, of a large number of readers' coils which have been examined in my laboratories at various times, most of these have been damaged by attempting to over-tighten the terminals,
with a result that these have been turned round bodily and the thin
leads to them from the windings have thereby been broken.
Do not Over-tighten Terminals. --It is unnecessary to make the terminals more than finger-tight, but this warning has not always been observed. I am strongly in favour of soldering connections wherever
possible, and if coil connections (and those to most other components
for that matter), were made by soldering the leads to small soldering
tags fitted under the terminal nuts many dificulties would at once vanish. Perhaps makers are in some measure responsible, due to the fact that many of them do not supply the necessary tags with their components, but it need hardly be mentioned that neat and useful
soldering tags can be bought for a few coppers per hundred. A very common reason for coil terminals being loosened is that thin
connecting wire, or even stranded flex, is used for making connections; this is very liable to lock the nut on the thread of the terminal shank, so that when an attempt is made to screw the nut right down the whole
terminal must work loose.
Short-circuited Terminals. --Even ifthe lead from the winding to the terminal does not break, there is another dificulty that might present
itself in the case of coils the terminals of which are mounted on apressed
metal baseplate fitted with insulating washers or bushes. When the terminal is loosened washers come apart and allow the terminal to slip
out of position, and thus make contact with the metal base. As the
earth terminal is generally connected to the base, one of the windings
204
ADJUSTING AND TESTING COILS
will be short-circuited. If it is the tuning winding that is afected, there
will be an absence of signals on either one or both wavebands, but if the
reaction coil is shorted it is possible that the H.T. supply might be short-
circuited, with serious results. A few instances have been noted where
the valve-filaments have been burnt out due to this. The reason is
ilustrated in Fig. 108, where it will be seen that the on-of switch is in the
combined H.T. -- L.T. -- lead, this lead being joined to the earth
terminal When H.T.-1- is inadvertently connected to earth due
to the coil short the voltage
"v4vAtv C7,A bAUeMSarPE7lS7,icAA/91Ce-w4AHIrzEOeNuoT.rT
Arr., ±
applied to the filaments is equal to that of the H.T.
tapping used to feed the
Stweent
detector and of the accu mulator in series.
It will be clear from
what has been said that,
if a coil terminal is loose,
it is very important that a
careful examination should
be made and the looseness
corrected. If the lead to the
terminal from the winding
appears to be badly twisted
the wire should be scraped
clean and re-soldered.
Measuring the Resist-
LT-
&nee of the Windings. --
hfr-
Fm. 188. --This circuit shows how the valve filaments might be burnt out due to one terminal of the reaction winding shorting to the coil screen
or baseplates.
Really accurate and scientific tests of tuning coils cannot be made by using the limited amount of ap-
paratus usually possessed
by the average service-man,
but it is generally suficient to take readings of the D.C. resistance of
the various windings, when any break, faulty operation of the wave-
change switch, or bad terminal connection can be traced. An ohm-
meter is required for taking these measurements, and an instru ment
such as the "Pifco " Rotameter, "Avo-Minor" or "Avo meter" is
useful. If the terminals of the meter are connected between the grid and
earth terminals of the coil, readings of approximately 2 ohms and
10 ohms should be obtained when the wave-change switch is in the
mediu m and long-wave positions respectively. These readings will not
necessarily be obtained, and the figures will vary to a certain extent, according to the particular make of coil being tested, but the ratio between the two readings should be about 5: 1. So long as one reading
205
PRACTICAL WIRELESS SERVICE MANUAL
is appreciably greater than the others, however, the exact figures may be ignored. When a pair of similar coils are employed in the same receiver the D.C. resistance of the windings of both should be the same; if not, it will be a fair indication that the coils are not matched, or that one of the m is faulty.
In addition to testing the tuned windings of the coil, the reaction winding should also be tested for continuity by placing the ohm-meter across its terminals. The resistance of this winding will probably be found to be equal to about half of the long-wave tuned winding, but
.4-Rha
Afroamf 14vs
tiaviv.r
d'amur
189. --A combined theoretical and practical diagram showing the points between which a dual-range tuner with loose-coupled aerial winding should be tested with an ohm-meter, and the (very
approximate) reading to be expected.
provided that there is continuity the resistance value need not generally be considered. Fig. 169 shows the various points, to which the meter should be connected, and gives arough indication of the readings which one might expect to obtain.
A Simpler Test. -- When an ohm- meter is not available it is possible to make asuitable instru ment by means of amiliammeter and various shunt and series resistances, as described earlier in this book.
Quite a simple and fairly reliable test can, however, be made simply by using a 100- mA, fuse bulb and a grid-bias battery. First of all, the positive and Ili-volt negative battery tappings should be used, and the two "test" leads connected to the two ends of the coil, as explained before. When the switch is in the medium-wave position it
206
ADJUSTING AND TESTING COILS
should be found that the bulb glows, but the light should not be so bright as when the two " test " leads are joined together, if it is, the windings, or connections from them, must be short-circuited. If the wavechange switch is turned to the long-wave position the light should either go out completely or become dim. If the light does go right out, increase the voltage of the G.B. battery in 11-volt steps until the light returns. If the bulb glows when the switch is in the mediu m-wave
-=*
170. --The diagram ilustrates the method described in the text for tracing the connections of a
typical coil.
Flu. 171. --Two common types of aerial coil.
position, and cannot be made to do so on long-waves, it is aclear indi-
cation that the long-wave winding is either disconnected from the terminal or broken at some point. The leads to the terminals can be traced, and if these are in order, examination should be made at those parts of the winding where the wire passes from one slot to the next. If the wire has been broken or fractured at one of these places it will probably be necessary to remove aportion of the winding until the ends of the break can be scraped clean and soldered together again.
Should it be found that the resistance is just the same regardless of the position of the wave-change switch, it will generally be found that the switch itself is at fault. Should the resistance remain low (equal to that of the medium-wave section only), the switch contacts are probably failing to "open." On the other hand, if the resistance remains high, the contacts are not "closing " properly, perhaps due to one of the contacts being strained or because it is dirty. The method of efecting a remedy naturally depends upon the type of switch employed, but
once the fault has been traced, the cure will generally be fairly obvious. Tracing Coil Connections. --It sometimes happens that the tester
has a coil for which the connections are not known. When this is the
207
PRACTICAL WIRELESS SERVICE MANUAL
case it is generally afairly simple matter to trace out the ter minals by applying the tests described above in the "reverse direction," as it were, that is, measurements must be taken between various terminals. For example, if it were found that aresistance of, say, 8ohms occurred between two particular terminals, whilst the resistance between the other pair being 1 ohms, it would be evident that the first two terminals were those joined to the ends of the medium-wave winding, and that the second two terminals tested were joined to the ends of the long-wave winding. In other words, the tuning condenser would have to be
.
FIG. 172. --The simplest modification of the coils
shown in Fig. 171.
T Fla. 178. --The usual coupling for an H.F. stage --the H.F. transformer it will be seen is almost iden-
tical with the aerial coil.
connected between the terminals from which the 1-ohm reading was obtained, the wavechange switch being wired between those terminals between which aresistance of 8ohms was measured.
This will perhaps be understood more clearly by making reference to Fig. 170, which shows the circuit of atypical coil with the terminals lettered from A to F, and with the resistance readings just mentioned indicated. After locating the tuned windings, the next step would be to find the tapping marked B; this could be done by connecting one side of the meter to terminal A, and moving the other lead about until a
resistance of about 1 ohm was obtained. The reaction winding could be located by finding --by means of the meter --two terminals which were connected together, but which were not in connection with the
tuned winding. In some cases, however, the lower end of the reaction coil (Terminal F), would be joined to terminal C, when areading would be obtained between terminal E and every other terminal on the coil.
208
ADJUSTING AND TESTING COILS
The reaction terminals would easily be located, however, due to the fact that the terminal showing the least resistance from terminal E would be
joined to the other end of the reaction winding. The tests just described could be carried out fairly satisfactorily
by means of afuse bulb and battery, as explained before, but the process
would be somewhat more laborious and slow. It is assumed when the service-man purchases a coil of reputable
make that there will be no dificulties in the operation of the receiver which will be directly attributable to the coil itself. Unfortunately, this is not true, and even the best coil which is made might give rise to troubles, but they are not due to faults in the coil per se, but are linked up with the method of using the coils. It is as wel, therefore, to con-
sider just what faults in a receiver can be traced to the coils, and thus to eli minate some of the dificulties which might be encountered by the set constructor. Firstly is the question of selectivity. The modern
coil is designed to ofer a certain degree of selectivity, and this may be obtained by including a primary winding, or by tapping down on the coil and thus remove the heady damping efect. Many service- men
might purchase such acoil, and thus using it in asimple detector circuit with avery large aerial, might find that the selectivity is not suficiently high for these particular set of conditions, and thus would blame the coil. Obviously, in such acase it will be found that the shortening of the aerial
will not weaken signals in any respect, but that as the length is reduced, so the selectivity increases, until a point is reached where any further
reduction in length is generally round about 40 ft. to 00 ft. and a ·modern coil wil be found to ofer adequate selectivity when used in
these conditions. Modifying a Coil. -- Where the coil is of the transformer or auto-
transformer type, that is, as shown in Fig. 171, it might be found that, owing to the fact that the local conditions are bad, or that the aerial is unfortunately rather short, signal strength is poor, and many distant stations can only just be heard. In such cases, an improvement can be efected without tampering with the actual coil, by joining the aerial to the " top " or high-potential end of the secondary winding. If the aerial is joined direct to this joint, the waveband covered by the coil
will be slightly higher than with the primary in use, and thus low tuning stations such as Fecamp, may not be received. If, therefore, it is found desirable to use this method of connection, and the results are satis-
factory except so far as the reception of the lower stations is concerned, it might be worth while to remove afew turns from that end of the coil. In the majority of air-core coils, this part of the winding is generally
wound in ordinary solenoid fashion, whilst the long-wave section (which is joined to earth, is pile wound, either as a duo-lateral winding, or in slots on the former. On iron-core coils the high-potential end of the winding is generaly of Litz wire, whilst the long-wave section is of
14
209
PRACTICAL WIRELESS SERVICE MANUAL finer wire. There should thus be no dificulty in finding the correct end, although Fig. 170 should remove any dificulties.
The H.F. Coil. --The coils used for coupling the H.F. to the detector valve (or for coupling together two H.F. stages), are very often similar to the aerial coil, and where the transformer type of winding is employed it may again be found that the selectivity is so high that serious loss of signal strength results. A simple modification in such cases is to cut out the transformer efect, and to employ a parallel-fed grid coil, which is made clear in Figs. 178 and 174. In Fig. 178 is shown the principal associated components, and in Fig. 174 the modifications have been made,
FIG. I74. --A modification of Fig. 64. It may be advisable to retain the resistance R1, and the condenser Cl to act as decoupling
components. and the old part of the circuit is shown in broken lines. Both the modification mentioned in the aerial circuit, and the H.F. transformer modification will give greatly increased signal strength, but naturally the selectivity will be very much poorer. It is therefore necessary for the constructor to decide which arrangement is most suitable for his needs, and in this respect a few nights' tests will soon enable him to decide.
Switching Defects. --A good commercial coil should not give trouble arising out ot the switching, but in view of the fact that certain materials do corrode, and that many coils are included in receivers which contain an accu mulator, there is the possibility of acid corrosion as wel as the normal oxidation. A good switch mechanism is self-cleaning, that is to
210
ADJUSTING AND TESTING COILS say, the contacts rub against each other as they come into contact, and this tends to wipe of any oxide and give a clean contact. Where it is found that long-wave reception is ideal, but that on switching to the medium waves results are poor, perhaps accompanied by scratching noises, or intermittent signals, suspect the switch mechanism. Carefulk examine the contacts, and make sure they are clean. If an accu mulator is kept near the coils, you may find agreen deposit on the metal, and this should be wiped of with arag dipped in strong soda water and the metal thoroughly cleaned. Operate the switch and watch the movement of the contacts. If it is found that there is a very minute wiping action, it may be possible to increase the efect by slightly bending the longer part of the switch arm. The actual method of attacking this will depend upon the switch, and care should be taken not to damage the actual contacts (which in some cases are silver), nor to break of the thin wires which may be soldered to the contacts.
211
CHAPTER XXX
SERVICING COMMERCIAL RECEIVERS
THE types of defect that can cause trouble in radio receivers are quite small in number compared to the number of possible faults. If the word "fault" is used in the sense as normally understood by technical people this statement is not as paradoxical as it may seem at first sight. A high-resistance soldered joint (dry joint) is awel-known defect but a dry joint at one end of a bias resistance must be regarded as avery diferent fault to adry joint at one side of an I.F. trimming condenser. So very great is the number of fault possibilities that it is auseless proposition to attempt to draw up acomprehensive list of faults with their symptoms, quite apart from the fact that combinations of faults can occur simultaneously, with correspondingly complex results.
A defect which, in itself, is simple enough may not always be very easy to track down, particularly if the fault symptoms do not give definite indication as to the type of defect, and its probable position. For any measure of success in fault tracing, it is absolutely necessary to work systematicaly, and one of the secrets of rapid fault tracing is that of dividing the work into distinct sections, each with a definite purpose. An example of atypical plan of campaign is :-
1. Ascertain the general fault symptoms and decide, as far as possible, what sections of the receiver are faulty.
2. If the fault symptoms are of multiple character, pick on one particular symptom feature as the first item requiring clearance.
8. Localise the particular circuit containing the fault concerned. 4. Trace the fault itself. 5. Clear the fault. If there are still other faults remaining, start over again at 1, and so on. There are exceptions to every rule, and it is not always possible to adopt the actual plan outlined above, but every efort should be made to get on to some such scheme whenever circumstances permit. Tackling the Dead Receiver. -- We assume the case of the receiver that gives no radio reception on any waveband. Adopting a very broad classification of the fault possibilities these can be regarded as being
in three groups :1. Battery supply faults, or mains supply faults; 2. Valve faults;
8. Faults in other components, or wiring. The first move should always be to check up on possibility No. 1.
Meter tests are not always necessary at this stage for it may be obvious
212
SERVICING COMMERCIAL RECEIVERS
immediately upon switching on the receiver that voltage supplies are not . lacking. No doubt should be permitted upon the matter, however.
The next move should be to check the output stage, including the
speaker, for "life," and attention should not be withdrawn fro m the
output stage until it is known that it is capable of responding to an L.F. input. The trouble with the receiver may prove to be in the output stage, or may be detected while the output stage is being tested, but if such is not the case the next job is that of locating the faulty stage.
There are various ways in which this can be done, the best method being determined by circumstances. Sometimes whole sections of the receiver can be checked by quite simple tests involving no testing gear. For example, if a straight receiver is provided with reaction in the detector circuit, and it is found that operating the reaction control produces the familiar sounds in the speaker caused by the receiver jumping into oscilation, then it is fairly reasonable to assu me that the detector valve itself, and all sections of the receiver following the detector, are in order. Again, bringing the aerial lead forward, circuit by circuit, towards the grid circuit of the detector as is explained in another part of this book, may enable trouble on the H.F. side of the receiver to be located. By applying modulated oscillations, at I.F. frequency, direct to the I.F. stages the latter can be checked over for response without any dependence having to be placed on the actions of the receiver oscillator and signal frequency stages. If the I.F. and
receiver oscillator stages are known to be in order, the signal frequency stages can be checked by employing the output of the testing oscillator, at signal frequencies.
The possibility of the receiver oscillator being out of action must never be overlooked where the case of a dead superhet is concerned, but it is advisable to make quite certain of the I.F. stages before going to any trouble in checking up on the receiver oscillator section. If it appears probable that the receiver oscillator is out of action a valve test should be made, or a substitution valve tried, and if there is no valve fault a
test should be made for H.T. on the oscillator anode. If this proves to be present there are now the alternatives either of making a close
search for fault in the oscilator circuit, or of making absolutely certain first that the oscillator really is out of action. If the exact arrange ments of the oscillator stage are familiar and coil ends, etc., are readily accessible it may pay to test round for afault immediately the suspicion arises that the oscillator is out of action. If, however, there is any uncertainty as to the oscillator arrangements, or ifthe oscillator coils and condensers, etc., are very inaccessible, then it may be advisable to prove definitely that the oscilator is not generating before exploring the " works."
Anode Current Value. --The fact that the mean oscillator anode current when the valve is oscillating difers from the current value when the valve is not oscillating indicates one possibility of testing for the
213
PRACTICAL WIRELESS SERVICE MANUAL
existence or otherwise of the local oscillations. A comparison should be made between the anode current value with the connections unaltered and the anode current value with the grid coil short-circuited, or with a large capacity condenser connected between oscillator grid and earth. A more indirect, but quite useful way of finding out if the receiver oscillator is out of action is to use the output from an external test oscillator. Unmodulated oscillations should be injected into the signal grid circuit of the frequency changer, or first detector, and the test oscillator and receiver should be tuned for signals as acombined outfit.
If signals can be brought in in this way there will be no doubt that the receiver oscillator has a fault. Incidentally this external oscillator test would disclose the fact if the receiver oscillator happened to be generating but to be hopelessly out as regards frequency ranges. It is very unlikely that the receiver would have been completely "dead " under this condition but, in any event, with the external oscillator in use it would be possible to pick up the beats between the two sets of oscillations if some searching were carried out with the test oscillator.
If the receiver oscilator does prove to be out of action, coils should be tested for continuity and short circuits; the possibility of an opencircuited or short-circuited condenser should also be allowed for. This does not cover all possibilities, but faults of more tricky character, will be considered at a later stage.
After this digression in connection with the oscilator of a superhet we will return to the general case under consideration and assu me that the position has been reached that a particular stage of the receiver has been located as faulty, but that the fault itself has not yet been tracked down. The valve itself must come under suspicion first and no further move made until it is definite that there is no valve trouble.
Testing the Valves. --Parts connected in any way to the positive H.T. lines of the receiver are more likely to develop defects than those in grid circuits, or low potential tuning circuits, for the reason that the former parts are subjected to greater stresses. A short-circuit across H.T. can be ruled out now because a fault of this nature would necessarily have forced attention upon itself at the commencement of the proceedings. It must be remembered that we have made the rule to get "life" in the output stage as one of the first moves, and any fault leading to absence of H.T. (or L.T. for that matter) would come up for investigation at once. It is right time now to make a voltage test between anode and chassis as an open circuit in any transformer winding, choke winding, or resistance connected in series with the anode circuit will then be shown up. If the valve is of the indirectly heated type a voltage test should also be made between anode and cathode to check up on the possibility of a break between cathode and chassis. If the valve is of the screen-grid or pentode type, the screen to chassis voltage should be checked as wel.
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SERVICING CO MMERCIAL RECEIVERS
If the fault still does not show up, a grid circuit break should be considered as a possibility, and tested for.
If the fault is stil elusive the most likely possibilities are: a shortcircuit across the anode load, a short-circuit across the grid circuit, or, if one is in use, an open-circuited grid condenser. As regards the anode load matter, think of the case of the primary circuit of an I.F. trans-
former with a trimmer condenser across the winding. If there happened to be a short-circuit through the latter it would have the efect of preventing any amplified I.F. oscillations being developed in the anode circuit and there would be no input available for the next valve. When the anode load is of high resistance type, as with resistance-capacity coupling, a short-circuit across the load should have been noticed with the anode voltage test.
When a short-circuit across a winding of comparatively low D.C. resistance is the fault, the occasion cals for a test with an ohmmeter that will give low resistance indications. If the ohmmeter available is of the type that does not give readable indications for low resistances the position may be alittle dificult.
An open-circuited grid condenser can easily be checked up by using another condenser on ashunt test.
It is to be hoped that by now the fault will have been found, but should the case still be obstinate it may pay to consider the possibility of an open circuit, or very high resistance contact, between a valveholder terminal, or socket, and the valve pin. A miliammeter test in the anode circuit should prove useful, as a fault of the above type will generally show up ifthe miliamps are measured. It is to be pointed out, however, that any fault afecting anode current is a fault that must incidentally afect the actual anode voltage. As to whether this would be noticed at the time an anode voltage test is being made depends not only upon the extent of the voltage discrepancy (which may in some cases only be very slight) but also upon the alertness of the tester.
" Diary " of a No Signal Case. --Receiver: A mains superhet of typical modern design. Report: No signals.
Receiver switched on and dial lights up. No signals obtainable on either of the two wavebands provided. Sound of mush heard in speaker. Output stage is alive, then, and probably I.F. stages working.
Jump aerial lead forward to signal grid of frequency changer. No result.
Decide to check I.F. stages. Apply modulated test oscillations at I.F. frequency to signal grid circuit. These signals come through well. Case beginning to look like a dead oscillator one. Switch out modulation of testing oscillator and adjust for signal frequencies. Connect aerial to normal receiver terminal, and search for signals, using tuning controls of both testing oscillator and receiver. Signals come rolling in. Sure that receiver oscillator is dead now.
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PRACTICAL WIRELESS SERVICE MANUAL
Try new frequency changer valve. Nothing doing. Wonder if there is any H.T. on oscillator anode ? Try with voltmeter. There isn't. On the track now.
Take chassis out of cabinet. Notice that directly connected to oscillator anode socket is a50,000 ohm resistance and a .002 mfd. condenser. Resistance obviously in H.T. feed line, so switch on receiver and test for volts at each end of resistance. Get voltage reading at end of resistance remote from valveholder. Surely fault is an open resis-
tance. Switch of receiver and make ohmmeter test of resistance. Get ajolt, for resistance is in order. (Ought to have taken closer notice of the voltage reading just now.) Try ohmmeter test from oscillator anode to chassis. Low reading obtained. Notice that condenser connects from oscillator anode to the oscillator coil assembly. Evidently it is a reaction feed condenser. Think it must be shorting through. Disconnect one end of condenser and test. Got the trouble now -- condenser is shorting through.
Replace condenser with similar type. Try receiver. Everything seems O. K., but don't like making any changes in an oscilator circuit without checking the trimming.
Hardly seems necessary here but decide to make sure. Connect up ganging gear and find that the oscillator trimming condenser can do with aslight alteration. Make the necessary adjustment.
Weak Reception. -- When the trouble is that the receiver as awhole is below its normal standard of sensitivity there are two checks which should be made at the outset: (1) on the valves, (2) on the valve operating voltages.
Once the valves have been proved to be satisfactory, and each to be working at its proper voltages, it will be advisable to ascertain
whether there is any lack of amplification on the L.F. side of the receiver, and a test with agramophone pick-up or an L.F. oscillator should
be made. When itisdefinitely known as to whether the trouble is on the L.F. side, or on the H.F. side (or on both), the best scheme is to consider possible types of fault, and to carry out aseries of elimination tests.
Dealing with L.F. trouble first: the speaker should be regarded
with suspicion at the start and atest with asubstitution speaker should be made. If the speaker does prove to be faulty some of the possible faults are:
Shorting speech-coil turns. High resistance in speech-coil circuit. Weak magnet (in permanent magnet type).
With a mains-energised type of speaker there may be shorting turns in the field winding. If the field winding is used for smoothing this fault would, however, probably draw attention to itself at an earlier stage of the proceedings.
With a moving armature type of speaker there may be shorting
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SERVICING COMMERCIAL RECEIVERS
turns or a weak magnet. The output transformer, if one is in use, should be tested for shorting primary or secondary turns, and for high resistance in the secondary. If there is choke-capacity feed to the speaker the choke should be tested for shorting turns, and the condenser for low capacity.
Where field, transformer and choke windings are concerned one should always be on the look-out for shunt condensers which may be shorting through, or have low leakage resistance, and may quite possibly cause suspicion to be wrongly directed to the windings. In the case ilustrated in Fig. 175, for example, ashort-circuit through C will throw the D.C. resistance of R across the primary of the output transformer. If R or C are not disconnected before making an ohmmeter test of the primary resistance the reading obtained will be somewhat lower than the actual primary resistance value.
Having cleared the speaker and any speaker coupling syste m it is advisable to work back, circuit by circuit, towards the detector. It is to be anticipated that the coupling between two L.F. valves or between the detector and an L.F. valve, will be one of the following types: resistance-capacity, choke-capacity, transformer, shunt-fed transformer, or shunt-fed choke, and faults in such systems are not usually dificult to trace. Test for shorting turns, resistances of too low values, shunt
leakage resistances, and also check up on coupling condensers. The H.F. Stage. --The high-frequency side of the receiver is not such
an easy proposition if the fault happens to reside there. Remembering that the valves and their operating voltages have already been covered, it is necessary to view all tuned circuits with suspicion.
The possibility of the group of tuned circuits contained on the H.F. side of the receiver being out of alignment (i.e., faulty ganging) must be considered and the adjustments of the various trimming condensers should be checked for correctness. Incidentally, the results obtained on the tests involved in a ganging check may very possibly reveal the location of the fault even though it may not be a fault of ganging adjustment. If it is found that the adjusting of any tri mming condenser fails to have any efect on the output of the receiver, or if no definite optimum adjustment can be found, then the circuit concerned should receive very thorough examination.
Any H.F. tuned circuit forming part of the amplifying chain in the receiver contributes something to the over-all amplification, provided that the circuit is in the resonant condition. As far as the tuned circuit itself is concerned its quota of amplification arises in the sense that the voltage built up by the oscillations in the circuit exceeds the value of the E. M.F. that is exciting these oscillations. This "circuit amplification "is greater the lower the efective H.F. resistance of the circuit. Thus, if the amplification of any particular circuit is below normal it may either be that the circuit is mistuned or that there is excessive
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PRACTICAL WIRELESS SERVICE MANUAL
H.F. resistance. Al this may seem to be very elementary but the point is that anu mber of fault possibilities become apparent if consideration is given to the above.
Taking the first reason for lack of amplification (circuit mistuned), some of the faults to look out for are:
Disconnected tuning or trim ming condenser. Wrong adjustment of the condenser. Incorrect capacity of any fixed condenser involved. In connection with the latter is it to be noted that although the simplest form of tuned circuit is as shown in Fig. 196, alarge nu mber of
Flo. 175. --A tone control circuit which can create dificulty when trying to locate a fault.
FIGS. 170 and 177. --A normal tuned circuit and a modification in common use which is
referred to in this chapter.
cases will be found in radio receivers where the circuit is of the character
shown in Fig. 177, the circuit containing a fixed condenser in addition to the variable condenser. From the diagram it is easy to appreciate that Cl is part and parcel of the tuned circuit, but anybody not especially alert might be liable temporarily to overlook the fact when exploring round an unfa miliar chassis. Any defect in Cl will naturally
efect the behaviour of the tuned circuit and, should the latter prove to be faulty, Cl should receive as much attention as any other part of the circuit.
H.F. Resistance. --Dealing with the second reason for lack of amplification (excessive H.F. resistance) some of the commonest faults are :
High resistance joint. Shorting coil turns. Leakage resistance through trimming condenser. Shunt leakage resistance of any kind.
There is rather a " snag " where shorting coil turns are concerned. Shorting turns will give a reduced D.C. resistance to a coil and the fault will probably be located by a careful ohmmeter test; but the actual efect of the fault in the radio sense is to increase the efective H.F.
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SERVICING COMMERCIAL RECEIVERS
resistance of the circuit. Of course, another effect of shorting turns is to
give mistuning, but as to whether this aspect will lead to the location of the fault will depend upon the particular testing methods that are
being employed. As usual, any grid coupling condensers must come up for investi-
gation, and also any condenser that forms the coupling unit of a capacity-coupled band-pass system. One item that is rather more important than is so meti mes appreciated is the anode by-pass condenser that is used in many receivers employing a triode or pentode detector. Where diode detector circuits are in use any H.F. by-pass condensers across the diode loads should not be ignored. By-pass condensers joined across bias resistances connected between cathodes and chassis
are not to be exempted from suspicion, until proved to be in order, as an open circuit will lower the amplification of the stage concerned.
This is a case where shunt testing with another condenser may save time.
There are degrees of weak reception, and the cases of receivers that give exceedingly weak reception (say, faint traces of local stations only) are to be regarded as very possibly containing faults of the kind that were dealt with on a previous page dealing with " dead" receivers. Some receivers wil give a trace of signals with faults which in other receivers would lead to complete absence of reception.
Broken Grid Connection. --The following is given as an actual example of a case involving very weak reception. It will serve also to ilustrate a rule of importance, namely, to be on the look out for any special features or peculiarities of the fault sympto ms with the object of taking advantage of any short cuts that may be indicated. The case was that of an all-wave superhet of modern design with which only faint sounds of local broadcasts could be heard. It was observed that the volume control had no efect upon these signals. Fro m the fault-tracing point of view the matter of immediate importance was the failure of the volume control action, and no attempt was made to carry out general faulty stage localising tests. All that mattered at the moment was that there was some fault either in the volu me control or at some point not
far after it. The receiver contained a double diode triode, functioning as second
detector, A.V.C. detector and L.F. amplifier. The element of the volu me control formed part of the signal diode load, and the grid of the valve was fed fro m the volu me control through aresistance and acondenser. An ohmmeter test was made of the volu me control and this was found to be in order. The next test was acontinuity test from the grid to the condenser mentioned above and the fault was at once discovered. The lead from grid to condenser was ascreened lead, and the wire had broken inside the sleeving. The volu me control feature of the symptoms gave the clue, however. It should perhaps be mentioned that the weak trace
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PRACTICAL WIRELESS SERVICE MANUAL
of signals that could be heard while the fault existed were to be heard
by virtue of stray coupling. Instability. --This will manifest itself in the form of whistling, howl-
ing, or motor-boating. In the case of the superhet receiver, however,
there are anumber of whistle possibilities not associated with instability,
but these will be dealt with separately in a later chapter. It is wel to keep very much in mind, when endeavouring to locate
the cause of receiver instability, that there are certain components and certain features of asse mbly design deliberately introduced for the purpose
of preventing instability; consequently
when instability does occur these items
should be gone over very thoroughly.
There are various ways in which instability can occur, but there is one
178. --Two H.F. circuits which may need decoupling.
fact common to all cases, and that is there must be one circuit which is producing an A.C. E. M.F. in another
circuit. Thus, unwanted couplings of some kind or other must be
ferreted out and "killed "if the instability is to be cured. Now, two
circuits will couple together if an alternating magnetic field, or an
alternating electric field, of one links with the other; also if there
is some impedance which is so connected that it is common to the
two circuits.
Deeoupling Methods. --Decoupling resistances and condensers are
provided to cut out the coup-
lings that would be set up
between circuits by the im-
pedance of a common H.T.
supply, etc. Screening cans,
screening partitions, and
screened sleeving are intro-
duced to cut out inductive
CHASSIS
couplings; also certain leads
may be well spaced, and some kept to aminimum of length with a view to reducing un-
Fro. 179. --How the two circuits are connected in the receiver.
wanted couplings to a negligible value. Also, certain by-pass condensers
will play an important part in preventing instability.
An open-circuited decoupling or by-pass condenser is easy to trace,
as atest with another condenser in shunt will remove the effect of the
fault. As a matter of fact, whenever instability is experienced it is a
good plan, as a first move, to work round the chassis, shunt-testing all
" likely " condensers. The contacts between screening cans, screening
partitions, and the chassis should be checked; also the contacts between
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SERVICING COMMERCIAL RECEIVERS
screened sleeving and chassis. Decoupling resistances should not be overlooked, and H.F. filter coils and condensers should be tested.
Elusive Faults. --Should the fault prove to be elusive the following paragraphs may contain the key to the solution:
The fact that two H.F. circuits may be shown in the receiver's theoretical diagram as completely separate does not necessarily mean that in the receiver itself they have no common part. If each of the two circuits has one side earthed it may well be that there is a wire in the chassis assembly which is acommon earthing wire for the two circuits. The designer will have seen to it that, under normal conditions, this
CHASSIS
Frn.-180. --Showing how a common earth can couple the two circuits through the resistance.set.up_in the defective connection.
wire has insuficient resistance to form an appreciable coupling between the two circuits. Suppose, however, that bad contact develops at either end of the wire, then there is quite a possibility that one efect of the resistance introduced will be to couple up the two circuits suficiently to cause trouble. If Fig. 178 represents the two circuits then Fig. 179 may represent the arrangement as it holds in the receiver itself. Should the common "earth " of the two condensers develop an appreciable resistance the arrangement is then as shown in Fig. 180, in which obvious coupling exists between the two circuits. If all" earthing" bonds are regarded as potential danger points, and given close attention, any such fault should not cause any bad delay in fault clearance work.
Any radical displacement of the wiring of H.F. circuits is apt, in some cases, to cause instability. This point, however, will probably need consideration only if there have been any component changes made in the receiver. A broken connection between the coating of a metallised valve and the valve pin is one of those often overlooked faults which will sometimes be responsible for instability.
Testing Fixed Condensers. --A fixed condenser may be faulty in one of the folowing respects :--internal short-circuit, internal open circuit, excessive D.C. leakage (i.e., resistance between terminals "down "), incorrect capacity value, power factor too high.
To take the last item first, the question of testing for power factor 21
PRACTICAL WIRELESS SERVICE MANUAL
does not usually worry the average service engineer who has limited testing gear. He usually relies on substitution checking to help him if he has any suspicions regarding acondenser on this score. An internal short-circuit is easy enough to locate as a simple ohmmeter test will reveal the fault.
Excessive D.C. leakage at low voltages will also show up on an ohmmeter test, but care should be taken that shunt circuits do not give rise to false indications if test is made while the condenser is still in place and connected up in the receiver. There is always the possibility that a condenser which proves to be in order on a low voltage test may develop excessive D.C. leakage at a higher voltage, and a test with a miliammeter connected in series with the condenser under working conditions may show trouble. With such a test as this, however, all due precautions should be taken to guard against meter damage.
For routine testing of condensers (as separate items) for excessive D.C. leakage a source of D.C. (batteries or an A.C. H.T. eliminator) is most useful on the service bench. A miliammeter should be joined in series with the condenser and the D.C. source together with a series resistance of suficient value to restrict the current to the value of the maximum meter reading, should the condenser choose to short. Alternatively, a voltmeter could be used, in series, as a current indicator. In this case the voltmeter range should embrace the voltage of the testing supply. In the case of electrolytic condensers it must be remembered that these have polarity and that any test voltage, including even that of asimple ohmmeter test, should be applied in the correct sense.
Capacity Values. --An internal open circuit can easily be detected if the condenser is of large capacity, but it is not such an easy proposition if the condenser is of small value, unless a capacity tester is available. In the latter event, of course, it is simply amatter of testing for capacity value.
Assu ming that acapacity tester is not available, an ohmmeter test of high capacity condensers will show whether or not there is an internal disconnection. If the condenser is in order, the pulse of charging current taken from the oh mmeter's voltage source will give anoticeable" kick" of the needle when the test leads are applied to the condenser. If the needle kicks over and then falls back it is asign that the condenser has taken acharging pulse and therefore cannot be internally disconnected. The dificulty with the testing of small capacity condensers by this method is that the charging pulse is too small to give any noticeable indication on the normal type of ohmmeter. Fortunately, a shunt capacity test with the suspected condenser connected in the receiver will overcome any difficulties that may arise in this way.
Checking for incorrect capacity value is an awkward proposition as far as accurate tests are conderned unless acapacity tester is available. Assu ming the absence of acapacity tester, the matter is not so serious
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SERVICING COMMERCIAL RECEIVERS
as it may perhaps seem, because actually it is only with fixed condensers used for high-frequency adjustment, tracking, etc., that critical values are necessary. In those cases, of course, a good capacity tester should be used.
Ganging Troubles. --Modern receivers with H.F. amplification invariably have ganged tuning, and poor performance can be due to the high-frequency circuits being out of alignment. This trouble may be found to have been caused by misguided eforts on the part of an inexpert person who has been experimenting with the adjustments; sometimes acomponent fault will be responsible for the receiver being "out of gang ";sometimes it will be found that atrimmer condenser adjustment has shifted, under the efects of vibration. Finally, acomponent replacement made in a high-frequency circuit will very often put the receiver out of alignment, necessitating reganging.
To put the cart before the horse, the procedure for reganging will be considered first and faults that may make reganging necessary will be dealt with afterwards. For ganging work in any other than the simplest of cases a calibrated modulated H.F. oscillator is virtually a necessity. and the oscillator should have a variable output amplitude control.
This instrument makes it possible to apply to the receiver circuits modulated H.F. inputs of selected frequencies and of suitable amplitudes. Obviously it will be necessary to have some means of observing the efects of ganging adjustments, but the idea of judging the optimu m settings by listening to the sound of the signals in the speaker is to be deprecated as most unsatisfactory. A much used and good method is to use an output meter (A.C. voltmeter) connected across the speaker.
Eliminating A.V.C. --Before we go into details of ganging procedure there are anumber of general points which are worth noting. If A.V.C. is incorporated in the receiver it must be remembered that an A.V.C. action tends to counteract the efects of H.F. carrier amplitude changes, as far as the receiver output is concerned. To say the least of it, such an effect is most undesirable when ganging adjustments are being made. There are three alternative ways of overcoming this dificulty. The most obvious is to cut out the A.V.C. by temporary circuit modifications. Another solution of the problem is to use, instead of the output meter, a D.C. miliammeter, or aD.C. voltmeter, and to take the anode current, or the anode voltage, of one of the controled valves as the indication for ganging. Undoubtedly, however, the best way is to use the output meter and to keep the testing signal down to asuficiently low amplitude that the A.V.C. detector is not operated.
If the receiver ganging is badly out to begin with, care should be taken as the circuits are brought closer and closer into alignment that the increasing oscillation amplitude does not start up the A.V.C. action. A useful rule is to keep cutting down the oscillator's amplitude whenever the receiver circuit improvements permit.
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PRACTICAL WIRELESS SERVICE MANUAL
In the majority of cases only one setting of any particular trimming condenser will give a peak indication on the output meter and this setting wil be the correct one for ganging purposes, but there are exceptions. In the case of a superhet oscillator circuit, there may be two trimmer settings which give output meter peaks and one of these settings is definitely the wrong one to use. The other exception is one that can sometimes prove to be rather troublesome if elaborate test gear is not available, and this case arises with tightly coupled circuits designed for band-pass response. Here the tuning adjustment of each circuit, for any given frequency, is afected by the coupling influence of the other circuit; moreover the correct adjustments of the two circuits are those which will give the actual band-pass response characteristic for which the circuits and coupling were designed. Adjusting for a single maximum peak on the output meter will not be correct in such a case. With the average straight receiver, it will not normally be found that the presence of any coupled tuned circuits will introduce any dificulties, but with certain types of superhet receivers which employ bandpass I.F. transformers the latter may present rather a problem. A cathode-ray oscilograph, with" sweep "control on the testing oscillator frequency, provides, of course, the perfect solution of the problem, because with such apparatus the actual frequency response curve of any individual coupling, or of the receiver as a whole, can be made visible on the oscillograph screen. Thus, the complete efects of any trimmer adjustment can be watched. To have an oscillograph available is not, however, the good fortune of everybody, so we must consider how to get along without it although it must be admitted that in certain cases the lack of an oscillograph represents a considerable disadvantage.
It is to be noted that correct ganging is more than amatter of getting circuits into alignment, for perfect alignment could be secured and the receiver could still be incorrectly adjusted. The most obvious case of this would be that of lining up I.F. circuits on aganging signal adjusted to a frequency which is not the actual I.F. specified for the receiver. Where signal frequency circuits are concerned, one could possibly line up the circuits with too much capacity in every trimmer and the efect of this would be that the receiver would not then tune down to the lowest wavelength which it was designed to cover on the particular waveband concerned. It is advisable to aim at keeping each trimmer at as low a capacity as possible with the important reservation that the final setting must be an optimum one.
For adjustment of trimmers in signal-frequency circuits atest signal frequency corresponding to a wavelength near the lower end of the wavelength band should be used. A manufacturers' service manual usualy gives instructions as to the frequencies which should be used for reganging work on the receiver concerned, but if such information is not available, chose some frequency which tunes in near the minimum
24
SERVICING COMMERCIAL RECEIVERS
end of the tuning dial of the receiver. On the normal mediu m and long
wavebands frequencies of 1,400 kc /sec. (214 metres and 300 kc /sec. (1,000 metres), respectively, are generally satisfactory.
Re a ming the Straight Receiver. --An output meter should be connected across the speaker (a large capacity condenser should be used in series to keep D.C. out of the meter), and the testing oscillator's
leads connected to the A and E terminals of the receiver. The oscillator lead to the A terminal should be taken either via adummy aerial unit or via a .0002 mfd. condenser, if the former is not available.
Reganging the Superhet. --The work of reganging asuperhet, assu ming that it requires readjusting right through, must be divided into
several distinct parts: (1) Lining up the I.F. stages to the correct I.F. value and,
possibly, for a good shape of response curve.
(2) Oscillator trimming. (8) Signal frequency circuits' trimming.
(4) (With certain exceptions) Oscillator tracking. If band-pass complications are not anticipated the procedure described below should be satisfactory in all normal cases: Connect up an output meter to the speaker, and cut out the receiver
oscillator temporarily by joining alarge-capacity condenser from oscillator grid to chassis. Connect the testing oscillator output leads to the signal grid of the frequency changer, or first detector, and to E, respectively. The lead to the grid should be taken via a condenser (a 0.1 mfd. should do). Tune the testing oscillator to the correct intermediate frequency value and keep down the test signal amplitude to the lowest possible value, consistent with asatisfactory output-meter indication.
Adjust the I.F. trimmers in turn for peak response, starting at the last I.F. secondary and working back to the first primary. Next, run
the testing oscillator frequency through resonance and back, and and watch the output-meter needle for any signs of bad asymmetry of response. This step should be taken as a precaution against getting
"caught " should the I.F. transformers prove to be tighter coupled
than was anticipated. If it is known that the I.F. amplifier is designed for a pronounced
band-pass characteristic, there are two ways in which the adjustments can be tackled, assuming that oscillograph gear is not available: (1) Trim for peak response and then readjust by trial and error to get
as symmetrical and as steep-sided aresponse curve as it is possible to judge by watching the output meter while running the test-oscillator frequency to and fro through resonance. Care should be taken, however,
that in the enthusiasm of getting asymmetrical variation of the outputmeter needle, that the I.F. amplifier is not dropped in sensitivity to much. As amatter of fact, it will pay whenever aslight readjustment of atrimmer does not appear to be very helpful to let the trimmer" stay
15
25
PRACTICAL WIRELESS SERVICE MANUAL
put" as it was. (2) Impose fairly heavy damping on each I.F. circuit in turn while its companion is being trimmed for peak response. A shunt resistance of from 10,000 to 50,000 ohms should be tried, connecting it across aprimary while the corresponding secondary is being tuned, and vice versa.
Oscillator Trimming. --If it so happens that the receiver has avariable-coupling control on one or more of the I.F. transformers, then the latter should be tuned for peak response with the coupling kept at mini mu m.
As a reminder --always be ready to reduce the signal amplitude whenever circumstances permit.
When the I.F. circuits have been adjusted the oscillator tri mming must be tackled. Connect up the testing-oscillator leads to the A and E terminals, as described for the ganging of a straight receiver, and take
away the shunt condenser which was temporarily connected to the oscillator circuit.
Check the pointer setting of the receiver. The medium waveband should be dealt with first, and it is to be assumed that the rest of the adjustments described below apply to the one waveband. Apply atest signal as you would for astraight receiver, and turn the main tuning control of the receiver to the dial reading corresponding to this. Start with the oscillator trimmer of the receiver at minimum capacity and increase the capacity until an output-meter peak is obtained. By commencing at the lowest capacity and stopping at the first peak setting, the risk, which arises in some cases, of tuning to the second channel oscilator frequency will be avoided. The oscilator circuit must now be left severely alone while the signalfrequency circuits are lined up in the normal manner. This brings the position up to the matter of oscillator tracking. By the process already gone through the diference between the signal and the receiver oscillator frequencies has been made equal to the intermediate frequency. It is necessary that this relationship shall
hold reasonably closely all over the waveband. When the ganged condenser has identical sections the "tracking" of the oscilator is secured with the aid of extra series capacity in the oscillator circuit. In some receivers, and on certain wavebands, the series-tracking condenser will be afixed condenser. In such acase, ifthere is no component defect and the right value of the I.F. has been used, the tracking will come right when the oscillator trimming has been done. Assu ming, however, that the tracking condenser is variable, it should be adjusted on a test signal near the top end of the waveband. The tracking condenser should be adjusted for a peak response on the output meter, and since it is most important not to disturb the settings of the tri mmers while this job is being done, it is advisable to rock the main tuning control of the receiver slightly and obtain the best combination of
26
SERVICING COMMERCIAL RECEIVERS tracking condenser and tuning-control settings. The adjusting of the tracking condenser is liable to afect the optimu m setting of the oscillator trimming condenser, and areturn should therefore be made to the lower end of the waveband and the oscillator trimmer readjusted, if necessary.
Many receivers contain ganged condensers in which the oscillator tuning section differs from the others in the respect that it has specially shaped plates designed to give the correct oscillator tracking without it being necessary to use an additional series tracking condenser. " Shaped
NT
&VPOLeersn
· 181. --This skeleton diagram shows asuperhet frequency-changer stage, with Cl
and C2, the two-gang tuning condenser.
plate tracking," however, normally only holds good over one waveband (usually the medium), and extra tracking condensers are brought in for other wavebands. In a case of this nature the oscilator adjustment on the medium waveband is merely that of trimming, but other wavebands will involve both trimming and tracking adjustments.
Dealing with the general matter of reganging a superhet, it is to be understood that once the receiver has been adjusted right through on one waveband, the I.F. stages are finished with. The other wavebands, however, must be dealt with individually, as far as signal frequency and oscillator circuit adjustments are concerned.
Faults Which Affect Ganging. --There are two possible ways in which faults affecting ganging may show up. In one case it may be
27
PRACTICAL WIRELESS SERVICE MANUAL
found impossible to get an optimum adjustment on atrimmer. In the other case it may be found that while the receiver will gang up perfectly on one test frequency it gets out of gang at other frequencies. This type of trouble can occur with straight receivers just as much as with superhets.
When searching for faults afecting ganging, all coils and condensers associated with the high-frequency circuits must be regarded as potential trouble points. It is to be assumed, however, that any such drastic types of fault as breaks or short-circuits would most probably have drawn attention to themselves before one would have got so far as to bother about ganging. A disconnected tri mmer is apossible exception.
If any particular circuit refuses to give satisfactory response to trimmer adjustments, common sense will suggest that the trimmer itself and its connections should be inspected. If these are in order it is reasonable to assume that there is excessive shunt capacity across the circuit. A bad displacement or alteration of wiring may be responsible for this.
If ganging will not hold constant over a waveband the trouble may be due to a faulty ganged condenser. If the plates have been abnormally bent, if there is end play on the spindle, or if a set of " fixed " plates has shifted position (sometimes the" fixed "plates are adjustable) it is not to be expected that the various circuits tuned by the ganged condenser will keep in line, even if they have been lined up properly at one particular frequency. Close inspection of the ganged condenser may, therefore, be worth while. Certain ganged condensers have split end vanes, the segments of which can be bent to bring about capacity matching between the sections of the whole condenser. If, due to mishandling, one or more of these segments have been shifted from the proper position, trouble with the ganging is inevitable. The occasion calls for the job of bending the ofending segments back into place. In a bad case it will be necessary to go through a series of gangingcheck processes, working the condenser round from minimum to maximum and segment by segment and to make trial-and-error adjustments of the end vane segments until the matching is secured.
Tracking Adjustments. --In the case of the superhet oscillator circuit, bad tracking may be experienced and there are a nu mber of possible types of fault which could be responsible for this. A defect in the oscillator section of the ganged condenser is, of course, a possibility, and so is that of a defective oscillator coil. Where fixed tracking condensers are used it will need no more than a capacity change of one of these to throw the tracking out. Incidentally, it will sometimes be found that acombination of afixed and variable condenser is used for tracking. If, when carrying out tracking adjustments, it is found that the variable condenser will not give a peak setting, then the fixed
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SERVICING COMMERCIAL RECEIVERS
condenser associated with it should be suspected before anything else.
(Fig. 181.) It should be noted that the tracking will be wrong if the oscillator
has, by some mischance, been tuned to the wrong beat, and the advice given earlier in this article to work from minimum capacity of the oscillator trimmer towards the first peak setting is important, although it
must be admitted that in some cases it would not be found possible to tune to the second peak. However, it is as well to be careful on this point.
If coils have become unmatched there may be a little dificulty in locating the faulty one, although probably the clue will be obtained from the behaviour of the trimmer in the circuit which contains the faulty coil. Any suspected coil should be closely inspected for such faults as displaced turns, loose turns, or turn-insulation defects. With certain kinds of iron-core coils it is possible for trouble to be produced by acore displacement.
If any particularly obstinate case is met with when endeavouring to find the cause of ganging defects, the possibility of valve trouble may be well worth investigating. Due to the efects of inter-electrode capacity the valves do have an influence on ganging adjustments and it is even possible that incorrect valve-operating voltages may indirectly have some efect on the ganging. Again, the anode by-pass condenser associated with triode detectors has abearing on the actual tuning of the grid circuit and should not be overlooked in atroublesome case.
Dial Readings. --Check the pointer setting of the receiver. This is an important matter as the ganging work can be rendered unnecessarily tricky if the pointer is not set properly relative to the rotor of the ganged condenser. Adjust the oscillator frequency to the chosen test value and turn the receiver tuning control to the dial reading appropriate to this frequency; also adjust the oscillator's attenuator control to give a suitable initial reading on the output meter.
The next move is to make the actual trimming adjustments. If the receiver employs reaction it is advisable to do the trimming with the reaction control advanced to a setting that the user would normally employ when searching for distant stations. The first tri mmer to be adjusted should be that associated with the detector input circuit, and the other trimmers should be adjusted in turn, working back towards the aerial. Each trimmer should be adjusted to give a definite peak reading on the output meter. If, as progress is made, the meter reading starts to get high on the meter scale, cut down the oscillator output accordingly. Should the receiver jump into oscillation, reduce the reaction setting.
If it is found that one trimmer has to be adjusted to minimu m or maximum without reaching a" peak setting "try adjusting the tri mmer while the main tuning control of the receiver is varied slightly each way.
29
PRACTICAL WIRELESS SERVICE MANUAL
If an optimum setting of the trimmer can be found with the main tuning control slightly altered, leave the latter at the new setting, and attend to the trimmer adjustments all over again. The point here is that the initial setting of the ganged condenser may have been slightly out, particularly if the dial is not very accurate, or if the pointer setting is not critical.
When every trimmer has been given adefinite optimu m adjustment the ganging should be in order for the waveband concerned. To make sure that all is wel, however, the following checks should be made: verify that the receiver has not been lined up with a little too much capacity in all the trimmers, by setting the receiver tuning to the lowest wavelength of the band and resonating the test oscillator to the receiver. The oscillator should be at a frequency corresponding to this wavelength. Check up on the matching of coils and ganged condenser sections by testing the trimmer settings at awavelength, say, about halfway up the waveband, but be careful that the trimmer adjustments are left at the optimum setting found at the higher test frequency.
A Possible Problem with the Superhet. --If the other wavebands of the receiver involve separate trimmers each of these wavebands should be dealt with in the manner described. With most M. W./L. W. straight receivers, however, ganging on the M. W. band will be all that is necessary.
It will be noted in the foregoing that single peak indications on the output meter have been adjusted for, and it will be unusual if the adjustments are not satisfactory with astraight receiver of average type.
There are certain peculiarities in the case of ganging a superhet receiver. The I.F. stages must be attended to before any other sections of the receiver, and if the I.F. value is not known, the best thing to do is to apply an I.F. test signal from the testing oscillator and to tune the oscillator to give maximum response through the I.F. stages. Ignoring the possibility of the receiver adjustments having been tampered with, it is fairly reasonable to hope that all the I.F. circuits will not be so badly out of adj ustment that the test oscillator frequency giving the maximu m response will not at least be somewhere near the correct value.
Once the question of I.F. value is settled, the ganging of asuperhet, although more involved than in the case of astraight receiver, is not to be regarded as complex. The only dificulty (and only in certain cases) may be that already touched upon, namely, that of getting the right shape of response curve.
Distortion. --There are two diferent types of distortion. One is frequency distortion, which is produced by the receiver giving disproportionate amplification to diferent frequencies, i.e., "cuts " high notes, exaggerates high notes, shows frequency resonances, etc. It must be understood that frequency distortion is to be expected of every receiver of average type, and that it is to be regarded as a fault
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SERVICING COMMERCIAL RECEIVERS
only when the distortion is greater than normal. Incidentally, if
frequency distortion is greater than normal anon-technical owner of a
receiver does not usually say that the receiver is distorting. "The
tone has become poor" is the complaint usually made. The other kind of distortion is amplitude distortion. With this type
of distortion the modulation waveform of the signal is so badly treated
by the receiver that alien frequencies are introduced and the sound
emitted by the speaker thus contains components not present in the original. Very little amplitude distortion can be tolerated without the
reproduction being considered definitely most unpleasant. Amplitude
distortion will occur as a result of valve overloading; speakers, too,
can become overloaded if too great an input is applied and will produce
amplitude distortion. With some receivers overloading will occur ifalocal station is tuned in and the receiver worked at the fulest volume
of output. It would not always be fair to the designer, however, to
regard such a case as that as a fault. The minimising of frequency distortion is very much wrapped up
in the design of the receiver. The high-frequency response, the proper matching of valve and anode load impedances, the correct choice of a
large proportion of the fixed condenser values, and, of course, the design of the speaker, are all matters that have a bearing on audio-frequency
response. Certain receivers will be found to contain tone correcting
devices. The most familiar example is that of the compensating condenser, or condenser-resistance combination, shunted across the primary of the output transformer when a pentode output valve is in
use in the output stage. Naturally, tone correction arrange ments should be viewed with the eye of suspicion if frequency distortion
is bad.
·
To deal generally with the procedure in tracing causes of distortion --
if there is no particular clue immediately apparent --it will be advisable
to check up on the valves and the valve operating voltages, as a first move. It is important to check up on the operating conditions of the
valves as in some cases quite asmall departure from normal will introduce distortion. The process of valve and valve voltage checking will amount to a check up on quite a large nu mber of fault possibilities. With push-pul, Class B or Q.P.P. output stages, aspecial watch should be kept for any signs of inquality in the two halves of the system. A miliammeter test in each output anode circuit in turn should be made if there is any doubt on the matter.
If the valves are in order and have been proved to be working under correct voltage conditions, and if the trouble is that of amplitude distortion it is high time to consider the speaker as the possible seat of the
trouble. There may be a damaged cone (or cone suspension). With a
moving-coil speaker there may be filings or other particles in the gap, speech-coil former of centre, or loose speech-coil turns. Such faults as
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PRACTICAL WIRELESS SERVICE MANUAL
shorting turns in speaker or output transformer windings are to be mentioned, although these would probably call for investigation more on the score of weak reception. A similar remark applies to the possibility of a weak magnet with a P. M. speaker.
If areceiver incorporating A.V.C. shows evidence of excessive valve
overloading although the valves are in order, there is every possibility that something has gone wrong with the A.V.C. system. Al resistances, condensers, and connections associated with the A.V.C. network should be checked over.
The cause of excessive frequency distortion is not always easy to
find. As regards a detector or
L.F. stage it will be advisable
MIL LIAMMETER
to check all resistance and capacity values and the resis-
tances of any transformer
windings should also be closely
checked.
More General Testing Notes.
TEST LEADS 182. --Method of using a milliam-
meter for low-resistance testing.
--In an earlier chapter mention was made of the importance of having means of carrying out low-resistance ohmmeter tests.
There is a great diference between a low-resistance coil in normal
condition and the same coil with anu mber of the turns shorting, yet
the diference would not be appreciable on an ohmmeter test if the instru ment were not designed for low-resistance measurements.
If any reader is experiencing dificulty where low-resistance tests are concerned, but possesses a low-resistance milliammeter, he can very easily make a device which will give comparative indications on low resistances.
In Fig. 182 amilliammeter is shown connected up to a D.C. voltage source through avariable resistance. The latter should be adjusted until the meter is giving full scale reading.
Suppose the milliammeter happens to be a0-80 mA. instru ment with aresistance of 5ohms. If a5-ohm shunt were placed across the meter
the reading would fall back. As to the exact position where the needle will settle, this will be partly dependent upon the value of R and the voltage regulation of the supply, but it will be somewhere above the 15 mA. scale reading. A 4-ohm shunt would give a lower reading; a 8-oh m shunt lower still, and so on.
Thus we have got an arrangement whereby diferent low resistances will give certain and diferent meter readings. Furthermore, with decreasing values of shunt resistance readable indications can be obtained down to a minimum value that may be regarded as surprising by those who have not used such a device before.
232
SERVICING COMMERCIAL RECEIVERS
To use the arrangement as atesting device two extra leads should be taken from the rnilliammeter terminals and used as the test leads. For making comparative tests the device can be used as it stands; for actual resistance measurements calibration would, of course, be necessary. The lower the resistance of the meter the lower will be the minimum value of resistance that will give readable indication on the meter. The usual care to guard against meter damage should be exercised during the rigging up of the arrangement and the initial adj ustments.
Poor Selectivity. --The overall selectivity of areceiver is something upon which each of the H.F. tuned circuits has a bearing. Thus, when the selectivity of a receiver is below normal it is necessary to take an interest in any factor that afects the selectivity of each individual H.F. tuned circuit. The superhet receiver has certain selectivity peculiarities of its own, but these wil be dealt with later.
If selectivity is below normal a ganging check may be useful in some cases, not only because the actual trouble may be directly associated with ganging adjustments but also because the efect of some particular test readjustment may give a clue as to the particular circuit which contains the fault.
It is a fundamental fact that the selectivity of any given tuned H.F. circuit is lower the greater the efective H.F. resistance of the circuit. Very frequently the search for the cause of low selectivity will develop into a hunt for a fault that is putting up the H.F. resistance of acircuit. It is important to note that the H.F. resistance of acircuit can be increased either by an increase of resistance in series with the circuit, or by a reduction of resistance in shunt with the circuit. Thus it is necessary to take into account not only the series circuit itself but also all components which are connected, either actually or efectively across the circuit.
The possibility of a valve being to blame must be allowed for and this, again, is one of the occasions when valve substitution may solve the problem.
If the valves are O. K. and it has come to acase of trying to track down the cause of excessive damping in some particular H.F. circuit the actual procedure will be greatly dependent upon the actual arrangements of the circuit and the accessibility of the various parts. If the circuit has M. W./L. W. switching (shorting out the L. W. winding on the medium range) and the selectivity is below normal only on the medium-wave range the switch contacts should be closely examined. Any excess resistance in the contact must inevitably have a bad efect upon the medium-wave performance.
Superhet Peculiarities. -- With normal reception the I.F. amplifier of a superhet receiver is fed by the frequency-changing stage with a signal, the carrier frequency of which is the diference between the
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PRACTICAL WIRELESS SERVICE MANUAL
wanted signal carrier and the local oscillation fundamental frequencies. The I.F. stages, although selective, give no discrimination at all between an input signal properly produced as described above, and any other signal at intermediate frequency which may be produced by signal combinations other than that of the wanted carrier and the oscillator fundamental. For example, if the diference between an unwanted carrier and the local oscillation frequency comes within the range of the I.F. amplifier the latter will, unless prevented, amplify the resulting I.F. signal just as readily as the one produced by the wanted signal.
Thus, with the superhet principle of operation, there is the danger that undesirable frequency combinations may produce I.F. signals, and it is to be noted that the I.F. amplifier itself ofers no protection, apart from its normal selectivity. It will be obvious, therefore, that the selectivity of the signal-frequency circuits (i.e., those in front of the frequency changer) is a matter of very considerable importance, in view of the fact that it governs to agreat extent the number and amplitude of unwanted signal frequencies reaching the frequency changer.
If the I.F. amplifier does happen to be carrying more than one I.F. signal, then the beats between these will be taken by the second detector and L.F. section of the receiver to produce awhistle in the speaker.
Second-channel Interference. --The interference case already mentioned, namely, that of an unwanted signal combining with the local oscillation fundamental frequency to produce an I.F. signal, is the wellknown "second-channel" case. When the receiver is tuned to awanted station the frequency of the latter is, in all normal cases, below the local oscillation frequency by the amount of the I.F. value. Secondchannel trouble, if present, will be caused by an unwanted station above the frequency of the local oscillations by the amount of the I.F. value.
There are a number of other whistle possibilities. If the oscillator has any pronounced harmonics an unwanted signal and an oscillator harmonic may combine to give an unwanted I.F. signal. Combinations of incoming signal frequencies may give unwanted I.F. signals (without the oscillator coming into account). Harmonics in the frequency changer open up further possibilities.
We must now proceed to tackle the case of the superhet that is producing whistles. It must be noted that the progress of superhet design during the past few years has included whistle reduction
among the improvements made, and that where a superhet of early design is concerned, the case may be one where it is not so much a matter of a fault as that of the "vintage" of the receiver.
Eliminating Whistles. --First and foremost aganging check is most important. As far as second-channel interference is concerned, some receivers incorporate special suppression-coupling systems, so, in these cases, the suppression system should be very carefully gone over if second-channel trouble is experienced. This is really a case where it is
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SERVICING COMMERCIAL RECEIVERS
important to have the receiver service manual in order that the exact
signal frequency-coupling arrangements can be closely studied. Valves and valve-operating voltages must (yet again) not be ignored,
particularly as excessive harmonic generation is a fruitful source of whistle trouble. In view of what has been stated about the signal-
frequency circuits, it will be realised that any faults which lower the
selectivity of the signal-frequency side of the receiver will tend to cause trouble, so every care should be taken to make absolutely certain that
the signal-frequency circuits are in order.
Apart from the causes of whistle generation already mentioned,
there are two others. First, there is the "common or garden" case of H.F. instability, and this should be handled in the manner described
earlier.
The remaining case is one that can be quite trouble-
some. The importance of keeping high frequency out of the L.F.
section of any receiver, owing to the possibility of feed-back to the
earlier stages, has already been emphasised. In the case of the super-
het receiver it is not so much a feed-back of the fundamental intermediate frequency which will cause trouble as a feed-back of the
harmonics of the I.F. Any H.F. filtering arrangements associated with
the second detector are, therefore, items to suspect if there is trouble on the above score; also screening generally.
Mains Hum. --In view of the fact that a mains receiver contains components (the smoothing components) specially concerned with the minimising of hum, it is obvious that, when hum is experienced, these
are the very first items to check.
An open-circuited smoothing condenser is a simple fault that can cause hum. Shorting turns in a snoothing choke (or field winding, if
this is used for smoothing) is another possibility. Both these faults can be easily checked by test. If the smoothing system is in order, valves should be the next to receive attention; in particular, an insulation
failure between heater and cathode of an indirectly-heated valve may
be the fault. Other possibilities are a breakdown of one half of a centre-tapped
filament potentiometer, or wrong adjustment if the latter is variable. Mains-transformer insulation defects can be responsible for hu m, but it
is to be expected that such atype of fault will come up for investigation upon some other score than hum (possibly smoke).
There are many possibilities involving inductive coupling between the mains transformer and an L.F. transformer, or even grid wiring,
which need hardly be anticipated with commercially-built receivers, but the point may be worth considering if any non-standard replace-
ments have been made in the receiver. There is one cause of hu m
which is particularly exasperating, and that is the case where excessive hu m only comes on when a signal is tuned in. This case is called
modulation hum, and is dealt with in Chapter XXVII on Hu m Troubles.
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CHAPTER X XXI
SECOND-C HANNEL INTERFERENCE
TI'S Chapter describes various ways and means of counteracting this form of interference in superhet receivers. The most popular receiver to-day is undoubtedly the superhet because of its great selectivity and range. Nevertheless, it sufers from a rather serious snag --that of whistles appearing at various settings of the dial. These are caused by the actual process of frequency changing.
A powerful station (usually the local) can produce even a small voltage at the grid of the frequency changer when the set is tuned to a station widely difering in wavelength from that of the local; and if this diference in wavelength happens to be equal to, a product of, or even an harmonic of the intermediate frequency, then abeat note is obtained, and a whistle is audible.
When the intermediate frequency is, as it usually is, 110Kc, it is apparent that when the oscillator is tuned to 1101 Kc, stations of wavelengths corresponding to 1000Kc and 1220Kc will both produce the required intermediate frequency of 110Kc. Interference will result and, if the unwanted transmission happens to be very powerful, then the interference will be serious and a loud whistle will be heard.
The obvious remedy for this trouble is to introduce suficient selectivity in the H.F. circuits to prevent the unwanted signals occurring at the grid of the frequency changer valve when the set is tuned to the desired trans mission. Unfortunately, if the unwanted signal is the
local station, second-channel interference is almost inevitable, as it is not apractical proposition to introduce suficient H.F. selectivity.
In London the frequency of the two local stations (London National and London Regional) is 1149Kc and 877Kc respectively, so that with intermediate frequency of 110Kc, second-channel interference will occur at 929Kc and 677Kc, the oscillator being tuned to 1089Kc and 777Kc respectively. This interference corresponds to wavelengths of 328 metres and 450 metres approximately, and if these are the only whistles that occur, they are not of very great importance.
Selective Tuned Circuits. --A complete remedy lies in the provision of several highly selective tuned circuits prior to the first detector valve, but this is an expensive proposition and not worth while, and it also re moves the advantage of a superhet in giving a high degree of selectivity in a simple and cheap manner through the action of the I.F. circuits.
236
SECOND-CHANNEL INTERFERENCE
Experience has shown that two H.F. tuned circuits (either of the band-pass type or two single circuits coupled by an H.F. valve) are
suficient to remove all whistles other than those caused by the local station. Even two local stations should produce only two whistles, corresponding to received transmissions lower than the frequency of the local stations by twice the intermediate frequency.
It sometimes happens, however, that whistles are produced all round the dial. If these do not disappear when the local stations are closed down, then they must be due to H.F. or I.F. instability and are outside the scope of this chapter. If, however, no whistles are apparent
when the local stations are not working, then it is safe to assume that they are caused by second-channel interference. It is important before tackling aproblem of this nature to make sure that it is second-channel interference, and not instability which is the cause of the trouble.
Correct Ganging Necessary. --Having seen that second-channel interference can be removed by the provision of extra highly selective circuits, it follows that the first thing to do is carefully to check the ganging of the tuned circuits and get this as accurate as possible, and also to see that the I.F. transformers are adjusted to the correct frequency. It is very possible that this check wil provide more accurate ganging and that greater freedom from whistles will result.
Many readers will no doubt have had the experience of continual retrimming of the H.F. circuits and adjustment of the I.F. transformers without any appreciable reduction of the number of whistles. If they are situated near apowerful local transmitter they are almost bound to have come across this efect, which is caused, not by misalignment of the tuned circuits, but by overloading of the first detector by the local station (or stations). With agood aerial, alocal transmission can easily impress anything up to 20 volts R. M.S on the grid of the first detector and part of this voltage must of necessity always be present whatever the setting of the tuning condensers. When this is added to the voltage of the received station it is apparent that overloading will easily occur and cause whistles.
Reducing the input to the receiver, either by using ashorter and less eficient aerial system, or by inserting asmall condenser in series with the existing aerial and the receiver, will efect a cure, but it will of course reduce the strength of all stations in the same proportion, so that more post-detector amplification will in all probability be necessary. Where A.V.C. is fitted, the set will, because the input is reduced, have to work under more sensitive conditions. The controlled valves will amplify more with the result that valve hiss and atmospherics will be increased and reception badly marred.
Avoiding Overloading. --A case in point was a superhet, with an H.F. stage preceding the first detector, in use in London. The I.F. transformers and the H.F. tuned circuits had been accurately adjusted
237
PRACTICAL WIRELESS SERVICE MANUAL by means of an oscillator and an output meter, but when the set was tried out on ordinary broadcasting transmissions with a good aerial whistles occurred all round the dial and reception of the local stations was almost impossible due to very severe distortion. The latter of course provided a very important clue to the cause of the whistles. It was not due to misalignment of the tuned circuits, but to overloading of the first detector. A very short aerial (just a yard of wire attached to the aerial terminal of the receiver) gave perfect reception of the local stations and the set was free of all whistles except the two normal second-channels to be expected at 823 metres and 450 metres, but of
Flo. 183. --A twin wavetrap for removing second-channel interference on medium waves.
A, Beginning of winding; B, End of winding.
course strength of foreign stations was greatly reduced. While a "local-distance" switch would, at first sight, appear to answer the purpose in a satisfactory manner, it was soon apparent that, although this would render undistorted reception of the local stations possible, it could not do anything towards removing the whistles. What was wanted was something that would reduce the strength of the locals without affecting other stations.
Many readers will remember the days of the wavetrap, and if they sufer from second-channel interference with their modern superhet, they should not hesitate to visit their "junk box" and fetch out awavetrap. Insert it between the aerial and the receiver, tune the set to the
238
SECON D-C HAN NEL INTE RFE RE NCE
local station, and then adjust the wavetrap for minimum signal strength. It certainly afected a complete and lasting cure in the case of the superhet referred to above, and had but little efect on trans missions of near wavelength to the locals. For instance, Radio Normandy is received at very good strength free of London National, while Hamburg is still a very good station just clear of London Regional.
A Twin Wavetrap. --If there are two local stations, as there were in the case referred to above, a twin wavetrap should be used. This was made up of two coils of about 50 turns of wire each wound on 21 in.
diameter formers, and each tuned by a .0005 mfd. bakelite dielectric condenser.
The condenser is connected right across the winding in each case, so that it tunes the whole coil. One end of the first coil is connected to the
aerial and one end of the second coil to the aerial terminal of the set, the free ends of the two coils being joined together. The whole assembly
is mounted on a small baseboard with the axis of the coils at right angles to prevent interference and breakthrough. The sketch (Fig. 188) makes the whole assembly very clear.
Tune in one of the local stations, and adj ust one wavetrap condenser for minimum response. Now tune in the other local station and repeat the procedure with the second condenser. This will result in the
strength of the local stations being reduced and will therefore remove
the second-channel whistles caused by them. So far we have only considered an intermediate frequency of 110Kc,
.000/ /%41-0, FIG. 184. -- Circuit diagram of a465Kc longwave filter applied to an H.F. transformer; A, I.F. filter coil; B, Medium-wave coupling coil; C, Long-wave coupling coil ;D, Mediumwave tuning coil; E, Long-wave tuning coil.
and its efect in producing whistles. It is apparent that if we can arrange the intermediate frequency so that the second-channels normally produced fall outside the normal broadcast wavelengths, then we can produce a set which will be free of whistles. This gave rise to the development of an I.F. of 465Kc, and this frequency is now being widely used, as it renders the mediu m wave band free of secondchannel interference, and enables the H.F. circuits to be much simpler. In fact it is common practice with a465Kc intermediate
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PRACTICAL WIRELESS SERVICE MANUAL
frequency to use only one tuned H.F. circuit, thus dispensing with a coil and a condenser, and making the initial cost of the receiver considerably cheaper.
When we come to long waves, however, the matter is somewhat diferent. It is possible, particularly near to a local station, for a small
amount of mediu m-wave signal to creep through the self capacity of the coil when it is actually tuned to some long-wave station. Since the oscillation frequency when receiving long waves falls within the mediu m waveband when using an I.F. of 485 Kc, it is clear that any creep through of this nature will give rise to whistles.
Aerial Circuit Filter. --This is minimised by including afilter in the aerial circuit, this filter being switched in only when receiving long waves. This filter only permits signals up to about 850Kc to pass through and hence no creep through of mediu m wave signals occurs. A coil of about 800 turns layer-wound on a former of 1in. diameter is suficient. The size of the coil wil vary with diferent sizes of aerial and diferent localities, but the actual value required can best be found by trial, it being easy to make a small coil, and then take off a few turns until the correct value is found. The ideal to aim at is a coil of just suficient size to give good signal strength in daytime without excessive second-channel whistles at night. Fig. 184 shows a suggested method of connecting the filter. Note that the .0001 mfd. condenser is essential. A small mica fixed condenser is the type to use, or a .0002 mfd. pre-set may be inserted, as this will enable accurate control to be obtained.
CHAPTER XXXII
CHEC KING PERFORMANCE
THE Radio Manufacturers' Association has taken an important step by publishing for the benefit of its members two
booklets. Both are called "Specification for Testing and Expressing Overall Performance of Radio Receivers"; Part Iis devoted to "Electrical Tests", and Part I to "Acoustic Tests". These publica-
tions are not intended as instruction to members, but as suggestions,
with the object of bringing the tests down to a uniform basis. No doubt the suggestions will be generally adopted, in which case the
science of radio receivers will have been taken a step further. It is not intended to repeat the large amount of information, the
various formulae and the graphical expressions of results which are
given, but simply to explain in simple language some of the tests which are described. In many cases, the details given are of ahighly technical
nature, and could not be adequately dealt with in this chapter, but their meaning is interesting and instructive.
Overall Sensitivity. --The R. M.A. suggest that tests for sensitivity of the receiver as a whole should be made when the set is delivering an output of 50 m W --which is the lowest loudspeaker output for enter-
tainment value in the average room of ahouse. This output is referred to as the "standard" output, and the primary object of the sensitivity test is to measure the signal input, in microvolts (millionths of a volt)
required to produce such an output. In theory this is simple enough, but there are many practical dificulties in the way of making the
appropriate measurements. The first is that the output is dependent, not only upon the voltage
of the signal input, or of the carrier wave, but also upon the amount of modulation --sound impulses --imposed upon that carrier wave. Thus,
it is recommended that the steady signal, which must be provided by a signal generator of accurate design, should be modulated to the extent
of 80 per cent. This means, roughly, that the oscillations representing the sound " mixed " with the carrier wave should be of about one-third the strength of the carrier. It is very important that the signal be
constant, and this means that it must consist of a single note; an ordinary signal consisting of speech or music would not do, because of its constantly and rapidly-varying intensity. It is also important
that the measurement be taken at a standard frequency --a note of uniform pitch. Thus, it is suggested that a 400-cycle note should be employed for modulation. Such a frequency is representative of the average of the notes given by most musical instruments and of the
16
241
PRACTICAL WIRELESS SERVICE MANUAL
human voice. For example, the range of the human voice is from about
60 to 1,000 cycles per Second, and that of the cornet is from about 150
to
1,It200cancyclbeesseepenr
second. from the
above
particulars
that
by
measuring
the
input to the set in microvolts of a" standard signal "required to produce
an output of 50 milliwatts, the sensitivity of any type of receiver can
be specified. Moreover, the test indicates the overall eficiency of the
H.F., detector and L.F. stages in a" straight "set, or of the frequency-
changer, I.F. stages, second detector and L.F. amplifier of a superhet.
As the sensitivity of a receiver varies according to the wavelength
to which it is tuned, tests must be made at various condenser settings
on the various wavelengths covered by the set. The results can then
be shown as asmooth curve on graph paper.
Selectivity Curves. -- When we come to consider selectivity, the same
general rule holds good. In this case, however, two signal generators
are used in series; one of these produces an "interference " signal of
the form mentioned above, and the other the "required "signal which
is not modulated. The only reason for not modulating the latter is
that it would then be impossible to measure the output which is repre-
sentative of the extent of interference. The output is measured first
when the two signals are at the same frequency or wavelength, after
which the tuning of the "interference " signal is altered above and
below the frequency of the "wanted "signal, and the output obtained at
various settings of the signal generator is measured. A graph can then
be drawn from the results obtained; this is the reverse of the usual
"peaked "curves with which most readers are already familiar.
Automatic Volume Control --The recommendations also cover the
measurement of the A.V.C. action of aset, again using the "standard
signal." When making the tests the signal input is first set to 1 volt,
and the receiver volume control adjusted so that the set delivers one-
quarter of its nominal power output, which is normally one-quarter
of the rated maximum undistorted output of the last valve. After that
the input is progressively reduced in convenient steps, and the output
measured at each step. Theoretically, of course, the output should
remain constant but in practice ittails of to zero as zero input is reached.
Nevertheless, when using a highly-developed modern set, there is only
aslight change in output over the first half of the range of input reduc-
tion. Results of these measurements can be expressed on a graph,
where the vertical ordinate is marked as audio-frequency output, and
the horizontal ordinate in radio-frequency input. Alternatively, the
result can be given as a fraction representative of the "slope " of the
curve at any particular point. In this respect, the expression is com-
parable with that used to indicate the "slope " of the anode-current-
grid-volts curve of a valve (this expression is known as the slope or
mutual conductance).
CHECKING PERFORMANCE Response and Acoustic Output. --The "Acoustic Tests" include the preparation of agraph to show the change in" loudness "of the output according to the variation in frequency of the signal modulation. This graph takes into consideration the performance of the valves, intervalve couplings and of the loudspeaker, and is similar in appearance to the curves sometimes given by manufacturers in connection with pickups, microphones and speakers. . Other tests, which are too numerous and involved to be dealt with adequately here, are for distortion, acoustic sensitivity and hu m. Some of these are inter-dependent, as wel as being calculated by taking into consideration some of the results obtained from the tests described above. They call for alarge amount of delicate apparatus, which would be too expensive for other than manufacturers, whilst a considerable amount of mathematics is involved in producing the final figures. Suficient has been written to give an outline of the future possibilities with regard to the accurate expression of the precise performance of receivers of all types.
243
CHAPTER XXXIII
B A C K G R O U N D N OIS E S
0W NERS of short-wave sets are often disappointed to find background noises so pronounced that clear reception is practically impossible. The fact must be faced that interference is definitely more troublesome on the short waves, but this does not mean that it is necessary to tolerate incessant "hisses" and "crackles." In most instances it will be found that these unwanted noises have their source in the set, and that they can be prevented.
The fact which must be remembered is that aset operating on short waves is far more sensitive to bad connections and improper contact than is a broadcast receiver. This can often be proved when using an all-wave receiver, for it is noticed that although there is a " background " of silence when the wave-change switch is turned to the medium or long-wave position, this is far from the case when the set is adjusted for short-wave tuning. Some of the trouble is often due to outside sources --such as atmospherics and electrical interference -- over which the listener may have little or no control, but on the other hand apronounced improvement might be possible by carefully checking the wiring and components.
Greater Resistance to Higher Frequencies. --As an example of what is meant, a soldered connection might provide an apparently perfect contact at the lower frequencies (medium and long-waves), whilst it ofers afluctuating resistance of several hundreds of ohms to currents of very high frequency. The very same idea applies to looped connections, for it requires only a small deposit of oxide on the surface of the wire or on the face of aterminal nut to turn a good contact into a distinctly bad one.
When various "hissing " noises are heard it is thus a good plan first of all to disconnect the aerial and earth leads and then to listen carefuly while the various controls are operated Should it be observed that the noises continue it will be evident that the fault is within the receiver. If the noises do cease, and if they cannot be re-started by any adjustment of the receiver, it will be known that interference is being picked up by the aerial-earth system, and that "external " measures will have to be taken. That aspect is not to be considered here, for it has formed the subject of previous chapters, and is in itself somewhat complex. It is, however, worth while before turning attention to the set to make quite sure that the aerial and earth leads are not broken or fractured and that they are properly connected to the set.
244
BACKGROUND NOISES
Loose Connections. -- When it has been decided that the set is at
fault the first step is to try tapping it in various places with the hand;
if the noise is afected by this treatment aloose connection will at once
be indicated. The particular connection at fault can then be found by
the simple process of examining all joints in turn. All terminals must
also be checked for tightness,
and it must be ascertained
that the valves are properly
and tightly fitted into their
·1 /4.4,
-
holders. This check might appear to be too simple to
warrant ful investigation,
but it is usual eventually to
find that the trouble was
due to some very slight
- .
FIG. 185. --See that the wire-end connectors of fixed condensers are not loose and making bad contact by testing to see if there is any
possible end movement.
matter which had been overlooked. Just as with a
normal broadcast receiver, terminals, looped wire ends and valve-pins must be seen
to be clean, and should be lightly rubbed with fine emery or glass-
paper if any doubt exists.
With regard to terminal connections, it is not unusual to overlook
the fact that terminals attached to components may themselves not be
perfectly tight. Consequently, they should be checked, if necessary
removing the components from the set in order to tighten the terminals
as wel as to examine the
leads to them, where these
are exposed. It is also wise to scrutinise the windings of
nEXEXTRA
CONNECT/ON
coils and chokes when the
accumulator is housed in the
same cabinet as the set, be-
cause it has often been found
that acid fumes from the
accumulator have attacked
and " eaten "the cotton insu-
lated covering. If such wind-
ings do appear to have sufered it is best, where possible, to
BRONZE P16 TAIL
rewind the component; some-
times, however, it will be necessary to fit a new one.
Wire-ended Components. --
Another point which is often
188. --In some of the older types of variable condenser connection to the rotor is made by means of a copper spiral. Noise can often he stopped by replacing this with ashort
length of flex, as shown.
245
PRACTICAL WIRELESS SERVICE MANUAL
overlooked concerns the wire ends of components such as tubular fixed
condensers and fixed resistances. It is by no means rare to find that
the wire ends of a condenser have been loosened due to careless
soldering or due to their having been pulled loose when fitting the
component. As a test the condenser and each lead in turn should be
held, a careful note being made of any end movement of the lead.
Where this fault is suspected it is quickest to fit anew component --
which will probably cost only a few pence.
If it had been noticed that
the "fizzing" noises were
accompanied by a form of oscillation, special attention should be paid to condensers
OIL CO NTAI NI NG COLL OIDAL
GRA PHITE
used for decoupling the anode circuit of the detector or the
screening grid of an H.F.
valve. If a condenser is used
as a by-pass between the
detector anode and earth, this also should receive attention.
The Tuning Condenser. -- When the background noises
are in evidence only while the
SL Oh/
tuning condenser is being operated it will almost cer-
M OTIO N D RIVE
tainly be found that the
friction contact between the rotor and the correspond-
Fro. 187. --Using a feather for applying a little lubricant to the condenser bearing.
ing terminal is bad. Often the correct method of procedure will be
obvious, but if there is no form of tension or pressure adjustment it will
generally be found that a cure can be efected by the simple process of applying a trace of graphite grease or graphited oil to the bearing. The
oil sold by garages and known as" coloidal-graphite-treated lubricant"
is ideal for the purpose, and a spoonful is suficient to treat fifty
condensers.
Where a slow-motion drive is used with the tuning condenser this
might be the cause of the noises, since aform of electrolytic action some-
times takes place where there is a friction drive. Any trouble in this
respect can be overcome by applying the graphited lubricant mentioned
above to all bearings and points of friction contact. In some of the
older types of tuning condenser a spiral of wire or flat copper strip is used for making contact between the moving vanes and a terminal.
This can cause" noises "if the turns of the spiral touch as the spindle is
rotated. The best cure is to replace the spiral by ashort length of fairly
stif insulated flex, making sure that the ends are well soldered.
246
CHAPTER XXXIV
LOUDSPEAKER FAULTS AND RE MEDIES
W HEN areceiver is being serviced it is often found that all the existing tests fail to reveal anything wrong with the circuit or components. In such a case the speaker will be at fault, and many service-men now adopt the procedure of testing the speaker first --considering this as the weakest link in the chain. There are many obscure faults which may exist in this part of the equipment and which may fail to answer normal tests. Such items as shorted turns on the speech coil or field, for instance, may be very dificult to locate. Dealing with the usual faults in a more or less sequential order, one of the commonest troubles is that which gives rise to what may only be called a" paper and comb "form of reproduction. This may be due to several things, the most usual being a torn cone. This will not be readily seen if the speaker is mounted with its dust-bag intact, and the speaker will therefore have to be dismantled. This should, of course, be the first step in testing the speaker. A tear or rupture in the fabric of the cone should be easily seen unless it occurs round the edge of one of the types
of cone which has corrugations round the edge. A careful examination will in this case have to be made and the surrounding frame, with felt ring must be removed. The simplest cure for this trouble is to repair the fracture and in some cases this may be done merely by carefully coating the torn edges with a good adhesive. A patch may be used in some cases but will interfere with reproduction. In some cases, of course, this may be tolerated and only with a quality receiver will it be necessary, unless the fracture is very large, to replace the cone.
Speech Coil and Leads. --In some cases the speech-coil turns may have become loose and can give rise to a similar efect and the remedy is again to re-attach them with a good adhesive. On most speakers two flexible leads are anchored to the cone and taken down to the speech coil, and these should be kept clear of the cone. If, due to mishandling, they have been pushed near to the cone they may vibrate and come into contact with the cone at certain frequencies or at acertain volume and thus give a periodic "comb and paper" efect. Short-circuited turns on the speech coil will upset the matching and give rise to the type of distortion which is experienced with an unmatched output stage, and only by removal of the cone will it be possible to identify this type of fault unless the resistance of the coil is known beforehand and a good ohmmeter is available to read the resistance.
Distorted Cones. -- When aspeaker has been kept in adamp atmosphere there is a possibility of the cone becoming flabby and distorted,
247
PRACTICAL WIRELESS SERVICE MANUAL
and although this can give rise to distorted reproduction a more serious
efect is that the resultant sagging of the speech coil may cause the
coil former to foul the sides of the air gap. Again, this may be periodic,
occurring only at certain volumes. With some cones it is possible to
dry them out, but in most cases anew cone is called for. Re-centring
is a dificult task, but may be simplified by slipping strips of card or
ivorine at three or four points round the speech coil whilst the spider or
centring bolt is locked into position. Special small wallets consisting of a
number of strips of varying thickness are supplied by Messrs. Holliday
and Hemmerdinger and will facilitate this operation.
A broken web on the spider will be obvious and the only satisfactory
cure here is to replace it--
preferably by a similar spider
obtained from the makers of
the speaker.
Energised Speakers. --The
field winding of an energised
model is wound to a definite
resistance, and if over-run the
covering of the wire may char
and eventually give rise to a
short-circuit. If this occurs on
two or more adjacent turns it
will not make a great deal of
diference except perhaps to
result in aslight rise in tem-
perature. Where, however,
the short-circuits efectively
cut out many turns the resis-
tance of the field wil be
reduced and excessive H.T.
will be applied to the valves.
FIG. 188. --A service accessory for loudspeaker repairs.
The increased current resulting from this will cause an
increase in the temperature of the field and more charring will probably
take place. A new field winding is the most satisfactory solution where
it is definitely traced to over-heating, but where movement or some
other cause has resulted in a slight wearing of the insulation the field
winding may merely be impregnated with good shellac or if the coil
is unwound a small piece of insulation tape may be wrapped round the
defective part. Fairly elaborate apparatus is necessary in order to
identify short-circuited turns in afield winding and the use of an ohm-
meter which is realy accurate will in most cases enable the service-man
to ascertain whether or not these exist.
Mis-Matching. --The speaker is generaly provided with a matching
248
LOUDSPEAKER FAULTS AND REMEDIES
transformer mounted on the speaker chassis and this should be suitable for the valve in use in the output stage. Where distortion is present and all normal tests fail to reveal the trouble, the fact should not be overlooked that the output valve may have been replaced by one having a diferent impedance and thus the speaker is not matched. A good output transformer, with multi-ratio tappings or terminals is a useful accessory for the service-man and will enable him quickly to ascertain
whether the output transformer
BA FFLE BOARD
' n ilesadast sfhaouultl.d Tbhe edsispceoencnhe ctoeidl
from the existing transformer and the universal matching transformer then joined to the
DISTAA CE PIECE
speech coil and connected in the output circuit.
Where aspeaker and receiver combination produces a tone which is too deep, and it is
found that this is due to the thickness of the cone, it may be found that modifications in
the circuit do not enable the pitch to be raised satisfactorily. A simple remedy in this case is to remove the bolts holding the speaker to the baffle and to replace them, inserting large distance pieces between baffle
DISTA NCE PIECE
and speaker. This will leave an airspace between the speaker
and the baffle and will serve
to raise the tone appreciably
in most cases.
FIG. 189. --Reducing bass response by spacing the speaker from the bafle.
A "Buzz " Test --Finally, when a speaker is to be tested, one of the simplest methods is
to "buzz " it at a low frequency. The standard mains 50-cycle supply
is ideal for this purpose and will reveal many faults in a speaker which
may not be discernible by ordinary means. To enable the speaker
to be used on a mains supply without risk of damage an old mains
transformer may be employed. The primary is usually tapped to suit
mains from 200 to 250 volts and there will probably be 10 or 15 volts
available at one of the tappings when joined to your particular mains. The speaker may be connected to this spare winding or spare terminals
as shown in Fig. 198. An excessive voltage should not be employed.
249
PRACTICAL WIRELESS SERVICE MANUAL
Switch Faults. --Although really coming within the heading of bad
contact, the on-of switch has not been mentioned. If this is of the old
push-pul type it is quite feasible to expect that the contact strips
are making bad contact with the metal-ended plunger; the test is to
short-circuit the switch with
TO SPEAKER
a short length of wire. If it proves to be faulty it should
be replaced by another of the
more reliable quick -make -
break type. The wavechange
switch may also be the cause
of trouble when a two-range
220
·
coil is in use, but in most
cases the noises would then
2/0
0
be heard on one waveband
200
8
only. The method of effecting acure depends entirely on
the form and construction of
the switch, but if it is not
built into the coil it is wise
to replace it --again with one
of the quick-make-break pat-
tern.
MA NS PLUG
When all other tests fail to reveal the source of the
FIG. 190. -- A 'buzz" test for speakers,
using a mains transformer.
background noises it is wel to remember the possibility
(only slight, it is ad mitted),
of there being a faulty con-
tact inside the cap of the detector valve. This can be checked only
by replacing the valve. Fixed resistances, especially the grid leak, can
sometimes be tracked down as responsible for the trouble. The reason
is generally that the contact between the composition element and the terminals or connecting leads is not sound. Replacement provides the
simplest check, although the grid leak can often be tested by simply
disconnecting one end; if the noise then ceases the component should be replaced.
CH APTE R X X X V
EQUIPPING A SERVICE WOR KS HOP
AGLA NCE at Fig. 191 will give an idea of the layout suitable for a small service workshop. It is impossible, of course, to give a hard and fast layout (as the roo ms will vary in al most every case), but as we must start so me where, we will assu me that the roo m has four walls, a door and a windo w.
FIG. 191. --This Pictorial Chart shows the layout of the workshop.
Item No. 1. --The work bench. 2. --A useful cupboar d. 3. --The tool drawer. 4. --Sheet of zinc. 5. --The vice. 8. --The tool rack. 7. --The bench stool. 8. --The scrap box. 9. -- The testing bench.
10. --The battery compartment.
1. --Bookshelf. 12. --Universal terminal strip. 13. --Shelf.
251
Item No. 14. --Lead-in tube. 15. --Aerial lead-in. 16. --Insulated hook.
,, I7. --Stout galvanised wire. 18. --Sliding Clip.
,, 19. --Three-way adaptor. ,, 20. -- Wall-plug and switch. ,, 21. --Light plug and switch. ,, 22. --Adjustable lamp. ,, 23. --Electric soldering iron. ,, 24. --Loud speaker mounted
on bafle-board. ,, 25. --Headphones. ,, 26. --A set under test.
I92. --Above may be seen two ideal workbenches such as are found in the E. M.I. service range. That on the left is felt-covered to avoid scratching cabinets, and on the right is a gramo-
phone unit for L.F. circuit testing.
FIG. 193. --Two further valuable service accessories. An adjustable table lamp and an extension bench to carry additional tools. 252
EQUIPPING A SERVICE WORKSHOP
In this pictorial chart it will be noticed that the various " furnish-
ings "are numbered and indexes, so if we take them in nu merical order
we will have cleared up most of the points.
Item I. --The bench, amost important fitting in any workshop. The
construction of a bench of this description is very simple, as will be
appreciated from the drawing.
c-
whilst it is not necessary to
have a very hefty afair for radio work it is as well not to
go to the other extreme and
have a flimsy one that bends at each saw cut The supports
Rule.
FIG. 194. --A steel rule screwed to the bench is handy for measuring wire, etc.
should not be less than 21 in. square and ordinary floorboarding will be found very
suitable for the top. It will be noticed that the bench takes
up the whole length of the
wall in front of the window. Although this position is clai med at the expense of a short aerial lead-in, it will be justified by better lighting
on the job in the vice. Item 2. --A cupboard, housed under the bench, wil be found very
useful for storing the enormous amount of oddments which only awireless fan can hoard. If this cupboard is judiciously sub-divided a re-
markable degree of orderliness may be maintained. Item 8. --The tool-drawer a"close-up" of which is shown in Fig. 195
is supported on wooden runners which are screwed to the underside of the bench. The sliding tray, shown here, will be found very handy for
smaller tools, such as gauges, callipers, small screwdrivers, etc., which
otherwise have a nasty habit of hiding themselves under the larger tools in
the drawer. Item 4. --Asheet of zinc
covering one end of the bench wil be found useful
for the soldering department.
Item 5. --The vice.
Although this need not be of cumbersome dimensions, for radio work, it is as well to have something fairly sturdy, as some of our jobs (such as metal chassis
195. --The more delicate tools should be kept together in the sliding tray of the tool
drawer.
253
FIG. 198. Tools in the right place will simplify service work. Here are some hints for creating a tidy workshop. On the left may be seen how to store small tools in holes drilled in the end of the bench. Below is asuggestion for aerial and earth strips to avoid
untidy leads.
Fic. 197. A simple form of tool rack for the
bench end.
FIG. 198. A steel container into which the soldering iron may be placed to
avoid damaging the bench.
EQUIPPING A SERVICE WORKSHOP
construction) may call for a solid foundation. Just a word about the
mounting of the vice. Place it fairly near abench support for rigidity
but not directly over one as some awkwardly shaped job, which we
may have to tackle, may require space directly underneath the vice.
Item 6. --A rack for files, screw-
drivers and chisels. The latter
especially should be kept in this
manner, as the cutting edges will
soon become sadly out of condition
if the chisels are kept in the tool
drawer.
Item 7. --The bench stool.
Item 8. --The scrap-box, if regu-
larly and judiciously employed, will
maintain the tidiness of the work-
shop.
The Electrical Side. These
items -1 to 8-- give us the main
FIG. 199. --A handy rack for files, screwdrivers and chisels.
essentials for our constructional department. Now let us step into
the electrical and testing laboratory. It is here that the fruits of our
constructional labours come up for judgment, so let's be fair and give
them a good trial.
Item 9. --The testing bench. As may be readily seen from our
Sprtng Wine Curtain Runner-
1111 Supporting, I
&ache j!
I I it
Batt y, for f tforttriej
200. --The "power-station " or battery compartment.
chart this is really a com mon or garden kitchen table. A couple of drawers will come in handy for storing our logbooks, slide rules, pencils, pens, and all the "clerical " records of our " works."
Item 10. --The generating station, otherwise known as the battery cupboard, is shown in detail in Fig. 200. The shelf should, for preference,
255
PRACTICAL WIRELESS SERVICE MANUAL
be covered with a sheet of rubber (an old rubber mat will do nicely), so that any acid spilt, will do no damage.
Item 1. --The bookshelves, for our works of reference, can easily be put up, and this wil complete all the constructional additions to our testing bench.
Item 12. --The power distribution department, or to be more homely,
2'
A. E. ISO
ta te
0 00 0 0 0 0
L. v. 00 0 0
HT+
Wr-
1.7 LB
121
"R amona 00
Sflcaasrrmarrlio76coMmue-
Fm. 201. -- The lay-out of the universal terminal strip.
1/2.5ouuts DETAIL OF SupPoirrhve aucfrrer
the universal terminal strip. This is simply an ebonite strip mounted on wooden brackets and fitted with sockets. Fig. 201 shows the con-
structional details and layout. Two refinements will be noted; namely, the aerial series condenser and the potentiometer. The wiring up of the former is shown in Fig. 202, and by this arrangement we can either test a
set directly from the aerial (by connecting to A) or through the aerial series condenser by plugging in to A2. The object of the potentiometer wil be fairly obvious from Fig. 208. It iswired between the loudspeaker and the telephone terminals, in series and enables 'phones to be used in
Fig. 202. Connections for aerial series
condenser,
Fig. 203. Connections for potentiometers.
comfort, irrespective of the output of the set. A multi-flex battery cord connects the terminal strip to the power supply and thus a choice of
voltages is available for use. The L.T. side may, of course, be elaborated with further plugs and a switch, so that a choice of 2, 4 or 6 volts is available. As most sets incorporate the grid-bias battery, it is not worth while, in view of the additional amount of wiring, etc., to include gridbias tappings on the terminal strip. Fig. 204 shows aconnecting lead
suitable for use between the terminal strip and the set under test. If
256
EQUIPPING A SERVICE WORKSHOP
each of these leads is of adiferent colour it will obviate the risk of making incorrect connections.
Item 18. --As wil be readily seen, this is a shelf and as such
justifies itself. Its purpose, however, is twofold, as it also forms the
support for the aerial lead-in.
Item 14. --The aerial lead-in tube.
Item 15. --The aerial lead-in iscarried
under the shelf (as suggested above) direct
to the A plug on the terminal strip.
Item 18. --Insulated hooks support
the aei ial under the shelf.
Now we come to the lighting arrange-
ments of our workshop.
Item 17. --This isastout wire running
parallel to and directly over the bench,
and carries the bench light.
FIG. 204. --A flexible connection for use between the terminal
strip and the set.
Item 18. --A sliding clip of this description enables the light to be moved to any position over the bench. This clip,
as wil be seen by a glance at Fig. 205, is made from an ordinary
1
spring paper-clip, gripping apiece
of wood.
Item 19. --A three- way
adaptor. There are innumerable
ways of tapping alighting point,
but the three-way adaptor of
good quality will be found very
useful for our purpose. An
alternative suggestion is to fit a
two-way adaptor (which, perhaps,
will be more readily obtainable)
for supplying the two lights. The
wall-plug flex is then taken direct
from the ceiling rose, either above
or below the ceiling. This wiring should be encased in a lead
205. --The sliding clip for supporting the bench light.
covering if carried below the ceiling; it may be supported by staples
driven into the rafters above the plaster. When the position of the first
rafter has been found the subsequent staples may be carefully driven in
at distances equal to the pitch of the rafters.
Item 20. --This consists of a batten type lamp-holder and aswitch mounted on awooden base. This complete unit, when wired up, serves as a useful point of supply for the electric soldering-iron.
17
257
PRACTICAL WIRELESS SERVICE MANUAL
Item 2I. --A lamp-holder incorporating aswitch in its construction
will be found most saitable for the bench light.
Item 22. --This table-lamp (shown also in Fig. 206) is very handy on our testing bench, for the flexible tubular standard enables the light to
be directed in any required direction.
Item 28. --An electric soldering-iron will be found a great time,
labour, and te mper saver.
Item 24. --The loudspeaker (preferably of the moving-coil type)
should be mounted on a baffle-board
as large and as thick as possible. For
space economy reasons the loudspeaker
may be mounted in the corner of the
room as shown and supported by
brackets to the picture-rail. It is as
well to remember to have a space of
nine inches to one foot fro m the top of
the board to the ceiling.
Item 25. --A pair of headphones
wil be found almost indispensable for
short-wave work.
Item 26. --A set under test. This
brings us to the end of our list and
more or less completes the job.
These twenty-six items are, of
FIG. 206. --A useful lamp for the testing bench.
course, just the "bare bones" of our workshop. Maps, charts, lists of
stations, calendars, pipe-racks,
will all gradually find a place allotted to the m and give a
Pencil Line Marking Ran of Hex /
homely and individual atmos-
phere. Why not turn out that
spare room now, for when you've Testing
a proper room for practising your hobby --it will be twice as
interesting.
6)riTtasitions OMftnRafFitneerlsy Pointed
A Distribution Board. --It is
an excellent plan to have abench equipped with asuitable terminalR
61 -mlet Ia-- fters
board and meters, which acts as
amain distribution panel between
the batteries and the receiver; where mains equipment is used it might be possible to arrange the panel between a standard
ILeafldexCovered
Sta,oles
supply unit and the set, but there are more difficultiesinthis respect.
207. --These pictures show how to support the lead covered flex.
258
EQUIPPING A SERVICE WORKSHOP The bench itself can be quite asimple affair consisting of an old table about _8 ft. high, the top of which is covered with green baize or even light-coloured oilcloth. As to the distribution panel, this may be as simple or complex as desired, but details will be given of an arrangement which is fairly inexpensive, although combining all of the most important features. First requirements are that rapid and accurate measurements can at any time be taken of the H.T. voltage applied to a particular set tapping of the current taken at any point of the L.T. voltage
A
= GB
LT
F poim.nt208at.w--hiTchihsvodilatgargaemandshowcusrretnhet readings have normally to be taken. and current, and of the G.B. voltage. Fig. 208 shows the points at which measurements frequently need to be taken, the points at which current readings are required being indicated by crosses, and those between which voltage measure ments are needed being marked by arrows and a letter V. This is the simplest arrangement of battery leads, and yet no less than eight meters would be required if simultaneous readings were to be taken at every important point. It would obviously be an expensive matter to buy eight reliable and accurate meters, besides which they would occupy a considerable amount of space. Use of Plug and Jacks. --An alternative and better sche me, ifit could be adopted, would be to use a single multi-range meter and to make rrovision for quickly applying it to any one of the eight points. Actually, there is little dificulty in this if the meter is fitted with a plug, and if jacks of suitable type are included in the wiring. The meter might be
259
PRACTICAL WIRELESS SERVICE MANUAL
one of the many good types now available for about £2 and four single-circuit and four closed-circuit jacks are required. Single-circuit jacks are used for the voltage readings, the others being for taking current measurements. The reason for using the latter is that the circuit must be broken when the meter is connected and re- made when the meter plug is removed.
Jack Connections. --The idea will be more easily understood by referring to Fig. 209, where the jacks are shown in pictor ialform. It can be seen that there are three connections to the closed-circuit jacks used for current readings, and that when the plug is not inserted points marked 1 and 2 are short-circuited due to the spring contacts. The
R7 0
lef7+2
LU.Ji
IX
LT+ 0
·
M ULTI -RA KE 1.4B
METER
u+
). .17-4-
a-0
209. --This ilustration shows the connections to the jacks at the same points as indicated in
Fig. 208.
other jacks have only two terminals, these being joined one to each point between which the voltage is to be measured; thus, the jacks have no efect when the meter is not connected.
Suppose now that it is wished to measure the voltage being applied to the terminal marked H.T. +1, all that is necessary is to set the meter to read up to, say, 150 volts and to insert the plug into the jack marked B. Similarly the voltage to H.T. +2 is found by inserting the plug into jack E. The current passing to H.T. +1 lead is found by adjusting the to read meter in milliamperes and placing the plug into jack A. In the same manner the G.B. voltage can be found by using jack F, the
260
EQUIPPING A SERVICE WORKSHOP L.T. voltage by using jack H, the L.T. current by using jack G, and so on.
In connection with the wiring of the jacks, it should be noticed that the leads are the opposite way round to jacks D and G to those to jacks A and C. This is because these are in the negative leads, and it is important that the correct polarity of the meter be employed. With the
210. --This shows how the refinement of a choke-capacity output filter can be
added to the distribution panel. wiring shown, the tip of the plug should be connected to the positive terminal of the meter, and the main arm to the negative lead.
Panel Layout. --The method of arranging the jacks is not important, but a panel such as that shown in Fig. 201 is very convenient. This is made from a strip of ebonite, and is provided with six terminals --for aerial and earth, loudspeaker and low-tension leads, and with Clix sockets for the grid-bias and high-tension connections. The jacks are arranged between the terminals and sockets, those for current readings being above the centre line, and those for voltages being below, as indicated. This ebonite strip can be attached to the back of the bench, and should preferably be covered in at the back so that stray. wires cannot fall on the terminals and connections and so cause shortcircuits.
Another refinement consists of a .0008-mfd. bakelite-dielectric 261
PRACTICAL WIRELESS SERVICE MANUAL
condenser mounted between the aerial and earth ter minals, and connected in series with the aerial lead-in; this can be used for varying the degree of selectivity when the set under test is not provided with a series-aerial condenser. If a D.C. mains unit is employed for H.T. supply, another refinement might consist of a 1-mfd. fixed condenser wired between the earth terminal on the panel and the earth lead.
An Output Filter. --The idea of using the loudspeaker terminals on the panel is that astandard loudspeaker, placed in what has been found to be the best position in the room, can be permanently connected to them. The speaker should preferably be fitted with atapped matching
transformer, but an alternative is to mount a choke-capacity output filter on the panel, connected as shown in Fig. 210. It will be seen that the
AERIAL
EqfttH
e ft
18"
"._·/,·7 SPARE
0C
O_ LT e:_
oo03mid
VAR CON D
B HI ®
.
:
®+2 ®+1
1,ic. 211. --A suggested layout for the distribution panel described. The jacks are lettered in the same order as in
Figs. 208 and 209.
TEST PRODS.
two ends of a tapped output transformer are connected to the two speaker terminals on the panel, the speaker itself being connected to any
one of the other terminals by means of a flexible lead with spade terminal, and also to one terminal of a4-mfd. condenser, the other terminal of which is joined to the earth terminal on the panel.
Reverting to the meter and its plug, it is best to connect a .1-mfd.
non-inductive condenser in parallel with the meter to act as a by-pass when current readings are being taken. If this were not done instability might be introduced due to the comparatively high resistance of the
meter. The meter can then be mounted on a suitable bracket above the test panel or arranged to hang from ahook on the wall. In any case it wil be found convenient to arrange yet another open-circuit jack on the panel and to attach a couple of leads with test prods to this. Then any measurements which have to be taken inside the set, or when testing
components, can be taken simply by inserting the meter plug into the "spare "jack and using the test prods in the normal manner.
Modifications. -The general arrangement which has been described
262
EQUIPPING A SERVICE WORKSHOP can be modified in various ways to suit individual requirements and it would be a perfectly simple matter to include additional jacks for extra H.T. and G.B. leads. Those who have an output meter of so me kind could also arrange to plug this into ajack connected to the loudspeaker terminals. In the case of mains operation some of the leads might carry A.C., in which case it must be remembered that an A.C. /D.C. meter would be necessary in order to take all measurements. Generally speaking, however, the system is more suitable for battery-set users, and unless the reader has a fair knowledge of electricity he is not advised to attempt to modify it for use with mains sets. This does not apply when an eliminator is used in conjunction with battery sets, since the layout, as described, is perfectly satisfactory in such a case, the mains unit simply being considered as being identical in principle with the H.T. battery shown.
203
CHAPTER X X XVI
WIRELESS CALCULATIONS
THERE are many who do not seem able to make the few simple calculations necessary to determine, say, the correct value for a
decoupling or bias resistance, the wavelength range which can be covered when using a certain coil and variable condenser, or the sizes of resistances required to form afixed potentiometer. It is often thought that the arith metic involved is of adificult nature, or that the equations are for mathematicians only, whereas the calculations are often agood deal simpler than those required to draw up a cricket analysis or in working out the su ms that are regularly done at school by boys of twelve; the only real diference is that a little knowledge of wireless practice is required in addition to that of arith metic.
Oh m's Law. --Of all the calculations that must be made time after time that in connection with resistance values is the co mmonest,
as wel as being the easiest. All calculations involving resistance, current and voltage are based on Ohm's Law, which states that the current flowing in a circuit is always equal to the voltage causing the
flow divided by the resistance which tends to oppose it. Thus, if a voltage of 100 were applied to the simple circuit shown in Fig. 212, the
current passing through it would be 100 divided by 20,000, which is 1/200th of an ampere. It is generally more convenient to work in terms of milliamperes, which are one-thousandths of an ampere, so that
A MAAA/ /0, 000 sz A
the figure becomes 5 mA. The simple circuit shown
is typical of all valve anode
,- circuits, and the fixed
I
resistance might be a coup-
/0,f2000
ling resistance or it might
/00
represent the resistance of a
20,000 R
transfor mer or other
co mponent.
The formula for Ohm's
Law which we have just
used is written in mathe-
matical terms, thus :C = E '
Fin. 212. A simple circuit which shows the relationship between current voltage and
resistance. 264
where C is the currents in amperes, E is the voltage,
WIRELESS CALCULATIONS
and R is the resistance in ohms. This simple and useful formula can be re-written in at least two other ways in order to make it more convenient when the voltage or resistance is required, the other
two factors being known. For example, we could write: R = E--C,,
or E = C x R.
Bias Resistance Value. --Let us see how it works out when we want to find the value of the bias resistance shown in Fig. 218. In this case
it is known that the current passed by the resistance (the anode current of the valve) is 10 mA., and
HT* that the required voltage drop
across the resistance --the bias voltage --is 20. The required
resistance is obviously found by dividing 20 by 10 and multiplying by 1,000 (to change the milliamperes into amperes), and the answer is 2,000 ohms.
Now suppose in the circuit
/70A f · 8/ A.5
RE5/5TANCE E
2DV
HT--
shown in Fig. 212 that we know that 5 mA. is required to flow through the valve and that the resistance of the valve is, say,
5,000 oh ms, while the anode
Fm. 213. --The method of determining the correct value of a bias resistance can be
understood by reference to this circuit.
resistance has a value of 10,000 oh ms; we want to know the voltage necessary to
ensure the correct current. All that we need to do is to multiply the current in amperes by the resistance in ohms, and we get
5/1,000 x 15,000, which is 75 volts. Once we have seen these applications of Oh m's Law it is not dificult
to apply it to all resistance, voltage and current calculations, when two
of the values are known and the third is required. Resistance Wattage Rating. --There is another application of Ohm's
Law which must be used when it is required to find the correct wattage rating for a resistance. Power, in watts, is actually the product of the voltage and the current, in amps., but we often know the current
passing and the resistance value of, say, a coupling resistance, without knowing the exact voltage dropped across the resistance. But it is not dificult to see from the above equations that wattage can be deter-
mined from the formula: W =C 2R. In words, this formula reads: the
wattage is equal to the current in amps. multiplied by itself and by the resistance in ohms. Thus, in Fig. 214 we have a resistance value of
25,000 ohms and acurrent of 10 mA. or 1/100 amp. We see, therefore.
265
PRACTICAL WIRELESS SERVICE MANUAL
that the power in watts which is dissipated (or lost) is 1/100 x1/100 x
000 25,000, or 2150,,0,00 which is obviously 2.5 watts. Having made this
calculation we know that the resistance used in this circuit must be
rated at not less than 2.5 watts, and we should generally use a 8-watt
component to provide asu ficient
MT?'
factor of safety; aresistance of lower wattage rating would be
liable to burn out in use, due to
the resistance being overloaded.
It might not be clear to some
/0mA
readers how the expression C2R was found from the original
Ohm's Law equations to be the
same as ER --voltage multiplied
by resistance --or, in other words,
how C2 was found to be the same
as the voltage drop. But we saw
that the voltage drop (E) is equal
to C multiplied by R, and if also
we multiply this by C we get
214. --In order to determine the the expression C2R.
wattage rating of an anode resistance, it is necessary to know its resistance rating, and
the current passing through it.
Finding the Range.-- Now let
Wavelength us turn to
another simple piece of
arith metic which is not dificult to folow. Suppose it is re-
quired to find the highest wavelength that can be reached when a coil
of 2,200 microhenrys (a typical long-wave coil) is used in parallel with
a .0005-mfd. tuning condenser. The wavelength is found by using the
equation: wavelength =1,884 VL xC where L is the inductance of the coil in microhenrys, and C is the capacity of the condenser in micro-
farads, the wavelength being in metres. We have first of all to multiply together the inductance and capacity, and then to find the square root of the result, and finally to multiply this by 1,884. Taking the first
step we get 2,200 x-0005, or 2,20° x5, which works out to 1'1; when 10,000
the square root of 1.1, which is 1.05 approxi mately, is multiplied by 1,884 we get as the result, 1978.2, this being the wavelength in metres.
If the required wavelength and coil inductance were known, the correct capacity could be found by reversing the calculation and in the same manner the correct inductance could be determined from a knowledge of the ,wavelength and capacity. These calculations are slightly more involved, however, and since it is rarely necessary to use them, we will not consider them further.
266
WIRELESS CALCULATIONS Screening Grid Potentio meters. --In the opening paragraph mention was made of finding the values of resistance required in forming afixed potentiometer, and this is aproble m which often crops up in connection with an S.G. valve of which the screening grid is supplied from apotentiometer as shown in Fig. 215. If the screening grid did not pass any current, the two resistances would have values proportional to the maximu m supply voltage and the voltage required. In other words, if the H.T. supply delivered a voltage of 200 and 80 volts was required for the screening grid the upper resistance, marked RI could have a value of 60,000 oh ms, and the lower one, marked R2, avalue of 40,000 oh ms. Alternatively, values of 80,000 and 20,000 ohms could be used.
HT* 200V
HT-
FIG. 215. --Several interesting points arise when calculating the values of resistances required for a fixed screening-grid potentiometer.
It will be seen from this that the first step is to decide upon the approximate total resistance; with a mains set a maximu m value of 50,000 oh ms is generally suitable, and with a battery set, 100,000 ohms can be used successfully.
In the above assu mption of no current we simply made the lower resistance of such avalue that its proportion to the total was the same as the proportion between the required voltage and the total H.T. voltage --2 to 5, but let us see what would happen if the screening grid passed 1 mA. The voltage drop occasioned by the upper resistance would be 1/1,000 multiplied by, say, 80,000, or 80, and thus the voltage actually applied to the screening grid would be 80 less 80, or only 50 volts. Consequently, either the resistance marked RI must be reduced
267
PRACTICAL WIRELESS SERVICE MANUAL
in value, or that marked R2 must be increased. This apparent peculi-
arity is due to the fact that the cathode-screening-grid circuit is in
parallel with the lower resistance, thus reducing its efective value. It
is possible to evolve an equation from which the exact values of resis-
tance could be determined, but it is generally better for the non-mathe-
matical service man to use trial-and-error calculations and working on
the lines indicated above, until suitable values are found.
Series and Parallel Resistances. --In dealing with resistance calcula-
tions above we did not consider the efect of connecting resistances in
series and in parallel. If resistances are placed in series the efective
value is, as might be supposed, equal to the su m of the resistances.
Thus, if resistances of 500, 1,000 and 20,000 oh ms were joined in series
the total value would be 21,500 ohms. When they are joined in parallel
the result is entirely diferent, and the efective resistance of the co m-
bination is, mathematically speaking, equal to the reciprocal of the su m of the reciprocals. This expression is simplified if written:
Total R --
1 1/R1+/R2+1/R8,etc. This means that if the three
resistances mentioned above were connected in parallel the efective
value could be found by adding together: 1/500, 1/1,000 and 1/20,000, which equals 61/20,000, and reversing this to 20,000/01, which gives the result as approximately 830 ohms. It will be noticed that this is less than the value of the smallest resistance.
Condensers in Series and Paran a -- When two or more condensers are used together the efective capacity is found by reversing the methods described in respect of resistances. Thus, when condensers are joined in parallel, the total capacity is equal to the su m of the capacities of the individual components. When condensers are in series the resulting capacity is equal to the reciprocal of the su m of the
reciprocals. If, for example, a .0005- mfd. condenser is connected in
series with one of .0003 mfd. the resultant capacity is
1
1/.0005 +1 0003,
which is. 1/8 or .0015, which is. .0002 mfd. approxi mately.
.0015
8
USEFUL FOR MULAE
Frequency Formula
For mula for frequency is: f= 10 6 where L = inductance in 2ir A/LC,
microhenrys and C =capacity in microfarads. 268
WIRELESS CALCULATIONS
Wavelength of Tuned Circuit
For mula for the wavelength of a tuned oscillatory circuit is: 1884.96 A/LC, where L =inductance in microhenrys and C =capacity in microfarads.
Inductance
The formula for inductance in microhenrys is: L- 9.86/D2N2 K, 1000
where L =inductance, D =dia meter of coil in centi metres, 1= length of coil in centi metres, N =nu mber of turns per centi metre, and K constant. See table.
Inductive Reactance
Calculated fro m the for mula 2;T X, where I-frequency and L =the inductance.
This Table shows the Value of K, which must be Calculated fro m D
I )
/
4.00 3.75 8.5 3.25 3.00 2.75 2.5 2.25 2.0 1.75 1.5
K
-3654 .3743 .3944 .4111 .4292 -4545 .4719 -4972 .5255 .5579 .5950
I i
1
1.25 1.00
.90 -80 .70 .60 .50 .40 .30 .20 .10
K
-6381 .6884 .7110 -7351 .7609 .7885 .8181 .8499 .8838 .9201 .9588
Capacity of a Fixed Condenser
C- .0885 AKN 1,000,000 d.
W here K =Specific Inductive Capacity of dielectric. N =Nu mber of dielectrics. S=Area of overlap of plates in square centi metres. d=Thickness of dielectric in centi metres.
269
PRACTICAL WIRELESS SERVICE MANUAL
Another Formula:
C --
AKN 4,500,000 d.
Where A =Area of one plate in square inches.
K =S.I.C. of dielectric.
N =Nu mber of plates minus one.
d-Thickness of dielectric in inches.
Capacity of Variable Condensers
C-- .0885NS 1,000,000 d.
Where N =Nu mber of moving vanes. S = Area of both sides of one moving vane in square centimetres. d = Air gap between moving vanes and fixed vanes in centimetres.
H.F. Transformer Ratio. (n) n2 = --,, o
R being the dynamic resistance of the tuned circuit and R, the A.C.
resistance of the valve.
Stability in Screen-Grid Stages.
One Stage.
Stable if
g Co is. less than 2.
ai(az -Fal))
Co =residual anode-grid capacity in farads.
=.001 x10-12 for Cossor S.G. Valves (all types).
= .0045 x10 --la for Cossor MS/Pen A.
= co n d ucta nce of grid and anode ·circuits respectively.
=1/R where R= dynamic resistance in oh ms.
av =anode filament conductance of valve.
=1 /R 0.
Two Stages.
Assu ming identical tuned circuits throughout, and ignoring damp-
ing efects of valves on tuned circuits.
Stable
4 It,
to
Cog
is less than
1.14
(Tuned
Anode).
a2
or if--43 Co gis less than 1·14n 2 (Tuned Transformer). a'
where a=conductance of tuned circuit (secondary)
and n=transformer ratio.
·In the case of transformer coupling, or its equivalent, replace as
! by n2a, where n=transformer ratio, a=conductance ( =- -I of
tuned secondary. 270
For D.C.
WIRELESS CALCULATIONS
OHMS LA W
For A.C.
I= E
I=-- where Z =impedance of circuit.
Watts dissipated =/ 2/2-- E/.
Capacity of Condensers in Paralel.
Watts dissipated = /2R = E Icos 0
where 0 =phase angle
between E and I
Capacity of Condensers in Series.
1 1 + --1 C, C,
-- C.· C, C, ±C,
Resistances in Parallel.
R=
1
Resistances in Series.
--1 + --1
R1
R2
R = R 1+ R
R iR 2 R I± R,
Resistance, Capacity and Inductance in Series. Resulting Impedance.
1
2 =
Z
N I R2 + (CO L
R2 + x2
Reactance of Coil. 2fL. is, 8.14; f, frequency; L, inductance in henrys 1
Reactance of Condenser' 2niC.
C, capacity in farads.
Net Reactance. X = X L-- Xc
At Resonance, f=
1 ,or
1
27c A/LC
LC
Wavelength. --.1884 A/ LC. A, in metres; L, in microhenrys; C. in microfarads;
xf =300,000,000. Resistance of aTuned Circuit at Resonance (Dynamic Resistance).
R= C xr
r being the equivalent series resistance. Magnification of Tuned Circuit. m-- ca-rL
271
PRACTICAL WIRELESS SERVICE MANUAL Current in Series Circuit at Resonance. I res.
where r is equiv. series resistance of circuit at wavelength concerned (high-frequency resistance).
Peak Separation (Band-pass Tuners).
v.2 m 2_r2
arrL
cycles (inductive coupling).
co=2'nf ;M, mutual inductance in henrys; r, equivalent series
resistance of tuned circuit; L, inductance in henrys.
p= N
1
Oa
--r 2
Cm 2
(capacity coupling). cycles
2nL C =coupling capacity in farads.
Inductance of Single Layer Coil. L =n 2n2D 21k +10 -- 2
L, in microhenrys ; r, 3-14; D, diameter in cms.; nu mber of turns to the cm.; 1, length in cms.; k, afactor depending upon the length/diameter ratio.
When -- =0-1 0-5 1-0 2-0
3-0
4-0
k =0-96 0-82 0-69 0-526 0-429 0-365
Resonant Frequency of Tuned Circuit.
1 or 0)2= 1
27r A,7LC
LC
Wavelength of Tuned Circuit at Resonance.
=1884 LC A. in meters; L, in microhenrys; C, in microfarads
xf =800,000,000. Resistance of a Tuned Circuit at Resonance (Dynamic Resistance).
= /. C xr
rbeing the equivalent series resistance.
Selectivity of Tuned Circuit. Voltage across tuned circuit
100 \411 +4m2(//_)
/ Where f--Frequency of resonance in cycles.
8f =Amount of detuning in cycles. m = Magnification of tuned circuit.
per cent. of voltage at resonance.
272
WIRELESS CALCULATIONS
Optimum Coupling between Two Coils. (For maximum signal). Vr ir, M = 27rf rir, are the H.F. resistances of the two circuits. f=Frequency of resonance. M =Mutual inductance between coils.
Coefficient of Coupling. k-- M _ VL ILi
Where M =Mutual inductance. L1= L2 Inductance of two coupled circuits.
18
273
CHAPTER XX XVII
COLO U R CO DES
M ANUFACTURERS of complete receivers often adopt a code for the wiring of the receiver so that their service engineers may more easily locate certain leads when testing the receiver.
By adopting a colour code for standardisation it becomes a simple matter for a service-man or experimenter instantly to identify components or other details in any make of receiver, and such items as mains transformers, resistors, condensers, fuses, and battery leads are
now also identified in this manner.
Resistance Codes.-- For fixed resistors the code consists of three
colours, and although these are someti mes given in the form of
three bands or dots on the
BLACK
RED
component, the standard way
.
.
which is most generally adopted is to colour the entire
,
ORANGE
Pio. 215a. Example of resistance colour coding. This isa20,000 ohm, component.
body of the component for the first colour, one of the tips for the second colour, and to display the third in the form of adot or band on the centre
of the body. This is the order in which the colours are read, and the
colours in each case stand for the figures from 0 to 9. The complete reference table is as folows:
Colour Black Brown Red Orange Yellow Green Blue
Figure 0 1 2 8 4 5 6
No. of Noughts None 0 00 000 0000 00000 000000
Violet
7
Grey
8
White
·· . 9
An example will make the scheme quite clear. Suppose we have a
resistance with a red body, a black tip, and an orange spot or band. Then the value will be 2 (red body), followed by one nought (indicated
by the black tip), folowed by three noughts, or, in other words, 20,000
ohms. If it is found that a resistance does not bear a dot or band
274
COLOUR CODES
on the body it indicates that the dot is of the same colour as
the body. Condenser Codes. --In the case of fixed condensers the code is
exactly similar, but the basis, instead of the ohm, is the micromicrofarad. Thus, a .001 mfd. condenser (which is the same as a 1,000 m mfd.) would be indicated by a brown dot followed by a black dot folowed by a red dot. In addition to small fixed condensers, however, a code is drawn up for multiple condenser blocks, provided with cable or flexible leads for connection. The following is the recom-
mended code:
The highest capacity positive voltage
·· Red
The second highest
do.
·· Yelow
The third highest
do.
·· Green
The fourth highest
do.
·· Blue
The fifth highest
do.
Violet
Principal negative connection
Second
do.
Third
do.
Black
·· Brown
.
·· Grey
Centre connection for voltage doubler condensers White
Where a condenser has two terminal wires only, they are red and
black (for positive and negative respectively), irrespective of the
capacity and voltage rating of the condenser. Where two capacities
are equal, the higher voltage shall be given the higher colour in the
table. In addition to the colours it is also necessary to mark condensers
in which two or more complete units are embodied, and for this purpose
the folowing marks are adopted:
Common positive junctions shall be marked .
,, negative Series connections shall be marked
+ ±
Unconnected Sections :
Thus 8+ 8denotes 2sections with aco mmon positive.
8-- 8 ,, 2 ,,
IP
,,
IP negative.
8 ± 8 ,, a series voltage doubler connection.
8 & 8 ,, 2 isolated sections.
Fuses. --Fuses for use in battery or mains receivers are also coded to indicate the value, the colour usually being shown by a strip of
coloured paper inside the glass tubular container. The code is as follows:
60 mA. . 100 mA. 150 mA.
250 mA. 500 mA. 750 mA.
Black Grey Red
Brown Yelow Green
1 amp. .. 1iamp. . 2 amp. . 8 amp. .. 5 amp. .
276
Dark Blue Light Blue Purple
White Black & White
PRACTICAL WIRELESS SERVICE MANUAL
Mains Transformers. --In the case of modern mains transformers
flexible leads are used for connection in place of ter minals, and
these leads are covered either with coloured cotton or passed through insulated sleeving with the appropriate colour embodied. The primary
has ablack lead for the 0 connection (one side of the pri mary), and for
the remaining tappings a combination of black and another colour is
employed. Usually the lead is striped for this purpose. Black and
green indicates 10 volts, black and yellow 210 volts, black and red
GREEN
230 volts, and black
A
and brown 250 volts.
BARE WIRE FOR SCREEN
LEAD -1kt,
GREEN & YELLOW GREEN
The screen some-
J times interposed between pri mary
FLACK & BRO W2N50 RED 23V0
· ,.YELLOW21!0_4
RED RED & YELLOW
and secondary is
generally a bare wire. For the secondaries red isemployed
for the high volt-
BLACK AND GREEN10V
R ED BRO WN
age, the centre tap
C
being red and yel-
BLACK o
BROWN & YEU_OW
low. The rectifier
Fin. 215B. Diagra m ilustrating the code for mains transfor mer leads in which A is the rectifier heater, B High Voltage secondary (red and red, yellow for Westinghouse or Half-wave rectifier), C Valve- Heaters, and D additional L.T. windings.
heater is green, with
BROWN
green and yellow
BLUE
D 1 centre-tap, and for
BLUE & YELLOW
the heaters brown,
BLUE
with a brown and yellow centre-tap is
used. Any additional L.T. winding is coloured blue with blue and yellow
centre-tap.
Battery Leads and Plugs. --In the case of combination leads for
battery voltages, or for the plugs which are fitted to the ends of such
leads, the code adopted to indicate the various tapping points is as follows:
Highest positive voltage (H.T.)
Red
Second
ditto
Third
ditto
Fourth
ditto
Low tension positive (L.T.)
Common negative (L.T., H.T., G.B.)
·· Yelow
·· Green ·· Blue ·· Pink ·. Black
Maximum G.B. negative .
· · Brown
Second
ditto
.
Grey
Third
ditto
.
. White
Any additional point, such as fourth greatest G.B. negative, or fifth
greatest H.T. positive, or positive bias is violet, and any centre-tap white.
276
Valve Base Connections for Standard (B.V.A.) Valves
See page 280, for definitions of abbreviations used.
Valve Type.
Pin Conneasons.
Base.
1
2
3
4
5
6
7
8
Triode
· ·
.
Triode
· ·
.
Triode
· ·
.
Tetrode .·
.
Tetrode .· ·· ·· .
H.F. Pentode, or Tetrode
H.F. Pentode, or Tetrode H.F. Pentode, or Tetrode
H.F. Pentode, or Tetrode
H.F. He:ode
.· ··
Output Pentode .
.
Output Pentode
Output Pentode
Output Pentode
.
Heptode .
.
Heptode .·
.
Octode
. ·
.
Octode
. ·
.
Class B Output
is Double-diode
-1 Double-diOde
.
-a Double-diode-triode
Double-diode-triode
Triple-diode-triode
Single-diode-tetrode Triode-pentode . Triode-pentode .
Triode-hexode
.
Triode-hexode
.
Double-pentode .
.
· ·
Double-pentode .
.
. ·
Double-diode-H.F. Pentode
· ·
Double-diode Output Pentode ·.
Double-diode Output Pentode ··
Rectifier, Half-wave
.
. ·
Rectifier, Half-wave (high voltage)
Rectifier, Half-wave
.
.
Rectifier, Half-wave (high voltage)
Rectifier, Full-wave
·· ··
Rectifier, Full-wave
·· .·
Rectifier, Full-wave
.
.
Rectifier, Full-wave
Baretter .
·· ··
C.R. Tuning indicator .
D.H.
4
I.H.
ID..HH..
7 4
I.H.
5
D.H.
I.H.
I.H.
7
I.H.
7
D. H.
7
D. H.
4
D.H.
5
LB.
5
I.H.
7
D.H.
7
I.H.
D.H.
7
I.H.
7
D. H.
7
D.H.
4
I.H.
5
D. H.
5
LB.
I.H.
9
I.H.
7
D. H.
9
I.H.
9
D.H.
I.H.
7
D. H.
7
D.H.
9
I.H.
9
I.H.
I.H.
7
D.H.
4
D.H.
4
I.H.
5
I.H.
4
D. H.
4
I.H.
4
I.H.
5
I.H.
7
4
7
A A
G2 G2 G2
A A A AO AO G2 G2 G2 DI DI A DI D2 G2 G2 AO AO GI' GI' G2 DI DI A
GI
GI
GI
GS
GI
GI
G3
A
GS
A
GS
GI
GI
GI
GI
G2
GO
G2
GO
G2
G
GS, G5
GI
G3, G5
GI
Al
D2
D2
D2
F,M
D2
D3
GI
G2
Al
GS
Al
G3
GO
G2
GO
G2
Al
Al
G2'
A
A
D2
D2
A
A
A
A
A
A
A
.C.T
Al
H, C H, C
Cl
Targ.
A
G2
C, G3
G2
G2
G4
G2
G2
A
A
A
M. GO
A
C, M, G6 A
A2
DI
A
A
Dl
C,
AO
GO
AO
GO
A
A
G2', G2'
A2
G2'
A2
C, G3
DI
D2
G2
A
G2
Cs A
Top
9
Cap.
A A A A A GI GI G2 G2 GI GI G4 G4
A GI GI GI GI Gli GI GI GI A A
PRACTICALIWIRELESS SERVICE MANUAL
4 P/N
2
S PIN
2
3 2
-1/" TYPE 2
5
4-
LOCATING M ARK
OCTAL
4
5
A CORN
LOCAT/NG MARK
DEAF A/D
CON TINENTAL
No/ TYPE
2
3
No 2 7koe
2
3
Fm. 216. -- Valvecontact arrangements for all types of valves. (See tables on pages
277, 279 and 2e0.)
DE4F 4/0
278
PRACTICAL WIRELESS SERVICE MANUAL
Valve Base Connections for Continental Valves
See Fig. 216
Type.
Pin Connections.
Base
Top
I
2
3
4
5
6
7 Cap
H.F. Pentode
.I
G2 H
H
C
A
G3 --
G1
H.F. Pentode
. 2 G2 H
HC
A
M
G3 GI
Triode
. ..
2 --
H
HC
A
M
G1 --
Double-diode
.
2 G2 H
H
C
D1 --
--
D2
Frequency Changer..
2 G2 H
H
C
A
AO GO GI
Output Pentode .
I G2 H
HC
A G1 G3 --
--
Output Pentode . 2 G2 H
HC
A
G3 GI --
Rectifier
.
.
1 Cl H
H
C2 A2 A I --
--
Rectifier
.
. 2 Cl H
H
C2 A2
-- A1 --
Valve Base Connections for Hivac Midget Valves
Type.
Base
Pin Connections.
1
2
3
4
6
Tetrode (S.G.) Triode .
1
A
1
G2
G1
Output Pentode
2
A
GI
G2
Top Cap
A
Mallard Universal Side-Contact Valves
Valve Type.
Contact Connections.
Base
I 2 3 4 5 8
7
Top
8 Cap
Triode ..
·
H.F. Pentode .
Output Pentode
.P M H HC -- -- --
AG
. P M H H C G3 --
G2
A GI
. P -- H H C, -- --
G2
A G1
G3
Octode .
. ·
Double-diode .
.P . V
MH D2 H
H C, G2 GI G3,
G6
G5
H C, M DI --
--
A G4 -- --
Double-diode
(alternative)
. V M H H C DI --
--
Rectifier, Half-wave . P -- H H C -- -- --
-- D2 A --
Rectifier, Voltage Doubling .
. P Cl H H C2 Al --
--
-- --
Visual Tuning Indicator
(TV 4;
.
.P
--
H HC
-- G Target A
--
279
PRACTICAL WI RELESS SE RVICE MANUAL
Abbreviations used in Table on page 277
I.H. --Indirectly heated.
D. H. --Directly heated.
M. --Metallising.
F. --Filament.
H. --Heater.
C, Cl, etc. --Cathodes.
A,A1, etc. --Anodes (Ao --Oscillator anode).
G or GI --Control grid (Go --Oscillator control grid).
G2 --Screening grid.
G3 --Suppressor grid.
D, D1, etc. --Diodes (anodes).
Targ. --Target.
Valve Base Connections for American (Octal) Valves
Valve Type.
Base
Pin Connections.
I2345 6 7
Top 8 Cap
Triode .
. I.H. Oct. M H A -- G -- H
C --
Triode .
. I.H. Oct. M H -- A -- -- H
CG
H.F. Pentode . I.H. Oct. M H A G2 G3 -- H
C GI
Output Pentode
I.H. Oct. M H A G2 G1 -- H
C --
Heptode
. I.H. Oct. M H A G3, GO AO H
C G4
G5 (G1) (G2)
Double-diode
I.H. Oct. M H D2 C2 D1 -- H
Cl --
Double-diode-triode I.H. Oct. M H A D1 D2 -- H
CG
Doubli Triode
I.H. Oct. -- H Al Cl G2 A2 H
C --
,, sep.caths. I.H. Oct. -- H Al Cl G2 A2 H
C2 Cl
Rectifier, Full-wave I.H. Oct. M H -- A -- A -- H. C --
Rectifier, Ful-wave I.H. Oct. -- H H A2 C2 Al -- H, Cl --
Rectifier, Ful-wave I.H. Oct. M H A -- A -- H
C --
Rectifier, Full-wave,
Gaseous .
Oct. M -- A -- A -- --
C --
Valve Base Connections for Acorn and Deaf-Aid Valves
Type.
Base
ACORN
Triode
.
1
H
H.F. Pentode..
1
H
DEAF AID
Triode (D. H.)..
2
A
Triode (I. H.) .
2
A
Triode (Mullard) 3
A
PUBLIC ADDRESS
Triode
.
4
A
Pin Connections.
2
3
4
5
op. otom
C
H
G
A
--
C
H
S
SG
A
G
G
F
F
--
C
H
H
G
F
F
F
G
F
--
280
PRACTICAL WIRELESS SERVICE MANUAL
VALV14:-LEG SPACING DIAGRAMS
217.--Valve-leg apscn for 4. End 5-pin valves. The diameter of the valve pins is 3.2 mm.
'e mote trio mid li07774 moot Fit. 218. Valve-leg spacing for 7-pin valves.
S M.
740
FIG. 219.-- Valve-leg spacing for 9-pin valves.
Fics.220.and 221.--The two diagrams above show the valve-leg spacing of the Hivac midget valves (see table on page 279)
281
Copper Wire Data
Resistance in Oh ms per
Yard.
Resistance in Oh ms per Pound.
Pounds per Ohm.
Weight in
Pounds per 1,000
Yds.
Yards per
Pound.
Turns per Inch.
Single Double Single Double
Ena mel Silk
Silk Cotton Cotton
Covered. Covered. Covered. Covered. Covered.
10
.128
.001868
.0120 83.3
148.8
6.67
11
.116
.002275
.0200 500
122-2
8.16
12
.104
-002831
.0280 35.7
98.22
10.23
13
.092
.003617
.0550 18.1
76.86
13 00
14
.080
.004784
-0820 12-2
58-12
17.16
15
.072
-005904
.1400
7.14
47.08
21.23
16
.064
.007478
.2021
4.95
37.20
26.86
15 0
17
.056
-009762
.3423
2.38
28.48
35.00
17.1
18
.048
.01328
-6351
1.56
20.92
47.66
19.8
19
.040
.01913
1.315
-757
14.53
68.66
23-7
20
-036
.02362
2.012
.497
11.77
85.00
26.1
21
.032
-02990
3.221
.309
9-299
107.6
29.4
22
-028
.03905
5.498
.181
7.120
1.40.6
33.3
23
.024
.05313
10.14
098
5-231
191.6
38.8
24
.022
.06234
14.38
.069
4.395
228-3
42.1
25
.020
.07653
21 08
0471
3-632
275.3
46.0
26
.018
.09448
32.21
0309
2.942
340.0
50.6
27
.0164 .11138
46.55
-0215
2.442
410 0
55.9
7.64 8.41 9.35 10.5 12.1 13.3 14.9 16.9 20.0 23.8 26.3 29.4 33.3 38.5 42.1 46.0 50.6 55.1
7.55 8.30 9.22 10.4 11.8 13.1 14-6 16-5 19.4 23 0 25-3 28.2 31.8 36-4 40-0 43.5 47.6 51.6
7.35 8.06 8-93 10.0 11.4 12.5 14.1 15.9 18.5 21.7 23.8 26.3 29 4
33.3 35.7 38 5 41.7 44.6
7.04 7.69 8-48 9.43 10.6 11.6 13.2
14.7 17.2 20-0 21.7 23 8 26.3 29.4 31.3 33.3 35.7 37-9
,Z
.ts 'AO
4. t,
es t ·-
Copper Wire Data
Resistante in Ohms per
Yard.
Resistance in Ohms per Pound.
Pounds Per Ohm
Weight in
l'ounds per 1,000
Yds.
Yards per
Pound.
Turns per Inch.
Single Double Single Double
Enamel Silk
Silk Cotton Cotton
Covered. Covered. Covered. Covered. Covered.
-
28
.0148
.1398
70.12 .0141
1.989
503.0
61-4
60.4
56.2
48.1
40.2
29
4)136
.1655
98.65 -0101
1380
596.6
66-2
65.2
60.2
51.0
42-4
30
-0124
.1991
142.75 .0069
1.396
716.6
73.3
72.0
67.1
54.4
44.7
81
4)116
.2275
185430 .0054
1.222
820-0
77.8
76.3
70.9
56.8
46.3
82
.0108
.2625
248.20 .0040
1359
943.3
834)
81-3
75.2
63.3
50.5
33
4)100
.8061
337.50 -0029
.9081
100
884)
87-00 80-0
66.7
5243
84
-0092
.3017
4714)0 4)023
.7686
1800
98-0
98.4
85-5
70-4
54.9
85
-0084
.4338
676.50 .0014
.6408
1556
106
101
91.8
80.6
614)
86
.0076
.5300
1009
.00098
.5254
1903
116
110
102
86.2
64.1
37
-0068
.6620
1574
-00064
.4199
2380
128
120
110
92.6
67-6
38
-0060
3508
2598
.000385
.3269
3056
143
133
121
100
71.4
39
.0052 1.132
4645
.000217
.2456
4066
168
149
134
109
75.8
40
.0048 1.328
6860
4)00156
.2092
4766
180
159
142
14
78.1
41
.0044 1.581
9020
-000112
.1758
5700
194
169
150
42
-0040 1.913
13150
.000076
.1453
6866
211
191
167
53
3036 2362
20120
-000050
.1177
7500
230
206
179
44
3082 2.989
82210
.000030
.0929
10766
253
225
192
45
.0028 3-904
54980
300015
3712
14066
282
247
208
ABBREVIATIONS S. W.G.--Standard wire gauge; B. W.G. -Birmingham wire gauge; D.C.C. -double-cotton covered;
S.C.C. -single-cotton covered; D.S.C. -double-silk covered; S.S.C. -single-silk covered. '
PRACTICAL WIRELESS SERVICE MANUAL
Wavelength-Frequency Conversion Table
Metres to Kilocycles
Metres.
Kilocycles.
Metres.
Kilocycles.
5 6 7 8 9 10 25 50 100 150
200 205 210 215 220 225 280 285 240 245 250 255 260 265 270 275 280 290 295 800 810 820 830 340 350 860
60,000 50,000 42,857 87,500 83,383 30,000 12,000
6,000 8,000 2,000 1,500 1,463 1,429 1,895 1,864 1,338 1,304 1,277 1,250 1,225 1,200 1,177 1,154 1,182 1,111 1,091 1,071 1,084 1,017 1,000 967.7 987.5 909.1 882.4 857.1 838-8
870 880 890 400 410 420 430 440 450 460 470 480 490 500 510 520 580 540 550 560 570 580 590 600 650 700 750 800 850 900 950 1,000 1,250 1,500 1,750 2,000
810.8 789.5 769.2 750 781-7 714-8 697-7 681.8 666.7 652.2 688.8 625 612.2 600 588-2 576.9 566 555-6 545 4 585.7 526.3 517.2 508.5 500 461.5 428.6 400 375 852.9 383.3 315.9 800 240 200 171.4 150
Note. -To convert kilocycle to wavelength in metres, divide 800,000 by the nu mber of kilocycles.
To convert wavelength in metres to kilocycles, divide 300,000 by the nu mber of metres. 1 megacycle = 1,000,000 cycles or = 1,000
kilocycles. Thus, 80,000 kilocycles = 80 megacycles. 284
INDEX
A Abbreviations, 9 Acoustic output tests, 243 A.C. /D.C. receiver faults, 132 -- valves, use of, 132 A.C. sets, voltage tests on, 200
voltage, measuring, 50 Aerial circuit filter, 240 -- connections of universal receiver,
134 du mmy, using, 98 Aligning receivers, 106 Amplifier, low frequency, improving,
130 Analysers, 16 Anode current tests, 119
value, 213 Anti-interference units, 150 Automatic volume control, 242 -- eli minating, 223 A.V.C., checking, 67
Background noises, 244 Band pass tuners, peak separation of,
271 trim ming, 107 Battery leads and plugs, colour code
of, 276 Beat note audibility, 97 Breakdowns, causes of, 156 Bridges, 18 Broadcast signal, testing of, 95
Code, colour, 274 Coil, connections, tracing, 207 -- H.F., modifying, 210
modifying, 209 reactance of, 271 windings, measuring resistance of,
205 Coils, adjusting, 204 -- oscillator, 137
simple tests for, 192 -- testing, 37, 204 Colour code, 274 Commercial receivers, servicing, 212 Components, simple tests for, 191
testing, 36 Condenser, capacity of fixed, 269 -- colour codes, 274 -- reactance of, 271 -- tests, 183, 193 -- values, 51 Condensers, breakdown of, 158
in parallel, 271 series, 271 -- and parallel, 268 -- smoothing, 62 -- testing, 39 -- testing fixed, 221 Connections, testing, 35 Copper wire data, 282 Coupling, co-eficient of, 273 Crackling noises, 33 Current Consumption of receiver, 133 -- tests, 198, 202 -- anode, 119 -- value, anode, 213
Cabinet scratch remover, 164 Cabinets, polishing, 163
renovating, 162 Calibrating apparatus, 40 Calibration, frequency, 169 Capacity of fixed condenser, formula
for, 269 -- variable condensers, formula for,
270 measuring, 55 -- values, 222 Capacities, stray, reducing, 110 Car interference, 153 Cathode ray tube, servicing with, 165 Chokes, testing, 37 Circuits, checking, 161 Class B stage, current consumption of,
199
285
" Dead "receiver, servicing, 212 Decoupling methods, 220 Detector diode, testing, 101 -- stage, distortion in, 143 -- stage, measuring voltage of, 160
valve, replacing, 121 Dial readings, checking, 229 Diode, testing, 101 Direction finding, 147 Distortion, 140, 230
in detector stage, 143 Du mmy aerial, using, 98 Earth connections of universal receiver
134 Electrical symbols, 11 -- units, 1
PRACTICAL WI RELESS SE RVICE M A N UAL
Electron beam deflection, 165 Emission testing, 116 Energised speakers, 248
Fault tracing, 31 Filter, aerial, 240 Formula, 268 Frequency calibration, 169
formula, 268 -- wave-length conversation
284 Frequencies, musical, 85 Fuses, blowing, 36
colour codes, 275
table,
Gang condenser connections, 102 Ganging, faults in, 227
intermediate frequency transformers, 137
-- on long wave band, 110 -- receivers, 106 -- superhet, 225 -- troubles, 223 -- uneven, 109
using meter for, 108 with cathode ray tube, 174 Generator, signal, 20, 89 Graphs, using for calibrating, 96 Grid bias voltage, measuring, 66 -- connections, broken, 219 -- tests, 32 Hand capacity, 127 Hartley, oscillators, 78 -- circuit, 89 High frequency response curves, 170 note cut of, 140 H.F. circuit, checking, 65 coil modifying, 210 coupling, too tight, 143 -- stage, replacing valves in, 121 -- resistance, 218 -- stage, faults in, 217 stage, testing, 36 -- transformer, formula, 270 Hum, in radiogram, 189 S.G. stage, 186 -- tracing, 180 Inductance, formula, 269 Inductive reactance, 269 Instability, 104, 220
286
Instruments, choosing, 13 general purpose, 15 types of, 13
Insulation, breakdown, 132 Inter-electrode insulation, 113 Interference, 102
caused by cars, 153 -- external, 148
mains, 155 -- second channel, 234, 236
tracing sources of, 146 Intermediate frequency, incorrect, 102 -- transformers, ganging, 137
Life test instruments, 21 ILIcal distance switching, 141 Long wave band, ganging on, 110 Loud speaker cones, faults in, 247
faults, 247 Loud speakers, energised, faults in, 218 Low frequency amplifier, improving,
130 oscillator, 78 -- calibrating, 83 -- overloading, 144 -- response curve, making, 87
Magnification of tuned circuits, 271 Mains hu m, eli minating, 235
interference, 155 -- transformers, colour code of, 276 Meter, universal, using, 61 Milliammeter, D.C. multi-range, 40 Millia mmeters, adapting, 159 Motor car interference, 153 Multi- meter, 14 -- -purpose instruments, 23 Musical frequencies, 85 Noises, crackling, 33
0 Oh ms law, 254, 271 Oscillation, 141 Oscillator coil, 137 -- testing, 102
tri m ming, 226 using, 136 Oscillators, Hartley, 78 Oscillographs, 18 Output valve, current taken by, 64 Overloading, avoiding, 237 -- L.F., 144
IN DE X
Pentodes, substituting for triodes, 119 Piano key board and frequencies, 85 Pick-up connections of universal re-
ceiver, 134 Plastic wood, repairing cabinets, 162 Point to point testing, 49 Polishing cabinets, 163 Portable apparatus, 22 Potentiometers, screening grid, 267 Prefixes, 1
Q.P.P. stage, 199
Radiogram, hum in, 189 Reactance, inductive, 269
net, 271 of coils, 271 -- condensers, 271 -- table of, 60 -- values, 59 Reaction, efective on one wave band
only, 126 -- faults, 123 -- faults and remedies, 128 -- insufficient, 124
partially efective, 127 -- too fierce, 126 Receiver comparison, 178 -- performance, checking, 135, 241 Receivers, old, improving, 129 Reception, weak, 215 Re-ganging, straight receiver, 225 Repairs, 156 Resistance, breakdown, 157
colour codes, 274 -- measuring, 48 -- of coil windings, measuring, 206
wattage rating, 255 Resistances in parallel, 271
series and parallel, 268 -- testing, 195 Response curve, making, 87 -- curves, high frequency, 170 -- tests, 243
Screened grid stage, hum caused in, 186 Screening grid, potentiometers, 267
voltage, 14,2 Screening leads, 190 Second channel interference, 234, 238 Selectivity, curves, 242 -- improving, 130
poor, 233 Self oscillation, 141 Sensitivity, checking, 241
Service equipment manufacturers, 82 Set analysers, 16 Short circuit, testing for, 36 Signal generator, 89
generators, 20 strength, weak, 34 Signals, lack of, 31 lack of, 215 Smoothing condensers, 62 Speech coil, testing, 247 Straight receiver, ganging, 225 Stray capacities, reducing, 110 Substitution, 158 Superhet, converting to straight cir-
cuit, 100 -- peculiarities, 233
reganging, 225 -- tracing faults in, 99 Superhets, ganging, 106, 110 Switching defects, 210 -- local distance, 141 Symbols, 10, 11
Test signal, obtaining, 136 Time based system, 167 Tone correction, 120 Tracking adjustments, 228 Transformer, breakdown, 157
intermediate frequencies, ganging, 137
ratio, H.F., 270 Transformers, simple tests for, 192 -- testing, 37 Theoretical symbols, 10 Trimming, efect on wave range, 109 -- oscillator, 226 -- receivers, 106 Tuned anode coupling, hum caused by,
188 circuits, wave length of, 269 Tuning condenser, noises caused by, 246 -- flat, 127
Units, electrical, 11 Universal instruments, 23 -- meter, using, 61 -- receiver, aerial-earth connections
of, 134 -- receiver faults, 132 -- receiver, pick up connections ot
134 V
Valve base connections, 277 -- detector, replacing, 121 -- H.F., replacing, 121 -- leg spacing diagrams, 281 287
PRACTICAL WI RELESS SE RVICE MA NUAL
Valve pins, connections for, 278 -- replacement, 118 -- tester, 13 -- testers, 22 -- voltmeter, 14, 68 -- voltmeter, calibrating, 73 -- -- using, 76 Valves, breakdown of, 158 -- loss of eficiency, 18 -- testing, 113, 195, 214 Variable condenser, capacity of, for-
mula for, 270 Voltage dropping resistance, 133 -- grid bias, measuring, 06 -- screening grid, 142 - - tests, 197 -- tests, on A.C. sets, 200
Voltages, A.C., measuring, 50 Volume control, adding, 131 -- controls, testing, 38 Wattage consumption of receiver, 133 Watts dissipated, 271 Wave length range finding, 206 -- formula for, 271 -- frequency conversion table, 284 -- of tuned circuits, 269 -- of tuned circuit, 272 Wave trap, 239 Wire data, 282 Whistling noises, eliminating, 234 Workshop, Equipping, 251
akin tCYL A181. Th
LNOVERSITV
LA WRY OS MANCRES4PFS
288
The ANS WER
TO EVERY SER VI CI N G
PR OBLE M
Also available: Model 7 Resistance Range Extension Unit (for measure-
ments down to 1-100th oh m.
BRITISH MA DE
A FULL RA N GE OF TEST METERS
No service man, service depart ment or laboratory need be without the advantages of simple yet highly accurate test meters. The 'AV O' range also includes:
The 40 Range Universal 'AvoMeter' The Universal 'Avo Minor' (22 ranges) The D.C. 'AvoMinor' (13 ranges) The High Resistance 'Avo Minor' The 'AV O' Valve Tester The All- Wave 'AV O' Oscillator The 'AV O' Test Bridge The 'AV O' Bonding Meter The 'AV O' Low Resistance Oh m meter
SERVICING difficulties are halved and complications vanish before the unfailing
reliability, accuracy and simplicity of the Model 7 Universal AvoMeter. A single instru-
ment, yet, at the touch of two simple switches, it
provides for any one of 46 ranges of measurement --A C.
or D.C. Current and Voltage, Resistance, Capacity, AudioFrequency power output, and Decibels --everything that the serviceman or technician demands. Entirely self-contained, essentially portable (it weighs only 6 lbs.), it is, above all, simple to use. On the bench or out on a job," it is always ready to find the answer to
every electrical test. Designed to simplify (and render foolproof) testing under the difficult conditions of servicing, the meter is fully protected by an automatic cut-out against damage from over-load.
Ae 46-RANGE UNIVERSAL
Write for fully descriptive literature and current prices of
'Avo' Test Instrurnrnts.
Electrical Measuring Instrument
7
Sole Proprietors and Manufacturers:
THE AUTOMATIC COIL WINDER & ELECTRICAL EQUIPMENT CO., LTD.
Winder House, Douglas Street, London, S. W.1.
Te/ephone: VICtoria 3404-7
1)
'2Si 9
Building aNew Set? Servicing an Old?
then you'll be needing ERIE
High Resistance Products
Nothing is skimped in the manufacture of Erie Resistance Products. Only the finest raw materials, that are so essential for efficient operation, are used. Even In the face of rising production costs, cheaper substitutes, that are "al most as gooa", don't find their way Into Erie Products. That's why their performance has been steadily Improved through the years.
That's why to-day they give unparalleled all-round balanced performance. That's why you will find them used by all the leading manufacturers, who recognize the advantages of using only quality component parts.
CARB O N RESIST ORS.
V OL U ME C O NTR OLS.
TO NE C O NTR OLS.
VITRE O US EN A MELLE D WIRE- W O U N D RESIST ORS.
SUPPRESS ORS
CER A MIC DIELECTRI C C O N DE NSERS
ERIE RESISTOR LI MITED
CARLISLE RD., THE HYDE, LO N D O N, N. W.9. Tel.: Colindale 8011
ELECT R A DI X B A RG AI NS
GALV A N O METERS FOR ALL PURP OSES Horizontal Table Galvos by Siemens, Silvertown, etc.; brass body, glass top, 7;6. Vertical Telegraph type for circuit testing. S 6. Lab. Galvos. vertical ballistic moving coil, with mirror by Tinsley, 60,-. Brass Cased Lab. Galvos, by Paul, Onwood, moving coil. 90,'-. A.G. Galvos, moving coil, LS. Sullivan, Marine moving coil Galvos, LS. Multiplyer shunts, 15 -to £3 10s. Large range of other Lab. equipment.
Oscillators for Radio Set Testing. We have a nu mber of types in stock from time to time at low prices. Radio Outfit Test Set, complete in Case with five Multipliers, £4 10s.
BATTERY CH ARGI N G on A.C. Mains.
"NIT N D AY" Metal Rectifiers --Steel Chassis. Model N, A 21. Radio Home Charger. To charge 2volts amp., 12 6. Model N A 6i, Trickle Charger. To charge 6volts, amp., 17 6. Model N ,B6,1, Car Charger. To charge 6volts, 1amp ,24/-. Model N B6,1I, Car Charger. To charge 6volts, 1 amps., 27/6. Model N/C6/2, Car Charger. To charge 6volts 2amps., 37 -. Model N/1312/1, H.M. Car Charger. To charge 12 volts 1 amp., 39/-. Model NI 012/2, N.K. Car Charger. To charge 12 volts, 2 amps., $51-. Model N/E2, Doubler. Meter Car Charger. To charge 6 volts and 12 volts, 2 amps., 65,-. All Models for A.C. 200 250 volts input. MEG GERS & O H M METERS. Evershed Bridge with decade res. box 10,000 oh ms. Meggers, 100 volts, 250 volts, and small MEG. for D.C. mains. N.S.C. Oh mer. 500 volts to 20 megs. Ev. Edg. Metrohm, 250 volts, .01 to 20 megs. Silvertown Portable Test Set. Bridge type, .001 ohm to 1 meg.
Write or 'phone for further details and prices of our wonderful bargains.
Send 2d. stamp for a copy of our latest Sale List "P.S."
T U N G AR C H ARGERS. Two types of these famous sets. One for 70 volts, 6 amps., with meters and controls, etc., will handle 100 cells a day. £7 17s. 6d. Another fine Tungar for two 5 amp. circuits with meters and variable volt controls. 75 volts 10 amps. for 200 cells, Bargain. £12 15s. Morrison Valve. Rectifier Sets for Arcs, or Charging. 230 A.C. to D.C. 60 volts 12 amps., £14.
ROTARY C H ARGERS. We carry a good range of these. A.C. and D.C. Prices from £4 17s. 6d.
Write for Charger List "P.C." TEST METERS, LAB. BRIDGES, GALV OS AND SCALES, RELAYS,
CONTACT ORS, FLOAT SWITCHES, MOTOR PUMP SPRAY COMPRESSORS, RHE OSTATS AND
SWITCHB OARDS.
ELECTRADI X RADI OS
218 UPPER TH A MES ST, LO N D O N, E.C.4
Telephone: Central 4611
2911
RADIO BOOKS BY F. J. CAMM
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RADI O ENGINEER'S VEST POCKET BOOK
Contains in easily-consultable form nearly every fact, figure and formula
which service engineers, students, circuit designers, radio-operators,
transmitters, constructors and manufacturers require. It deals with Valve
Symbols, Wavelength of Tuned Circuit, Inductive Reactance, Capacity of
Variable Condensers, H.F. Transformer Ratio, Resistances in Parallel,
Condensers in Parallel and Series. Resistances in Series, Capacity and
Inductance in Series, Reactance of Coil, Wavelength Formula, Valve Base
Connections, Reflector Aerials, Long-wave Coil Data, Medium-wave Coil
Data, Short-wave Accumulator Data,
Coil Data, Transformer Data, Miscellaneous International
3/Crkystal
Combination, NET
Abbreviations, etc., etc.
%.0 (By post 3,9)
THE SUPERHET MAN UAL
Just published, this useful book for your wireless library deals with Funda-
mental Principles of Radio. Problems of Selectivity, Valve Fundamentals,
the Principles of the Superhet, General Design, Aerial Design, Variable
Selectivity, Noise Suppression, and A.V.E. Superhets with the Cathode-ray Tube, etc.
Tonesi.Contr(Nol,ETpostServ5/i6c)ing
RADI O TRAINING MANUAL
FOR THE SERVICES AND THE TRADE
This immensely helpful book will aid you in adopting one of the important
branches as a profession. The contents are comprehensive and deal with
Radio as a career (including Radio in the Forces, Production Servicing,
Laboratory, etc.), an Outline of Prospects, Electrical Units Explained,
Radio Formulae Simplified, Principles of Receiver Design, a Guide to
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ri
NET
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'291
IMPORTANT RADI O BOOKS
WIRELESS TRANSMISSI ON
FOR AMATEURS
By F. J. CA M M
A book which covers fully and clearly the whole field of wireless transmission for the amateur enthusiast, to the construction and operation of a transmitter. With 120 ilustrations.
5,- net (By post 5 6).
THE
MATHE MATICS
OF WIRELESS
By RALP H STRA NGER
This Wireless writer makes even
the most abstruse topic interesting
--and amusing! The book deals with: Arithmetic, Algebra, Powers
and Roots. Diferential Calculus. Integral Calculus. Trigonometry.
The Slide Rule.
Logarithms.
Curves and Graphs in Wireless.
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THE PRACTICAL
WIRELESS ENCYCL OP/E DIA
By F. J. CA M M
Forms acomplete guide, in alphabetical order, to the construction, repair and principles of every type of wireless receiver; including formula and complete instructions on the making and testing of various wireless components. 400 pages. 484 ilustrations.
10/6 net. (By post 1/-).
THE OUTLINE OF WIRELESS
By RALP H STRA NGER
(On the Official Admiralty List, and recommended to Wireless Operators
in the Navy)
Covers completely the theory and practice of Wireless Reception. The contents include: Matter and Energy, Magnetism and ElectroMagnetism, Batteries and Accumulators, Wireless Communication and Broadcasting, Modern Valves, Wireless Receiving Circuits. 832 pages. Over 500 ilustrations. 10 6 net (By post 1/3)
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COMPONENTS of QU ALI TY
TUNING COILS I. F. TRANSFOR MERS L. F. TRANSFOR MERS L. F. CH OKES PO WER TRANSFOR MERS THER MAL DELAY SWITCHES
The House of Varley has for many years enjoyed a reputation for supplying products of the highest quality. This same high standard of workmanship and material is being maintained to-day. Fit Varley components and oe sure of satisfaction.
Wiley
Write for Catalogue giving details of components suitable for service work.
Varley (Oliver Pell Control Ltd.), Ca mbridge Row, W ool wich, S.E. 18
Tel.: W O OL wich 1422.
20:3
AN IMPORTANT CONTRIBUTION TO THE LITERATURE OF RADIO
NE WNES SH ORT- WAVE
MA N U AL
Edited by F. J. CAM M
This complete work on the whole subject of shortwaves is an important contribution to the literature of radio. Itdeals thoroughly with the special problems underlying the design of short-wave apparatus, the special circuits which yield the best results, and designs for receivers based on those circuits. It covers the whole subject of short-wave aerial systems, eliminating interference, bandspread tuning, H.F. amplification, the morse code, aerial couplings, coil design, measuring wavelengths, the ultra-short-waves, and concludes with designs of several satisfactory shortwave receivers. Many useful tables are included.
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294
AN IMPORTANT NE W BOOK FOR ALL TECHNICIANS
DICTI ONARY OF METALS
AND THEIR ALLOYS
Edited by F. J. CAM M
This valuable work --the only one of its type --deals with every known Metal and Alloy: It gives the physical characteristics and historical facts, as well as details of the purpose to which most of the metals may be employed. Special sections deal with hardening and tempering, polishing and finishing metals, chemical colouring, electro-plating, etc., etc., and useful tables of sheet metal and wire gauges. It is a book which every engineer needs, and arrives at a time when the engineering trade is engaged in working in almost every known type of metal. The alphabetical arrangement of the contents enables the book to be rapidly consulted.
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2!)5
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EVERYMAN'S WIRELESS BO OK
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SIXTY TESTED WIRELESS CIRCUITS
By F. J. CA M M
Modern circuits of every type. Diagrams and instructions for assembling and wiring. Details of components and notes on operation. Including Chapters on Circuits for Battery and Mains- Operated Receivers. Short- Wave Receivers. All- Wave Receivers, etc., etc.
3 6 net (By post, 4,-)
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By F. J. CA M M
Chapters on Coils: Types and Principles. Resistance, Inductance, and Capacity. Choice and Use of Coils. A simple Tubular Coil. Screened Coils: Materials and Construction, Circuits, A Band Pass Unit, Ganging and Switching. Screeled Superhet Coils. Short- Wave Coils. Screened Short- Wave Coils. Adjusting and Testing Coils, etc., etc.
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PRACTICAL WIRELESS SERVICE MANUAL
F.J CA M M
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