M820 00 100 T800/T800 SERIES 1 MANUALS/M820 100/B5.1 B5.14 B5.1

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T825 Fault Finding

M820-00

5

B5.1

T825 Fault Finding

The following test procedures and fault finding flow charts may be used to help locate a
hardware problem, however they are by no means a complete fault finding procedure.
If the fault still exists after having progressed through them in a logical manner, contact
your nearest authorised Tait Dealer or Service Centre. Further assistance may be
obtained from the Customer Support Group, Radio Infrastructure Division, Tait Electronics Ltd, Christchurch, New Zealand.
Refer to Section 6 where the parts lists, grid reference index and diagrams will provide
detailed information on identifying and locating components and test points on the
main PCB. The parts lists and diagrams for the memory and VCO PCBs are in Part E.
The following topics are covered in this section.

Section

Title

Page

5.1

Visual Checks

5.3

5.2

Component Checks

5.3

5.3

DC Checks

5.4

5.3.1

Power Rails

5.4

5.3.2

VCO Locking

5.4

5.3.3

Mute Operation

5.4

RF Checks

5.5

5.4.1

VCO Frequency

5.5

5.4.2

RF Sensitivity

5.5

5.4.3
5.4.3.1
5.4.3.2

Oscillator Stability
TCXO
Second IF

5.6
5.6
5.6

5.4.4

Demodulator Output

5.6

5.4.5

IF Distortion

5.6

Fault Finding Charts

5.8

5.5.1

Regulator

5.8

5.5.2

Synthesiser

5.9

5.5.3

Noise Mute

5.11

5.5.4

Carrier Mute

5.12

5.5.5

Receiver

5.13

5.5.6

Audio

5.14

5.4

5.5

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B5.2

T825 Fault Finding

Figure

M820-00

Title

Page

5.1

RF Test Cable

5.5

5.2

IF Swept Response

5.7

5.3

Ceramic Filter Swept Response

5.7

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T825 Fault Finding

M820-00

5.1

B5.3

Visual Checks

Remove the covers from the T825 and inspect the PCB for damaged or broken components, paying particular attention to the surface mounted devices (SMDs).
Check for defective solder joints. If repair or replacement is considered necessary, refer
to Section 3 of Part A.

5.2

Component Checks

If a transistor is suspected of faulty operation, an indication of its performance can be
assessed by measuring the forward and reverse resistance of the junctions. First make
sure that the transistor is not shunted by some circuit resistance (unless the device is
completely desoldered). A 20k ohm/V or better multimeter should be used for taking
the measurements, using only the medium or low resistance ranges.
The collector current drawn by multi-junction transistors is a further guide to their performance.
If an IC is suspect, the most reliable check is to measure the DC operating voltages. Due
to the catastrophic nature of most IC failures, the pin voltages will usually be markedly
different from the recommended values in the presence of a fault. The recommended
values can be obtained from either the circuit diagram or the component data catalogue.

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30/04/96

B5.4

T825 Fault Finding

5.3

DC Checks

5.3.1

Power Rails

M820-00

Refer to the test points & options diagrams for test point locations, and to the regulator
fault finding chart (Section 5.5.1) for fault diagnosis.
Check the 9V (TP2) and 13.8V (TP1) power supply test points in the audio compartment with a DMM.
Check the 20V regulator output at the test point (TP3) in the regulator compartment.
Check the 5V regulator output at the test point (TP4) in the regulator compartment and on pin 4 of IC303.

5.3.2

VCO Locking
Using a DMM, monitor the VCO control voltage at PL3-1 or the junction of L1 &
R1 on the VCO PCB.
If the synthesiser is locked and the VCO aligned, the voltage at this point should
be between 5 and 13V.

If the VCO is not locked, refer to the synthesiser fault finding chart (Section 5.5.2).

5.3.3

Mute Operation

The front panel LED will show the status of the mute circuitry. It will be lit when a signal is received above the threshold level. It should always be possible to open the mute
gate by rotating the mute potentiometer fully clockwise, or by enabling the monitor
with the front panel switch.
If the mute fails to operate correctly, refer to the mute fault finding chart (Section 5.5.3).

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T825 Fault Finding

M820-00

5.4

RF Checks

5.4.1

VCO Frequency

B5.5

Check that the VCO is phase locked (refer to Section 5.3.2).
Connect a frequency counter (level +17dBm) to the VCO input to the mixer
(IC301).
Monitor the local oscillator frequency and check that it is 10.7MHz above the
required receive frequency.
Refer to the synthesiser fault finding chart (Section 5.5.2) for further information.

5.4.2

RF Sensitivity
Ensure that the VCO is on the correct frequency and the receiver correctly aligned.
Check that the sensitivity into the front end is typically:

Frequency Range

Wide Band

Narrow Band

66-76MHz

-119dBm

-120dBm

76-88MHz

-118dBm

-119dBm

If the sensitivity is poor, the fault can be traced by measuring the sensitivity into
successive circuit blocks. Prepare a test cable by connecting a 1nF capacitor to the
end of a length of coax cable as shown in Figure 5.1.

1nF

To Test Cable

Coax

Connect To Earth
Figure 5.1 RF Test Cable

Before using the test cable, ensure the coax braid is connected to an earth
point on the PCB.

Note:

Using the RF test cable, apply a modulated 10.7MHz signal to the test breaks in
the IF section, or an on-channel RF signal to the front end test breaks.
Check that the sensitivity at each test break is within 2dB of the levels shown on
the circuit diagram.

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B5.6

T825 Fault Finding

M820-00

Poor sensitivity indicates a fault in one of the circuit blocks following the test break.
Poor sensitivity into the mixer can be caused by lack of drive level from the
VCO (the drive level should be >+17dBm).

Note:

Refer to the receiver fault finding charts (Section 5.5.5) for further information.

5.4.3

Oscillator Stability

5.4.3.1

TCXO
While maintaining a low level unmodulated RF input to the receiver, loosely couple into the first IF an additional high level signal at 10.7MHz - a constant low frequency beat note should be heard.
Tap the TCXO with a finger and replace it if the beat note permanently changes.

5.4.3.2

Second IF
While maintaining a low level unmodulated RF input to the receiver, loosely couple into the second IF an additional high level signal at 455kHz - a constant low
frequency beat note should be heard.
Adjust the frequency of the 455kHz signal for zero beat.
If the second IF is more than 300Hz off frequency, check IC302, X301, C361 and
C362 and replace if necessary.

5.4.4

Demodulator Output
Apply an on-channel RF signal modulated by 1kHz with 3kHz [1.5kHz] deviation
at an amplitude of -65dBm.
Connect an oscilloscope probe (DC coupled) to IC303 pin 7 (audio output).
Check that an audio signal of approximately 380mV peak to peak is present.

Optimum tuning of the quad coil (L319) for minimum audio distortion (with a "flat"
audio response) should coincide with maximum audio amplitude and a DC level of
approximately 1.3V.

5.4.5

IF Distortion

If after careful IF alignment (Section 3.6) the audio distortion is still high or there is
irregular 1kHz ripple (refer to Figure 3.3), the IF should be swept to investigate the
bandpass response.

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T825 Fault Finding

M820-00

B5.7

Apply an on channel RF signal modulated at 10Hz with 12kHz [6kHz] deviation
at an amplitude of -65dBm.
Connect the modulating 10Hz audio signal to the "X" input of an oscilloscope and
observe the 455kHz IF input to IC303 pin 16 via a suitable RF probe on the "Y"
input.
The X input should be DC coupled.

Note:

Check that the swept response has a rounded top and no sharp non-linearities
(refer to Figure 5.2).

Figure 5.2 IF Swept Response

Increase the RF level to -30dBm; the trace will now show the shape of the 455kHz
ceramic filter (&XF303).
Check that the response has no sharp non-linearities.
If sharp non-linearities do occur, replace the filter and sweep to confirm a satisfactory solution (refer to Figure 5.3).

Figure 5.3 Ceramic Filter Swept Response

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B5.8

T825 Fault Finding

5.5

Fault Finding Charts

Note:

The standard test point designations as used in this section are as follows:
TP1
TP2
TP3
TP4

5.5.1

M820-00

13.8V
9V
20V
5V

Regulator
Check for
overheating
components &
short circuits.

13.8V present on
IC202 pin 3?

N

Check continuity
from D-range
connector pin 9.

Y

N

5.3V present on
IC202 pin 1?

Replace IC202.

Y

9.0V present
on TP2?

N

Check Q200,
Q201 & IC201.

Y

20V present
on TP3?

30/04/96

N

Check Q202 is
switching at
approx. 180kHz.

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T825 Fault Finding

M820-00

5.5.2

B5.9

Synthesiser
A (Refer to VCO circuit diagram)

B (Refer to synth. circuit diagram)

No VCO RF output.

9V present at
VCO Q4/Q5?

VCO out of lock.

N

+5V present at
TCXO, IC3, IC5,
IC6, IC1 & EPROM?

Check 9V reg.

N

Check 5V reg. &
supply resistor
to each IC.

Y
Y
DC V present
at Q4 & Q5?

Check biasing
resistors, Q4 & Q5.

N

N

20V present
on IC4 pin 8?

20V inverter
supply faulty.

Y
Y
8.3V present
at VCO?

Check Q4 C
multiplier
(synth).

N

N

9V present on
Q1 divider buffer?

Check 9V supply.

Y
Y
DC V present at
VCO, Q1, Q2 & Q3?

Check biasing
resistors, Q1,
Q2 & Q3.

N

Y
Capacitors OK?
Check for cracks,
shorts, etc.

N

DC V present
on Q1?

Check biasing
resistors & Q1.

Y
N

Check TCXO
& reference
divider path.

N

Correct ref.
on IC3 pin 8?

Replace if faulty.

Y

Y

Prescaler output
freq. present on
IC3 pin 4?

Replace VCO.

N

Check at IC1
pin 3; replace
IC1 if no output.

Y

Check VCO tuning
caps & trimmer.

N

N

VCO trimmer
freq. range OK?

Does VCO trimmer
adjust for stable
control line
between 5 & 13V?

Y
Y
Check &C2, &C3, &C4,
VC1 & D1-D6 in VCO.

N

Is 8MHz VCO range
achieved between
5 & 13V?

VCO freq. correct?

N

Go to C.

Y
Y
VCO OK.

N

Y

Check signal
path from VCO
to prescaler.

N

Does VCO trimmer
adjust for required
frequency range
between 5 & 13V?
Y

Does IC1 pin 3 O/P
freq. change with
VCO trimmer when
VCO not locked?

Synthesiser OK.

Y
Check synth. data
loading ccts;
change EPROM;
change synth. IC3.

N

Does IC3 pin 1
PDA change level
at the programmed
synth. freq?
Y

Check loop filter
cct around IC4.

N

Does control line
O/P level change
on IC4 pin 1?

Y

Check continuity
between VCO &
IC4 pin 1 O/P.

Synthesiser OK.

Continued on next page.
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30/04/96

B5.10

T825 Fault Finding

C (Refer to synth. circuit diagram)

VCO locked on
wrong freq?

M820-00

D (Refer to synth. & VCO
circuit diagrams)

Noisy synth; poor
selectivity &/or
poor residual FM.

Y
Correct channel
selected?
(switches on
external lines)

N

Set correct
channel.

N

Repair tracks.

Go to B.

Correct DC V
present on Q1 VCO?

N

Check inductors,
biasing resistors
& Q1.

Y

Y
Does toggling
each of the 3 LSB
DIP switches
change the freq?

N

Y

Y
Continuity OK
between EPROM &
IC3 on 3 address
& 4 data lines?

VCO phase locked?
5-13V present on
control line?

N

Y

Check for edge
transition on
IC3 pin 14 for
3 LSB DIP switch
operation; if wrong,
check C15, C16 & C2,
C4, C6 on EPROM PCB.

Loop filter &
components around
integrator op. amp. OK?

N

Replace if faulty.

Y
Change trimmer;
still noisy?

N

Y
Does freq. update correctly
when supply
toggled off & on?

N

Check presence
of low level on
IC3 pin 14 for
>200ms on switch-on;
change IC3.

Change Q1;
still noisy?

N

Y

Y
Substitute EPROM
with known good
unit; is freq.
wrong?
Y
Replace IC3;
synthesiser OK.

Change TCXO;
still noisy?
N

Provide new EPROM.

N

Y
Change varactors;
still noisy?

N

Y
Replace VCO
assembly.
Y
Synthesiser OK.

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T825 Fault Finding

M820-00

5.5.3

B5.11

Noise Mute
Ensure PL104 is set
for noise mute operation.

Apply on channel signal
@ -117dBm* to receiver.

Y

Rotate mute pot. RV100
c/w & cc/w & view LED.

Audio on
IC303 pin 7?

N

Noise on
IC304 pin 7?

N

*Modulated 3kHz [1.5kHz]
deviation @ 1kHz.

Refer to Rx
fault finding
chart.

Check components
around active filter.

Y
Does LED
toggle on/off?

N

Amplified noise
on C386?

Y

N

Check Q306
& Q307.

Y

Switch SW100 to ensure
mute is not disabled.

N

Approx. 0.6V DC rect.
noise on pad 114?

Check D301 is
rectifying noise.

Y

OK

Y

Check Q103, Q105 &
Q106 for operation.

Vary RF level &
observe voltage
on pad 114.

Mutable audio
on outputs?

Pad 114 voltage
varies 0-1.8V?

Check adj.
of RV301
(gain adj.).

N

Y

N
Refer to audio
fault finding chart.

Adjust mute
pot. RV100 &
observe voltage
IC100 pin 3.

Voltage varies
0-2.5V?

Check 9V rail &
resistors R149,
R100 & R101.

N

Y
Set RF signal level to
-117dBm. Rotate mute
pot. RV100 c/w & cc/w.

Monitor voltage on
IC100 pin 1. Switching
0-8V as pot. rotated?

N

Replace IC100.

Y
Monitor voltage on
IC100 pin 7. Switching
0-8V as pot. rotated?

N

Check/replace
IC100 & D100.

Y
Monitor voltage on
pad 101. Switching
0-8V as pot. rotated?

N

Replace Q100.

Y
Y

Copyright TEL

Does LED toggle?

N

Check/replace
Q102 & D102.

30/04/96

B5.12

T825 Fault Finding

5.5.4

Carrier Mute

M820-00

Ensure PL104 is set
for carrier mute operation.

Apply on channel signal
@ -95dBm* to receiver.

Audio on
IC303 pin 7?

*Modulated by 1kHz signal with
3kHz [1.5kHz] deviation.

N

Refer to Rx fault
finding chart.

Y
Approx. 3.3V on
IC303 pin 5?

N

Check for short circuits
and/or replace IC303.

Y
Rotate carrier mute
pot. RV104 c/w &
cc/w & view LED.

Y

Does LED
toggle on/off?
N
Approx. 5V on
RSSI pad 115?

N

Check IC304 and
adjoining components.

N

Check/replace IC104
& adjoining components.

Y
Switch SW100 to ensure
mute is not disabled.

Approx. 5V on
IC104 pin 1?
Y
Adjust carrier mute
pot. RV104 &
observe voltage
on IC104 pin 6.

Check Q103, Q105 &
Q106 for operation.

OK

Y

Mutable audio
on outputs?

Voltage varies
0-6V?

Check 9V rail &
resistors R154,
R156 & RV104.

N

N
Y
Refer to audio
fault finding chart.

Set RF signal level
to -95dBm. Rotate
carrier mute pot.
RV104 c/w & cc/w.

Monitor voltage on
IC104 pin 7. Switching
0-8V as pot. rotated?

N

Replace IC104.

Y
Monitor voltage on
IC100 pin 7. Switching
0-8V as pot. rotated?

N

Check/replace
IC100 & D100.

Y
Monitor voltage on
pad 101. Switching
0-8V as pot. rotated?

N

Replace Q100.

Y
Y

30/04/96

Does LED toggle?

N

Check/replace
Q102 & D102.

Copyright TEL

T825 Fault Finding

M820-00

5.5.5

B5.13

Receiver
A
Poor sensitivity:
worse than -117dBm.

Ensure VCO RF
& IF correctly
aligned.

B
High audio
distortion.

High audio
distortion?

Note: T.B. = Test Break
(refer Rx circuit diagram)

Y

See Chart B
(below).

N

Quad coil L319
tuned for min.
distortion (flat
audio response)?

DC conditions OK?
(see cct diagram)

Adjust L319.
Retune all
IF coils.

N

N

Replace
faulty device.

Y

Y
VCO frequency OK?

N

Check sensitivity
into each stage
(see 5.4.2).

Readjust TCXO
frequency.

Y
IF alignment OK?
(see 5.4.5)

N

Check/repair faulty
stage or component.

Adjust IF coils.

Y
Distortion at Rx
audio pad 111 OK?

Y

Fault find audio.

N
Inject 10.7MHz signal @
-15dBm into 10.7 IF pad.
Is distortion 3% or less?

N

Y

2nd IF
frequency OK?
(see 5.4.3.2)

Check/replace
IC302, X301,
C362 & C361.

N

Y
Sweep shape of
whole IF (see 5.4.5).
OK?

N

Replace ceramic filter.

Y
Fault in IC303 or
surrounding circuitry.
Check/replace.

Inject 10.7MHz into T.B.
after diplexer & measure
distortion. OK?

N

Check sensitivity
into each IF stage.

N

TCXO stability OK?
(see 5.4.3)

Repair stage.

Y
Inject on-channel RF
into mixer RF input T.B.
& measure distortion.
OK?

Y

Refer to synthesiser
fault finding chart.

N
Y
Replace TCXO (IC2).
OK.

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30/04/96

B5.14

T825 Fault Finding

5.5.6

Audio

M820-00

Apply on channel signal
@ -117dBm* to receiver.

*Modulated 3kHz [1.5kHz]
deviation @ 1kHz.

Refer to Rx
fault finding
chart.

N

Audio on
IC303 pin 7?
Y

N

Audio on
IC101 pin 1?

Check components
around IC101.

Y
Audio at
top of RV102
& RV103?

N

Replace C112.

Y
Ensure mute
pot. fully
clockwise.

Refer to mute
fault finding
chart.

N

Mute LED on?
Y

Audio on
C115/C116?

Refer to mute
fault finding
chart.

N

Y
Audio on
IC102 pin 2 &
IC103 pin 2?

Replace
faulty device.

N

Y
Audio on
speaker?

Replace C122 &
check continuity.

N

Y
Audio on line
monitor (pad 117)
& line output?

N

Check/replace
output components.

Y
OK

30/04/96

Copyright TEL



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Subject                         : T820 Series I Service Manual
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