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

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

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M820-00 T825 Fault Finding B5.1

5 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 Elec-

tronics 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

5.3.1

5.3.2

5.3.3

DC Checks

Power Rails

VCO Locking

Mute Operation

5.4

5.4.1

5.4.2

5.4.3

5.4.3.1

5.4.3.2

5.4.4

5.4.5

RF Checks

VCO Frequency

RF Sensitivity

Oscillator Stability

TCXO

Second IF

Demodulator Output

IF Distortion

5.5

5.6

5.5

5.5.1

5.5.2

5.5.3

5.5.4

5.5.5

5.5.6

Fault Finding Charts

Regulator

Synthesiser

Noise Mute

Carrier Mute

Receiver

Audio

5.8

5.9

5.11

5.12

5.13

5.14

B5.2 T825 Fault Finding M820-00

30/04/96 Copyright TEL

Figure Title Page

5.1

5.2

5.3

RF Test Cable

IF Swept Response

Ceramic Filter Swept Response

5.5

5.7

M820-00 T825 Fault Finding B5.3

5.1 Visual Checks

Remove the covers from the T825 and inspect the PCB for damaged or broken compo-

nents, 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 per-

formance.

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.

B5.4 T825 Fault Finding M820-00

30/04/96 Copyright TEL

5.3 DC Checks

5.3.1 Power Rails

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 com-

partment with a DMM.

Check the 20V regulator output at the test point (TP3) in the regulator compart-

ment.

Check the 5V regulator output at the test point (TP4) in the regulator compart-

ment 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 sig-

nal 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).

M820-00 T825 Fault Finding B5.5

5.4 RF Checks

5.4.1 VCO Frequency

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:

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.

Figure 5.1 RF Test Cable

Note: Before using the test cable, ensure the coax braid is connected to an earth

point on the PCB.

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.

Frequency Range Wide Band Narrow Band

66-76MHz -119dBm -120dBm

76-88MHz -118dBm -119dBm

1nF

Coax

To Test Cable

Connect To Earth

B5.6 T825 Fault Finding M820-00

30/04/96 Copyright TEL

Poor sensitivity indicates a fault in one of the circuit blocks following the test break.

Note: Poor sensitivity into the mixer can be caused by lack of drive level from the

VCO (the drive level should be >+17dBm).

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 cou-

ple into the first IF an additional high level signal at 10.7MHz - a constant low fre-

quency 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 cou-

ple 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.

M820-00 T825 Fault Finding 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.

Note: The X input should be DC coupled.

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 satisfac-

tory solution (refer to Figure 5.3).

Figure 5.3 Ceramic Filter Swept Response

B5.8 T825 Fault Finding M820-00

30/04/96 Copyright TEL

5.5 Fault Finding Charts

Note: The standard test point designations as used in this section are as follows:

TP1 13.8V

TP2 9V

TP3 20V

TP4 5V

5.5.1 Regulator

Check for

overheating

components &

short circuits.

13.8V present on

IC202 pin 3?

5.3V present on

IC202 pin 1?

9.0V present

on TP2?

20V present

on TP3?

Check continuity

from D-range

connector pin 9.

Replace IC202.

Check Q200,

Q201 & IC201.

Check Q202 is

switching at

approx. 180kHz.

M820-00 T825 Fault Finding B5.9

5.5.2 Synthesiser

Continued on next page.

VCO out of lock.

Check 9V reg.

Check biasing

resistors, Q4 & Q5.

Check Q4 C

multiplier

(synth).

Check biasing

resistors, Q1,

Q2 & Q3.

Replace if faulty. Correct ref.

on IC3 pin 8?

Check 5V reg. &

supply resistor

to each IC.

20V present

on IC4 pin 8? 20V inverter

supply faulty.

9V present on

Q1 divider buffer? Check 9V supply.

DC V present

on Q1?

Check TCXO

& reference

divider path.

Prescaler output

freq. present on

IC3 pin 4?

VCO freq. correct? Go to C.

Synthesiser OK.

No VCO RF output.

DC V present

at Q4 & Q5?

9V present at

VCO Q4/Q5?

8.3V present

at VCO?

DC V present at

VCO, Q1, Q2 & Q3?

Capacitors OK?

Check for cracks,

shorts, etc.

Replace VCO.

Check biasing

resistors & Q1.

VCO trimmer

freq. range OK?

VCO OK.

Does IC3 pin 1

PDA change level

at the programmed

synth. freq?

Does control line

O/P level change

on IC4 pin 1?

Check signal

path from VCO

to prescaler.

Check synth. data

loading ccts;

change EPROM;

change synth. IC3.

Check loop filter

cct around IC4.

Check VCO tuning

caps & trimmer.

A (Refer to VCO circuit diagram)

NCheck continuity

between VCO &

IC4 pin 1 O/P. Synthesiser OK.

+5V present at

TCXO, IC3, IC5,

IC6, IC1 & EPROM?

Check at IC1

pin 3; replace

IC1 if no output.

Does VCO trimmer

adjust for stable

control line

between 5 & 13V?

Does VCO trimmer

adjust for required

frequency range

between 5 & 13V?

Is 8MHz VCO range

achieved between

5 & 13V?

Does IC1 pin 3 O/P

freq. change with

VCO trimmer when

VCO not locked?

Check &C2, &C3, &C4,

VC1 & D1-D6 in VCO.

B (Refer to synth. circuit diagram)

B5.10 T825 Fault Finding M820-00

30/04/96 Copyright TEL

Replace IC3;

synthesiser OK.

Substitute EPROM

with known good

unit; is freq.

wrong?

Does freq. up-

date correctly

when supply

toggled off & on?

VCO locked on

wrong freq? Noisy synth; poor

selectivity &/or

poor residual FM.

Correct channel

selected?

(switches on

external lines)

Continuity OK

between EPROM &

IC3 on 3 address

& 4 data lines?

Does toggling

each of the 3 LSB

DIP switches

change the freq?

Set correct

channel.

Repair tracks.

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.

Check presence

of low level on

IC3 pin 14 for

>200ms on switch-on;

change IC3.

Provide new EPROM.

Go to B.

Correct DC V

present on Q1 VCO?

Check inductors,

biasing resistors

& Q1.

Loop filter &

components around

integrator op. amp. OK? Replace if faulty.

Change trimmer;

still noisy?

Change Q1;

still noisy?

Change TCXO;

still noisy?

Change varactors;

still noisy?

Replace VCO

assembly.

Synthesiser OK.

C (Refer to synth. circuit diagram) D (Refer to synth. & VCO

circuit diagrams)

VCO phase locked?

5-13V present on

control line?

M820-00 T825 Fault Finding B5.11

5.5.3 Noise Mute

Ensure PL104 is set

for noise mute operation.

Apply on channel signal

@ -117dBm* to receiver.

Adjust mute

pot. RV100 &

observe voltage

IC100 pin 3.

Does LED

toggle on/off?

Check Q103, Q105 &

Q106 for operation.

Mutable audio

on outputs?

Refer to audio

fault finding chart.

Switch SW100 to ensure

mute is not disabled.

Voltage varies

0-2.5V?

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?

Monitor voltage on

IC100 pin 7. Switching

0-8V as pot. rotated?

Monitor voltage on

pad 101. Switching

0-8V as pot. rotated?

Does LED toggle?

Refer to Rx

fault finding

chart.

Check components

around active filter.

Check 9V rail &

resistors R149,

R100 & R101.

Replace IC100.

Check/replace

IC100 & D100.

Replace Q100.

Check/replace

Q102 & D102.

*Modulated 3kHz [1.5kHz]

deviation @ 1kHz.

Audio on

IC303 pin 7?

Noise on

IC304 pin 7? N

Amplified noise

on C386? Check Q306

& Q307.

Check D301 is

rectifying noise.

Approx. 0.6V DC rect.

noise on pad 114?

Vary RF level &

observe voltage

on pad 114.

Pad 114 voltage

varies 0-1.8V?

Check adj.

of RV301

(gain adj.).

Rotate mute pot. RV100

c/w & cc/w & view LED.

B5.12 T825 Fault Finding M820-00

30/04/96 Copyright TEL

5.5.4 Carrier Mute

Rotate carrier mute

pot. RV104 c/w &

cc/w & view LED.

Does LED

toggle on/off?

Approx. 3.3V on

IC303 pin 5? Check for short circuits

and/or replace IC303.

Ensure PL104 is set

for carrier mute operation.

Apply on channel signal

@ -95dBm* to receiver. *Modulated by 1kHz signal with

3kHz [1.5kHz] deviation.

Audio on

IC303 pin 7? Refer to Rx fault

finding chart.

Approx. 5V on

IC104 pin 1?

Adjust carrier mute

pot. RV104 &

observe voltage

on IC104 pin 6.

Voltage varies

0-6V? NCheck 9V rail &

resistors R154,

R156 & RV104.

Monitor voltage on

IC100 pin 7. Switching

0-8V as pot. rotated?

Monitor voltage on

pad 101. Switching

0-8V as pot. rotated?

Does LED toggle?

Check/replace

IC100 & D100.

Replace Q100.

Check/replace

Q102 & D102.

Monitor voltage on

IC104 pin 7. Switching

0-8V as pot. rotated? Replace IC104.

Check Q103, Q105 &

Q106 for operation.

Mutable audio

on outputs?

Refer to audio

fault finding chart.

Switch SW100 to ensure

mute is not disabled.

Check/replace IC104

& adjoining components.

Approx. 5V on

RSSI pad 115?

Check IC304 and

adjoining components.

Set RF signal level

to -95dBm. Rotate

carrier mute pot.

RV104 c/w & cc/w.

M820-00 T825 Fault Finding B5.13

5.5.5 Receiver

High audio

distortion.

Quad coil L319

tuned for min.

distortion (flat

audio response)?

Adjust L319.

Retune all

IF coils.

Note: T.B. = Test Break

(refer Rx circuit diagram)

Readjust TCXO

frequency.

VCO frequency OK? N

Adjust IF coils.

IF alignment OK?

(see 5.4.5) N

Fault find audio.

Distortion at Rx

audio pad 111 OK?

2nd IF

frequency OK?

(see 5.4.3.2)

Check/replace

IC302, X301,

C362 & C361.

Repair stage.

TCXO stability OK?

(see 5.4.3) Refer to synthesiser

fault finding chart.

OK.

Inject 10.7MHz signal @

-15dBm into 10.7 IF pad.

Is distortion 3% or less?

Sweep shape of

whole IF (see 5.4.5).

OK? Replace ceramic filter.

Fault in IC303 or

surrounding circuitry.

Check/replace.

Inject 10.7MHz into T.B.

after diplexer & measure

distortion. OK?

Check sensitivity

into each IF stage.

Inject on-channel RF

into mixer RF input T.B.

& measure distortion.

OK?

Replace TCXO (IC2).

Poor sensitivity:

worse than -117dBm.

Ensure VCO RF

& IF correctly

aligned.

High audio

distortion? See Chart B

(below).

Replace

faulty device.

NDC conditions OK?

(see cct diagram)

Check/repair faulty

stage or component.

Check sensitivity

into each stage

(see 5.4.2).

B5.14 T825 Fault Finding M820-00

30/04/96 Copyright TEL

5.5.6 Audio

Apply on channel signal

@ -117dBm* to receiver.

Audio on

IC101 pin 1?

Audio at

top of RV102

& RV103?

Refer to Rx

fault finding

chart.

Check components

around IC101.

Replace C112.

Ensure mute

pot. fully

clockwise.

Mute LED on? Refer to mute

fault finding

chart.

Refer to mute

fault finding

chart.

Audio on

C115/C116?

Audio on

IC102 pin 2 &

IC103 pin 2?

Replace

faulty device.

Audio on

speaker? Replace C122 &

check continuity.

Audio on line

monitor (pad 117)

& line output?

Check/replace

output components.

*Modulated 3kHz [1.5kHz]

deviation @ 1kHz.

Audio on

IC303 pin 7?

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

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

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

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