Tait T850 II Series, Service Manual

Page: 194 / 370

C5.18 T856/857 Fault Finding
M850-00
31/09/98 Copyright TEL
5.7.7 Audio Processor
N
IC240 pin 6
at 9V?
Key exciter &
lock synthesiser.
Connect 1kHz audio
to external CTCSS input.
Does exciter
modulate?
Audio output at
IC260 pin 8?
N Y
Check audio path to
VCO control line.
Check alignment
of modulators.
Y N
Audio output at
IC260 pin 1?
Y
Check buffer IC260
pins 8, 9 & 10.
N
Audio output at
IC260 pin 7?
Check low pass
filter stage IC260
pins 1, 2 & 3.
Y
Check stage IC260
pins 5, 6 & 7.
N
Demodulated audio
frequency response
flat between 50 & 300Hz?
Y
Check low
pass filter
stage IC260
pins 1, 2 & 3.
N
N
Y
Audio frequency
response IC260
pin 1 flat 300Hz
to 2kHz?
Audio frequency
response IC210
pin 8 6dB/octave
slope?
Check integrator
IC210 pins 8, 9, & 10.
N
Y
Audio superimposed
on 200kHz signal
IC720 pins 9 & 3 (synth)?
Check audio path
between integrator
& phase modulator.
N
Y
200kHz clipped
triangle wave at
IC720 pins 8 & 4?
Check IC720, Q710, Q720,
Q730, Q740, D710, D720,
C727, C729, C724 & C725
N
Y
Check loop filter
(IC750, pins 5, 6 & 7)
& associated
components.
Select standard
audio configuration.
Check pre-emphasis
network IC230 pins 12,
13 & 14 &/or low pass
filter IC260 pins 1, 2 & 3.
N
Y
Connect audio
to line input;
key exciter.
Does exciter
modulate?
Y N
Demodulated
audio frequency
response as per
Section 4.5?
Audio processor
OK.
Audio signal at
IC210 pin 14?
Y
N
Audio signal at
IC230 pin 1?
N
Audio signal at
IC230 pin 14?
Y
N
Audio signal at
IC240 pin 4?
Y
N
Y
Check Q260 is not faulty or
being turned on inadvertently.
Y
Check audio path between
limiter IC210 pin 14 &
summing amp. IC260 pin 7.
Check IC210
pins 12, 13 & 14.
Check IC230 pins 1, 2 & 3 or
audio path between pre-
emphasis network & limiter.
Check IC230 pins 12, 13 & 14
or audio path between multiplexer
& pre-emphasis network.
Check IC240 or audio
path between line trans-
former & multiplexer.