In article ,
says...
This set has no audio. I cannot get external audio in or out either.
There is also no SAP or stereo LEDS display when its on. I probed for
audio on the stereo amplifier board and found none. The audio
processing seems to be done on another large standup board off the
main board.

Raw +24 and +12 is rectified on the stereo amp board, then
fed from J403 amp board to P14 stereo board. The stereo board
regulates these down to +15 and +8 via IC4 and IC5 respectively.

4.5MHz comes from main board P8 to stereo board J33. IC304
demodulates baseband audio, output on pin 12, buffered by Q2
to TP5. IC1 decodes stereo with L and R outputs on pins 15 and
16 and fed through low pass filters LPF1 and LPF2 respectively.

Not satisfied with just stereo, the signals are combined by the
mess around IC6 to create L+R and L-R available on the wipers of
VR75 and VR76 respectively. IC3 analog switch, IC7 quad amp,
IC201 expander, IC203 amp, and associated circuitry take care of
the expander (dolby?) function, and outputs appear at IC3 pins
14 and 15 and are coupled to connector PD2 which passes L and R
on to the audio interface module, which eventually feeds the
signals to the amplifier board.

Fortunately, most of this stuff is usually reliable, and I bet
you found the problem in the first paragraph above. If not,
check back - I don't find time to read this group every day,
but I'll see your post eventually.