Yet another Zenith To identify

[wkand]

Rather than replace the module, do you have a Sam's for the set that might give you DC voltages around the audio IC in your pic?

Are there separate discrete transistors for audio IF, or are all functions built into that IC?

Another thought is look up that IC by number online, which should give clues as to the expected voltages and waveforms around that IC. If a DC voltage is off, either a resistor drifting high, or a leaking /shorted cap might be the culprit.

Is the IC getting hot?

Also, check the inputs to the module. The problem may not be the sound module at all. I'm thinking there will be sound "takeoff" point coming from the video IF, but I am generalizing and not as familiar as others with 70s SS sets.

There are not that many parts on the audio module. Can you check for ESR on the electros, as well as resistor values?

Your cleaner spraying of the volume pot should have changed the sound (even if only slightly) if it were the problem. Still, they are very problematic...

Good luck! 😀