Parasound Unstable Channel - Have Isolated Board But Not Component

[it_iz_what_it_iz]

DEVICE: Parasound HCA-2003 circa 1998 three-channel amp
SYMPTOM: 10+ seconds after power-up one channel's output voltage (as read by a DVM on the outputs) slowly starts to go unstable varying somewhat sinusoidally several millivolts positive then negative growing in magnitude to in excess of a couple of volts. (This, of course, can cause voicecoil over excursion/damage.)
ROOT CAUSE CIRCUIT BOARD: Don't know but have been able to swap leads and, with 100% certainty, isolated the culprit to be one of the far right hand circuit array on the Inputs Board. See pic.
ROOT CAUSE CIRCUIT/COMPONENT: I simply don't know. I've A-B-C's each and every component across the three identical circuits (for the three channels) using only an ohmmeter and voltmeter. Have found all NPN transistors functioning OK, all PNP transistors functioning OK, all diodes functioning OK, all resistors very similar in resistance, pots all very similar in min and max resistance, all pot's resistance vary smoothly and similarly with rotation, all capacitors ohm'ing out very similarly.

QUESTION 1: What am I not doing that I need to do to isolate and verify what component(s) out of spec?

QUESTION 2: What additional diagnostic devices (scope, signal generator...) might be needed?

 

[it_iz_what_it_iz]

Guess I need to try diyAudio or some place similar. No diy'ers here?

 

[levimax]

DEVICE: Parasound HCA-2003 circa 1998 three-channel amp
SYMPTOM: 10+ seconds after power-up one channel's output voltage (as read by a DVM on the outputs) slowly starts to go unstable varying somewhat sinusoidally several millivolts positive then negative growing in magnitude to in excess of a couple of volts. (This, of course, can cause voicecoil over excursion/damage.)
ROOT CAUSE CIRCUIT BOARD: Don't know but have been able to swap leads and, with 100% certainty, isolated the culprit to be one of the far right hand circuit array on the Inputs Board. See pic.
ROOT CAUSE CIRCUIT/COMPONENT: I simply don't know. I've A-B-C's each and every component across the three identical circuits (for the three channels) using only an ohmmeter and voltmeter. Have found all NPN transistors functioning OK, all PNP transistors functioning OK, all diodes functioning OK, all resistors very similar in resistance, pots all very similar in min and max resistance, all pot's resistance vary smoothly and similarly with rotation, all capacitors ohm'ing out very similarly.

QUESTION 1: What am I not doing that I need to do to isolate and verify what component(s) out of spec?

QUESTION 2: What additional diagnostic devices (scope, signal generator...) might be needed?View attachment 247127

I would try "freezing" components to try to isolate the bad one (or bad solder joint) with something like this https://www.chemtronics.com/ultimate-guide-to-diagnostic-freeze-spray#:~:text=Freeze spray (also called freezer,that super-chills isolated areas. A scope and generator would be nice to watch for changes when freezing but you can probably use the DVM like you are now. Good luck.