RCA CED Player Audio Troubleshooting

[samson7point1]

I'm working an an RCA SGT-250 CED Player and troubleshooting a tricky audio issue - tricky because I don't really understand how a lot of this works and I'm trying to use this to learn.

This player has a toggle switch to select between Stereo, Mono R, and Mono L.  In the Mono R position, the audio output level is normal (from both the L and R audio output jacks), in the Stereo position the audio output is about half the volume, and in Mono L, it's about 3/4 the volume.  Something about putting it into Stereo mode is causing the volume to drop significantly.

The player processes the audio signal using two different reference frequencies. The "R" audio is modulated to 716kHz, and the "L" audio is modulated to 905kHz.

The "R" channel is first processed by the U3601 "Audio FM Demodulator" on the PW3000S main board, and then the demodulated "R" signal is sent along with the modulated "L" signal to the PW4000 "stereo" board where the "L" signal is  demodulated by the U4101 "Audio FM Demodulator" then sent to the U4601 "Audio Output" IC along with the demodulated "R" audio.

To troublehoot:
- I have replaced both the U3601 and the U4101 IC's with new known good parts (originally U4101 was not functioning at all). 
- I have replaced the U4301 quad op-amp
- I have replaced the Q4301 and Q4302 transistors
- I have tried adjusting the R4109 potentiometer (no audible change)
- I have tried adjusting the R4303 potentiometer (no audible change)
- I have checked the resistance and capacitance of all of the resistors and capacitors that seem to be in the audio path.
- dismantled, cleaned and reassembled the SI Mode Switch (audio mode switch)

Someone shared a copy of the service manual with me: https://drive.google.com/file/d/1FWt4c4ilGRha7aB6PF5ePpMBX4zHe5AM

The schematic for the PW3000S main board is on Page 54, 55 and 56

The schematic for the PW4000 stereo board is on Page 48 and 49

The service manual is fairly complete, and if I knew what I was doing I'm sure it would yield the clue I need.  For example, I'm not sure what exactly the "SI Mode Switch" (Page 48) is actually doing when switched into "Stereo" mode, other than lighting the stereo LED indicator.

Can someone take a look at this and hazard a guess at where I should look next?

[m k]

Stereo Indicator Mode Switch is activating and deactivating parts of U4201.

U4301
part C is driving both channels, it's deactivated in I-2 operation, so I-1 is Mono.
Part A is deactivated in I-1 operation, so Right is Mono.
Part B is deactivated in I-2 operation, but it's Normally On, so Left is I-2.

Maybe those operation modes are mixed.

[samson7point1]

Stereo Indicator Mode Switch is activating and deactivating parts of U4201.

U4301
part C is driving both channels, it's deactivated in I-2 operation, so I-1 is Mono.
Part A is deactivated in I-1 operation, so Right is Mono.
Part B is deactivated in I-2 operation, but it's Normally On, so Left is I-2.

Maybe those operation modes are mixed.

Thanks for taking a look.  I did eventually figure out that the R4513 trim pot (which isn't listed in the electrical adjustments section at all) controls the output of the U4501 decoder when it's in "Stereo" mode. The pot is in the lower right corner of the PW4000 diagram and labeled "Decoder Adjust".

Adjusting that I was able to match the stereo output level to match the mono mode.

I'm not a fan of adjusting factory trim pots. It means that some other component has drifted over time and may only cover the symptoms rather than resolve the problem. I'm just at a loss of what component may have degraded because I've either checked or replaced nearly every component involved (including all of the electrolytic capacitors).

[m k]

Some resistor types can drift quite a bit over time.