Help troubleshooting an audio problem with an RCA SGT-250 CED Player

[samson7point1@yahoo.com]

The Problem: This player has a toggle switch to select between Stereo, Mono R, and Mono L. Regardless of the setting it outputs the sound to both the L&R audio jacks. 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 and I can't figure it out.

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/1FWt...5ePpMBX4zHe5AM

The schematic for the PW3000S main board is on Page 54, 55 and 56 (image attached)

The schematic for the PW4000 stereo board is on Page 48 and 49 (image attached)

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 to figure out why the audio volume is cut in half when the switch is in "Stereo" mode?

[samson7point1@yahoo.com]

This is now resolved (at least mostly). Leaving this here in case anyone else finds this trying to solve the same problem.

There's a trim pot "R4513" that's not mentioned in the electrical adjustments. It's labeled on the circuit diagram as "Decoder Adjust". It controls the level output ONLY when the switch is in the "NORM"/Stereo position despite the fact that the "decoder" IC controls the other output modes as well.

Adjusting that I was able to match the output levels in all three modes.

The only reason I say this is "mostly" resolved is because 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).