Denon PMA-850 Troubleshoot

[Beavs]

I've got a Denon PMA-850 with a weak right channel (also sometimes distorted, but mostly just weak). Balance pot resistance wasn't reading evenly between L/R channels so I replaced that with a new Alps pot thinking it would fix it but it didn't (VR4 in attached pic).

Issue is not in the power amp section, verified via Main In jacks which have even volume between L/R. Also probably rule out Relay contacts.

Issue is not in the tone/control/pre-amp section as it is common with the Direct Couple switch on or off...Direct Couple signal for right channel is traced in red in the attached pic as I understand it.

Multiple cleanings/deoxit of SW2 (function switch), SW4 (tape monitor switch), SW5 (mode switch), SW6 (direct couple switch), and SW7 (loudness switch).
Issue is common on Aux/Tuner/Tape1-2 inputs (SW2 switch should be okay).
Issue is common on Mono/Stereo/Reverse (SW5 switch should be okay).
Issue is common when Loudness On and Off (SW7 switch should be okay).
Issue is common when Direct Couple On and Off (SW6 switch should be okay, when this switch is off the signal would travel through the tone/control/pre-amp seciton).


The Volume pot (VR3) is sealed Alps pot and I wouldn't suspect that to be the issue but at this point I can't figure out anything else that it may be.

Any thoughts or help would be appreciated.

Note: Schematic is split on two pages in the SM copy that I have. The attached pic/file mostly lines up between the two pages but the Direct Couple to the Pre-out/Main-in should probably have some vertical lines in that area that are missing due to the page split. Picture is a bit blurry but the attached PDF is better resolution.

 

[Beavs]

Pulled and tested the volume pot. On average, about a 0.4K difference between the two channels on a 50K pot. It actually wasn't sealed as I suspected...after desoldering I could pull a plastic cover off the bottom to get deoxit in there, but also unscrewed to clean it and see if it made a difference in the readings but mostly stayed the same.

To actually balance the output between the two channels the balance pot has to be turned to about 3:30. Would this 0.4K difference on the volume pot (50K) cause that much variation when the balance pot is 100K?

 

[Beavs]

seems well with tolerance, have you seen any dry joints ? only alternative is to trace\track the signal and see where its being lost,

I looked for any suspect solder joints and also wiggled point to point connections but couldn't find anything. As far as signal tracing, would I just feed a a steady tone in and then test with a voltmeter (black to chassis ground, red to various points along the signal path)? Or do I need to use an oscilloscope and look for reduction in the amplitude?

 

[Beavs]

i would use an input on another amp and use a phono plug, unless you had any proper probes

I've already used the pre from another integrated amp to the main in on the Denon to eliminate an issue from the main in jacks through the main power amp section.

I have mini grabbers (not gator clips) for both DMM as well as oscilloscope, just have to dig the oscilloscope out of storage versus the DMM I have ready to go.

 

[Beavs]

Posting an update on this, the Alps potentiometer that I used was a 15A taper pot. It appears what you need for a balance potentiometer is a MN taper pot which @JoseHH helped me realize in this thread:

https://audiokarma.org/forums/index...tentiometer-replacement.979696/#post-15238230