Selective Ground Loop Hum... HELP Please.

[kasbah512]

This is somewhat me circling back to an issue I was having months ago. I've been working on a Marantz 1200 on and off for a long time. I'm convinced its possessed by the previous owner at this point...

The issue:
-Ground loop-ish hum on the right channel only (next to the transformer)
-Only when the selector is set to "System 1" or "System 2" (Not when set to "Both")

Remediation:
-Changed the filter caps as some have suggested
-Changed the output transistors (in case it was actually oscillation)
-Biased to 14 mv across bases
-DC offset in sub mv range

Notes:
-Bias adjustment did have some influence on the intensity of the hum
-Adjusting bias on left channel has significant influence on hum in right channel (Why??)
-When in my garage, Simply connecting my DMM to the base leads was able to eliminate the hum (prior to transistor swap)
-It is now only occurring in my room, not in my garage (makes it real fun to troubleshoot)
-Prior to filter cap swap, it was always audible through headphones. (weather set to Phones only/System 1/System 2/Both)
-Prior to the transistor swap, it was occurring in my garage, AND in my room
-Connecting the common ground screw on the back to earth ground in my room does NOT influence the hum (I thought it might)

I'm at a loss, and any help would be appreciated... Thanks yall!

 

[MHouseholder]

Likely it is oscillation. I eliminated a similar intermittent oscillation/buzz when I found that clamping driver transistor Q511 between my thumb and forefinger added enough capacitance to stop it. I experimented further to find that adding a .0068uF film cap across R518 (1.2K 2W) in the top, front corner of the board also stopped it.