Fleetwood model 4068... Advice?

[VintagePC]

Thanks, tried that and it has reduced the noise considerably.

I think I may also have located part of the balance problem... There's some weird stuff going on with the volume control that's not in the schematic. It may also be the result of differing resistors in each channel- while they are all within tolerance, one example is the 100K ones; one channel has a 100.6K, the other is about 107K... Should they be as closely matched as possible?

The pot has a 4th lug which appears to be some sort of center tap- resistance goes from 300K to 0 and back up again as the knob is turned.

The schematic just shows the wiper going straight to the 6AU6, but here, the wiper goes through a 220K to ground, as well as to the 6AU6. One channel is shielded, the other isn't, so that may well be the source of some hum.

In addition, the audio input from the function switch runs through a cap to the 4th lead of the pot, then to a 47K resistor, through another cap, and to ground. The same circuit exists on both channels.

Does anyone have an idea what purpose this might serve? The mounting for the volume pot is different than the others, so I wonder if someone changed it and added a hack to make it fit/convert a linear to a log pot? My meter doesn't go to 3 megs, but afaict, it is a 3 meg pot.

Does someone have an idea what this might be?