I'm no expert and the techs around here just don't get these older amps and seems I just pour $$ out with no real results. Since it got back I have replaced the rectifier with the recommended diode, found and replaced a funky tube that was messing with the bias and added two phone plug inputs on the front panel to bypass the existing narrow and fragile RCA's. Old RCA inputs are now unhooked. The symptoms persist even with the new input jacks so feel it is a ground, something in the mono/stereo switch or other mystery.
I plan to replace the caps on the PC board and the bias caps but this hum has me frustrated and stumped. Guess it works OK in the end when both inputs are connected but just not right and I'd like to get it fixed correctly. Seems so much knowledge and experience on the board, any suggestions? Thanks...IA.