Adcom GFA 555 hum. Probably a repair question.

I bought a used GFA 555 to use as a hold me over amp for my venture to separates. But it hums. 60 cycle imo.

Trouble shooting so far:

Unplugged everything.
Amp by itself, speakers, no hum.
Amp with anthing at all plugged into RCA ins, hum starts.

just a simple cable is all it takes. i have hooked up two preamps and both hum. i have swapped out the Adcom with a friends power amp, the hum stops. took everything to his house..exact same thing.

The amp hums with anything plugged in.

i am handy with basic electronics and a soldering iron.
Can i fix this?