H/K AVI 200 & Sony STR-525 Amp Problem???

Needed help on a few receivers I have acquired. I've been reading this site for 1yr> & found it helpful.
A little about myself & future plans.
I'm a disabled Vet and for the last 4 yrs I desperately needed a hobby that I love. Started out with re-foaming speakers(about 40-50 now) but trying to fix older amps & receivers. I have a degree for Computer & Telecommunications received 16yrs back. I've accumulated a 100Mhz oscilloscope, sweep-function generator, counter, duel track P.S. & dig multimeter. Some things are creeping back into my grey matter as I go along. :confused: Enough of this for now.
The receivers I mentioned share same problem. Speakers only work on either A or B inputs meaning hooked up properly the balance would work BOTH speakers. Now if on A, 1 speaker works O.K. but no output on other speaker. Switch leads around, same thing. So I'm thinking a channel blew out. I could be completely off on my analysis but I needed to ask to find out. Except for multimeter & P.S. running, I,m still relearning other equip.
Wow, didn't know I was such a windbag. :eek:
ANY help on this would do my heart good. I figure I will be asking other ?'s along the way. THANKS.