You must have a volt-meter, since you've already rebuilt the amp 4 times?
1) Power-on both amps (minimum volume and nothing connected to inputs) measure and report DC and AC differences between both amps output terminals.
Please be aware that this is new to me so I might be doing things the wrong way. For task 1 I measured the voltages with the probes on the +(Red) and -(Black) of the amp outputs which are what are connected to the speakers.
So:
Left DC =0
Left AC=0
Right DC=.003
Right AC=0
2) Report any DC voltage present (however small) at the output terminals on each amp.
I am assuming that you want the voltages measured with a reference to chassis ground.
So:
All are 000 with the exception of the Right DC=.005
3) Power-off both amps, disconnect from mains and wait a few minutes for amp PSU caps to discharge. Report resistance between positive and negative output terminals to chassis ground on each amp.
All readings are ohms
Left --- = 0k
Left +++ = 41.3k
Right --- = 0k
Right +++ = 41.7k
4) Still disconnected from mains, report resistance from the chassis of each amp to its wall plug connector (to see if open).
No connections are open and they report a small ohm reading.
These readings might give you a clue but they are meaningless to me. However I wonder where the small DC voltage on the right channel is coming from.
I removed the sub woofer from the chain and it made no difference. I routed the cable many different ways and was able to get the hum to a minimum level though I hear a strange tic, tic, tic sound like the sound at the end of a record being played. There is also a faint signal from a radio station. Weird !
The problem I have with the present routing of the cable has to do with an unhappy spouse. Admittedly the routing is ugly.