So today I had the bright idea of having my friend touch the battery ground to the collar around the RCA connector on the amp to see if that made the hum go away. It did, but it also made the sound really quiet, so it didn't solve the problem.
I'm wondering though, what caused this behavior? I thought it might remove the hum because of a ground potential difference causing the ground loop, so I figured the battery ground would bring the ground at the RCA input up/down to where it should be, but I can't explain why this would make all the sound, rather than just the hum, go away. Any ideas?
I also had the bright idea this evening to have him try a "ground lift", and simply disconnect the ground on the RCA cable. Without a shield in place this might cause it to be more susceptible to RF but right now I'm just hoping to get something resembling normal audio out of the thing. I'll have him try that in the morning.
And of course the ground loop isolator is in the mail so that will be getting here tuesday and we can try that.
But if the ground lift works, that would be pretty sweet, since that's a cheap solution. I am afraid it won't though given the lack of any success so far. I had him try a "partial ground lift" before with a resistor on the negative RCA input and it did nothing. I am also afraid that if that doesn't work, how can the ground loop isolator?