I'm hoping I can re-open this thread as my problem is similar. A B&K 125.7 Amp I used for about 5 years without problems - balanced connections. I boxed it up about a year ago and a friend wanted to buy it so he took it home and set it up. 4 of the 7 channels had low output using unbalanced connections. He tried the switches in both positions, no change.
I went over yesterday and we first took a mono white-noise signal directly into each unbalanced input with a single speaker hooked to that channel and measured the output. Then moved the source and speaker channel by channel and measured them all. Sure enough 4 channels are ~30 db lower - changing the switch on the "good" channels lowers their output by ~30 db, but changing the switch on the "bad" channels does nothing.
Then we did the same test using a preamp with balanced outputs, same results - balanced or unbalanced the same 4 channels are bad. These 4 channels happen to belong to 2 stereo pairs, so of the 7 channels 1 stereo pair is good, and the 1 mono amp is good, 2 stereo amps are bad (the balanced/unbalanced switches work in stereo pairs - the mono amp has it's own).
I took the top off and checked all the fuses on the top of the amp (15 in all). They all seemed good. The sound of the low output channels seems OK, though it was hard to get it loud enough to really evaluate distortion.
Seems that some are familiar with these amps, any other ideas beyond "the switch"? Thanks for your thoughts and any advice.
