B&K 200.5 S2 Power Amp problem - Please help

If you run a dedicated outlet of 20A breaker and outlet to an Amp that has a 15A fuse, I`m quite sure you will blow the main fuse on the amp due to allowing 20A come through a 15A fuse.

If you run a dedicated outlet of 20A breaker and outlet to an Amp that has a 15A fuse, I`m quite sure you will blow the main fuse on the amp due to allowing 20A come through a 15A fuse.

. . . very funny.

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.

Well, think I figured it out. Put the amp on my workbench and ran the same test as last night; this time only 2 channels were bad. Started playing with the switches and noticed some noise, sprayed some contact cleaner in and worked the switches several times.

Now all seems well, unbalanced all the inputs produce the same level, pushing the switch in to balanced causes the level to drop ~30db. Running balanced all the channels also the same, letting the switch out to unbalanced actually causes the level to go up a little.

So if anyone else has this problem try cleaning the switches (once you're sure they're in the right position).

Sorry to resurrect an old thread but @bdgregory would you happen to know where to find the fuse you linked above now that Radio Shack is dead?

I've got a 7250 that had one channel (I suspect) blow a rail fuse last night while watching TV. All of the sudden that channel went very quite. I swapped speakers, wires, rcas all to troubleshoot it wasn't any of those. When you turn the preamp nearly all the way up you can quietly hear sound from that channel, so like another post above we know it's not the main or speaker fuses.
I'm going to open it up in a while and check the fuses for that rail, but if blown I still need to know where to find a replacement. 

Thanks for any help!