Repairing 2 Peavey 2000.

[Andrew58]

The trouble with the first one is that channel B is not working. I looked at channel A on the scope first and saw it was a very weak signal then i ran it loaded into a small amp and it wasnt loud at all. then i fienlly ran it on a spearker unloaded and all of a sudden after 20 min of testing and 5 min of it on the speaker it popped into full volyume and tottaly destroyed the speaker. it was the first time it did this during the whole testing. im going to test the preamp volume pot tomrorow at the shop.
The second amplifer has a sorta similier problem. Channel A works fine and when i bridge chennel b it gives a perfect 180 degree phase shifted channel A on channel b, but out of bridged mode the channel doesnt work or is so very weak i cant tell. I thought about checing jfet 111 which i think is for the muting circuit in case thats stuck on. .
If any one could help lead me in the right directions with either of these amps id appreiciate it very much. Ive just started and im learning actuelly to be a electronics technician. IM fresh out of high school so any tips would really go along way. thanks

[Enzo]

Well, good luck, electronics is a pretty big world to explore. And I am not so sure this amp is a beginner project, but...

The second amp first. The very essence of troubleshooting is to isolate the problem.

I am not clear on your symptom. Instereo mode only one channel passes signal, but in bridge mode? One input drives both channel outs? Or one inout provides inverted signal to the inputs of both, but no output from one? Just exactly what?

Apply a test signal to both inputs, and turn the volumes up. DOn't use a load, we don't need it yet. Now scope the outputs of the input boards. Pin 3 of the 8-pin connector on each input board is its output. Or go to the volume control and scope the middle leg. Got signal both places? if one is missing, then look through its channel. Only one op amp serves the full range - the other is for crossover operation. There are a couple ppush switches, they could be bad - one is subsonic filter and the other crossover enable. if they are not working berify there is +/15VDC getting to the little board.

Where is JFET 111? DO you mean Q101? With the amp powered, measure resistance across the JFET. If it is low, the JFET is shorted or stuck on. You can remove it and see if the amp comes alive.

If that isn't the p[roblem, move to the input stage of the power amp. Is there signal at the output of the op amp at pin 7?

Without a speaker, measure the resistance across the output of the amp - right at the speaker connections on the rear of the amp. Does it measure shorted across?

[Steve Conner]

On the second amp it sounds like the switch for selecting bridge mode is broken or dirty, try cleaning it.