I've run into an interesting problem on a Peavey 400BH. A friend brought it to me claiming that it immediately blows fuses after being turned on.

While checking out the power section I noticed that where CR13 (one of Peavey's 13886 diodes) should be there appeared to be a jumper. I did a diode check on it, and it registered 1.14 volts, which seems like an appropriate voltage for what the 13886's are supposed to be (correct me if I'm wrong, but I've read that they are two custom power diodes in series).

However, since CR13 is in parallel with the collecter and emitter formed by the darlington pair of Q3 and Q4, Q5, & Q6 in parallel couldn't that 1.14 V drop simply be from those transistors?

I'm not about to power this thing up with an apparent short across the power transistors, and am planning on contacting Peavey for a couple of 13886 diodes, but I'm hoping someone could shed some light on why this would have happened. Is it somehow part of a larger power supply fix that I am not seeing, or is this another case of people doing crazy things to amps??

Schematic and picture of jumper attached.

*Edited to note that I already know that the diode next to the jumper with the big crack in it is bad (CR11)!

Hey guys!

So a friend passed along an 80's solid state Sunn Coliseum 300 head to me that he claims has been blowing speaker cabs. Before it ended up in my hands he said that he took it too an amp tech that said it had a short but that he wouldn't work on it. I have some experience with doing moderate repairs on tube amplifiers but have yet to work on an amp that is entirely solid state and was hoping to get some input.

I built a light bulb limiter and when i turn the Sunn on the 100w light bulb i used momentarily is bright and then dims, but i have no reference as to what a functioning 300w solid state amp would look like in terms of the light bulb brightness. Would a 100w tube amp be comparable?

From looking around on the forum here it seems like a potential cause for blowing speaker cabs would be a shorted transistor in the last stages of the power supply that would cause DC voltage to be present across the rails of the speaker output. I checked most of the transistors (but honestly not all of them) for a short and could not find one. I also checked for DC voltage between ground and the speaker outputs and there was none.

Does anyone have any suggestions about what else I should check before passing this amp back to my friend? Are there any other problems that could cause a head to be consistently blowing speaker cabinets? Any input is appreciated! Thanks!

Highest quality schematic i could find is attached.