Peavey Musician Mark III series 400g troubles

[ricksdaname18]

Hey Enzo,
It is a negative 51 volts at the emitter of Q9. Pulled R59 and R60 and both look good. Stole the IC from the preamp board but it made no difference. Still positive 48 volts.

[Jazz P Bass]

Did you check Q2 for a short?
If it is shorted, that would explain the 48Vdc base measurement.

[ricksdaname18]

Yes Q2 was one of the initial problems I started with. I tried to do a "no load" test on the power amp and had the power transistors unpugged but didnt have the autoformer unplugged. It burned up R26 when I did this and I later found Q2 shorted. Im guessing that was my fault. I ordered the new Q2 (5331) and resistor and checked them when I got them. Im also confused about whether or not u can unplug the power transistor banks from the board and be ok applying power to the unit. I read some conficting info somewhere.

[Enzo]

Normally, any current draw from the output bus will be haqndled by the output transistors. If you unplug them, then any fault current must be handled by the tiny 47 ohm resistors and the small predriver transistors. I wouldn't power up the amp that way.

[ricksdaname18]

Makes sense, then the autoformer just exacerbates the problem. So what kind of reading if any at all should I be getting at the speaker jack? I dont know if it makes any difference what the circumstances were when the amp failed but... I had a microphone looped on a delay pedal in close proximity to the speakers. It was ok for a while but then suddenly seemed as though the sound kept backfeeding and the speakers got louder and louder until it quit and popped the fuse. This probably happened in 5-10 seconds. There was also a 2 into 1 input jack splitter that split the signal between channel A and B. I wonder if the speakers getting louder was related to me now having this 48 volts, like the voltage kept going up then the speakers got louder. I wonder if it overdrove the amp with the delay loop. This is also when it had those NTE replacement output transistors in place which evidently dont bias the same as the SJ6357 ones, making an unbalanced load and a weak link in the operation.

[Enzo]

ANy DC on the output would put DC acros the transformer, which would not like it.


When I work on these, if they ar blown, you know what I use for a test signal? MY finger. If I have DC going on, then I don;t need signal anyway. But once I have the amp stable without any offset, I just reach over and touch the input pin. I get a hum. Good enough to tell me if it is amplifying or not. Once it seems to work, I can then apply a more musical signal.

I usually also leave the preamp not conected. Not that it matters either way to the amp, but just fewer cables and stuff inthe way.

I have signal generators, but most times I just take a line out from my shop stereo tuned to a music station and use that signal. I use clip leads right to those corner connector pins.

[ricksdaname18]

I have tested the 15 volt circuit. At the preamp connector I tested for positive 15 volts at the red wire and negative 15 volts at the green wire. Had the positive voltage but not the negative. Went back to the regulators and found positive 34 volts and positive 15 volts at VR1. Also found negative 28 volts at VR2 but no negative 15 volts, bad regulator? Also understand that there should not be continuity between the collectors of the power transistors and ground. I have continuity on the first bank of transistors. I can switch leads and get continuity one way but not the other, diodes??? Also was advised that I could switch the 2 big filter capacitors around and see if I then get the negative voltage instead of the positive, wanted to see what u guys thought before I try anything else.

[Enzo]

REgulators have three terminals. If there is voltage to the input and none on the output, looks like time for a new 7915 to me. Missing one of the 15v rails means the amplifier ICs can;t work, and that surely is enough to upset the amp circuits. Fix that first and then see what else might be wrong. Without proper power supply, all other readings are pointless.

COntinuity. Learn the difference betwen continuity and low resistance, and diode connections. COntinuity really means a direct connection. If you have say 200 ohms between two points, that really is not continuity. We might informally use the term just to mean the 200 ohm resistor wasn;t open. But across an output transistor is not a case of that. If your continuity test is also your diode test on your meter, then if it reads different one way from the other, then you don;t have continuity, you have a diode or other semiconductor junction.

CR11 and CR16 are across the two output transistor groups. They act like flyback diodes, they prevent any inductive kickback from a speaker going past the power rail voltage. That would appear to the transistors as reverse voltage and they would not be happy about that. If you want to check for shorted transistors, the ohms feature will tell you zero ohms of not. The diode/continuity test will measure the junction drop, which is fine, but understand what it is. If the transistors were shorted, it would measure so both directions. If that checks like a diode, then those two are probably fine.

DOn;t bother swapping the big caps, they are not involved with the 15v supplies.


Pin 14 of U1 tells the power amp circuits where to go. It normally sits at about -0.87v. That requires both +15 and -15 on the IC's power pins. If one is missing, the voltages in teh IC will all slam over to the remaining power suplpy, and that will tell your output to swing wide as well.

[UnluckyFett]

Hi sorry to jump in but i have the same amp (400 b/g) with a similar problem. it doesn't blow fuses i just don't get any output. it powers on with a pop through the cab and no hum with the good ole finger check then pops when power off. i know tube amplifiers and tube electronics but im lost with solid state. i don't even know where to start measuring on this. it worked perfectly then was put in storage and now doesn't work. it looks like it got some moisture in storage. theres some minor corrosion on the foil side of the pcb where the main transistors are. all the components look ok. any help would be greatly aprecieated. i again am greatly sorry for barging in the thread. thanks

[Enzo]

Hi, and welcome to the forum. Here is how to avoid the barging in thing, just start a new thread for your amp. That way we won't have to try and keep track on two separate repair efforts intertwined in the same thread.

[UnluckyFett]

Thanks. sorry for the interruption its just on other forums I've seen people complain because theres already a thread for it and i didn't want to cause any problems. i started a new thread at http://music-electronics-forum.com/t26534/ like you've suggested and would greatly appreciate any help troubleshooting the amp and learning more about solid state. thanks and sorry again for interrupting the flow of the thread.

[Enzo]

No problem, no apology needed.

I think in many forums that just discuss topics, they don't want five threads all about "what is your favorite movie" or something. But here, the point of the threads are usually to discuss a particular repair, so we rather prefer them separate.

Besides, threads are cheap, we can start as many as we like.

[ricksdaname18]

Hey Enzo,
Replaced VR2 and took care of dc on speaker jack. Plugged in amp and working great! Thanks so much for your help. I feel confident now that I could fix another one of these amps. Now that that project is completed, do you know anything about televisions?

[Enzo]

I know a lot about televisions. They attract dust and dirt inside due to large static charges. Old ones were full of wax caps which got all over your fingers.. The TV at my house has the old lady's cop shows on 24/7. I can;t watch National Geographic to save my soul.

DO I work on the damned things? No.

[ricksdaname18]

Oh ok, well wish me luck!