Peavey Musician 400G

[nickbme07]

If your +15V rail is not working, the output of U1D will go towards the negative rail and upset the power amp. Until you get that rail back up, it's best not to power up for more than long enough to check the rail voltage. In fact, it might be a good idea to unhook the power amp supply while you fix the low voltage supply by unplugging the transformer connector that has RD-RD/YL-YL. That will allow you to repair the 15V supply and test it without damaging the amp.

Ok, will do. That seems to allow me to at least see the short happening without the high voltage side, thanks.

When I had the board out I made note of 2 diodes on the solder side. Where are those on the schematic, I can't seem to find them. CR14 is accounted for. One of them goes to the + of C40 and is grounded, so that sticks out to me. It seems to test both directions so wanted to make sure of the model, as the print is not clear. I drew where the diodes are on the solder side for both diodes.

Edit: So with 30v AC on mains, I get +7v on +C36 and -7v on -C37. I havent tried to push it harder, but seems stable without high power connected now. However the +15 output molex for preamp is showing -volts and goes lower as I apply more mains voltage. This pin shows 561ohm resistance to ground (without completely tracing circuit, im assuming the preamp gets its +15 from the +15 rail).
The negative side of C35 shows a steady -1.7v and seems to be consistent across the circuits and doesnt change. Can I put more mains on and get rail to +15 without messing anything up with high power disconnected?

[nickbme07]

Are you talking about the input or output voltage of the regulators being as expected? There should be + and - 15V supplies coming out of them.

Yes, one has the +7v while the other has -6.1v under around 25v mains

[Helmholtz]

So that little yellow cap (C26, 180pF) tests out of circuit as .40nF, or 490pF. It's resistance shows open.

I have some 181 ceramic 180pF 1kV (used for PRS treble bleed haha)(they also test as .49nF,, should this go in or leave it alone?

Leave the cap alone.

Forget your C-meter with low cap values.

[Helmholtz]

If I let it sit on the variac at a few volts, R61 and R26 do not take long to get to 100 degrees with ir gun. This is with the power modules not connected and the variac draw under .1amps.

If i hook up the modules, i get -9v on R61 and -0.1v on R26 and the variac at 20v draws .20amps

A side-remark:

Are you sure the autotransformer is/was out of circuit?
Don't forget the autotransformer is a DC short. So any DC imbalance would cause a large short-circuit current at the output without showing as DCV.

[nickbme07]

Are you sure the autotransformer is/was out of circuit?
Don't forget the autotransformer is a DC short.

Yes, ive kept the autotransformer disconnected now. I have the molex to the output jacks still connected (no load attached)

[Helmholtz]

Yes, ive kept the autotransformer disconnected now. I have the molex to the output jacks still connected (no load attached)

Just wanted to make sure.
It's not clear from the schematic if unplugging the molex actually disconnects the autotransformer.

[The Dude]

That diode you show in post #91 next to VR1 regulator goes from regulated output to ground, so it's likely a 15V zener diode. Not sure why they added that with a regulated output already existing. That diode could be leaky/shorted. Unhook one side of it and check it. Or, unhook one side of the diode and see if your +15V comes up.

[nickbme07]

That diode you show in post #91 next to VR1 regulator goes from regulated output to ground, so it's likely a 15V zener diode. Not sure why they added that with a regulated output already existing. That diode could be leaky/shorted. Unhook one side of it and check it. Or, unhook one side of the diode and see if your +15V comes up.

Those two diodes seem to be (possibly) the only components changed (added?? I cant seem to find on schemtic or layout) when i got it. The center connection on the orange molex had a splice made into it and shrink wrapped, maybe wire was damaged at some point in that section (just didnt want to disclude from my findings)

[g1]

That diode you show in post #91 next to VR1 regulator goes from regulated output to ground, so it's likely a 15V zener diode. Not sure why they added that with a regulated output already existing. That diode could be leaky/shorted. Unhook one side of it and check it. Or, unhook one side of the diode and see if your +15V comes up.

Sometimes reverse biased diodes are added across VR output as protection.
So it is important to find both the type and polarity of those added diodes.

[nickbme07]

Probably due to the VR's being a 78xx/79xx with the lock problem.

https://groupdiy.com/threads/peavey-...des-why.60850/

[nickbme07]

They just seemed to be 4004's. I changed them out and now things look more stable and not up and down everywhere (- to + swings) but I still cannot get +15 or -15 to get over 0.5v. I am able to get more volts on variac now, but something is still sucking the rail down.

[g1]

Have you tried with those diodes disconnected? If they are reversed they will hold the low voltage rails at approx. 0.5V
Is preamp still disconnected? How many IC's are there in power amp?

still cannot get +15 or -15 to get over 0.5v. I am able to get more volts on variac now

What do you mean by this? Either they will go over 0.5V or they won't. Until you can get +/-15V, nothing else matters. Do you measure low resistance across C40 or C41?
If so, you can track down the problem without powering the amp and possibly causing further damage.

[tdlunsfo]

Not sure if this helps but I had a similar issue with a similar amp.
https://music-electronics-forum.com/...gulation-issue

[nickbme07]

Have you tried with those diodes disconnected? If they are reversed they will hold the low voltage rails at approx. 0.5V
Is preamp still disconnected? How many IC's are there in power amp?


What do you mean by this? Either they will go over 0.5V or they won't. Until you can get +/-15V, nothing else matters. Do you measure low resistance across C40 or C41?
If so, you can track down the problem without powering the amp and possibly causing further damage.

Yes while i had the diodes out i powered up and was the same as with them in.

I mean that regardless of volts on variac, I can't get over the 0.5vdc on +C40. I installed diodes and +15 stays at 0.5v regardless of mains voltage. Same reaults with both (2) IC's out. Resistance between +C40 and ground is 4.1k. -C41 and ground is 5.5k. Both measure short on opposite side (ground to ground).

I could maybe use some help determining the proper orientation. I followed how they tested coming out, they went back in. Testing their solder points with them out, they showed a diode test as ok, so there are others they are in series with. After installing new ones, diode tests look the same as before.

Should those diodes cathodes both be on ground, or should they be opposites of each other? I ask because of your comment of 0.5v. One of the diodes is oriented with anode on ground, it's cathode is on the -15 pin of the preamp out. The other diodes cathode is on ground, with its anode on the +C40.


I see a little bit of AC (80mV) at 30v mains on -C41. +C40 does not. Also with the 30v mains, showing -1vdc (-C41) and +0.6vdc (+C40)

I don't plan to hook up preamp until I get +/-15 at the preamp molex.. which its + pin reads the same as the +C40

[nickbme07]

Diode output of +15 circuit looks ok. I can get +C36 to +15vdc. The -C37 follows with its negative dc, just a few volts behind, however I do see almost +1v ac at the -C37 as well. It tries to go higher but something is pulling it down to around +0.8vac as I turn up variac. VR2 doesn't seem to get lower than -1v dc, VR1 seems to output its +dc fine

Edit: VR1 input from diodes is fine.. I'm assuming the output to the R22, R24, etc path, and that does not have a good dc output. Seems VR1 may be bad (now). This wasn't an issue when it was 'half working'.. may have been what was affected when it shorted out to ground with pcb removal. What is a good replacement for the VR's (320/340 on schematic, the 340 is a 78M15C on board).

Edit: I ordered some 7815's and 7915's. The 79 seems to output negative as expected. I'll replaced the 78 when it comes in, thanks guys!