Peavey Musician 400G

[glebert]

I don't know why people are saying the layout might be wrong. Seems like some wrong stuff got put in at some point. This is pretty common module, MEF would be littered with "that schematic is known to have a layout issue" comments.

I may have missed something, but we are trying to get stable voltage rails at this point, correct? Considering what you are seeing I would basically be pulling every transistor out of power amp, going from biggest to smallest, and trying to get stable voltage rails. I'm not saying that is the smartest thing to do, but it's what I would do...

[nickbme07]

I don't know why people are saying the layout might be wrong. Seems like some wrong stuff got put in at some point. This is pretty common module, MEF would be littered with "that schematic is known to have a layout issue" comments.

I may have missed something, but we are trying to get stable voltage rails at this point, correct? Considering what you are seeing I would basically be pulling every transistor out of power amp, going from biggest to smallest, and trying to get stable voltage rails. I'm not saying that is the smartest thing to do, but it's what I would do...

Yes basically at this point, get stable rails. The main power transistor modules are not hooked up, nor is the preamp board. Its about as raw as you can get, and i even now have the U3 opamp out.

[The Dude]

The problem with that approach is that the transistor (power amp stage) is supplied by the +&-52V rails. The supply that is low is the +15V rail

[The Dude]

If removing the op amps doesn't bring up the +15V supply, try changing the regulator.

[g1]

Not sure why everyone keeps saying that. That blue resistor is a 47ohm 1/4w resistor I installed to R61. The same is in R26 now.

Sorry, I re-read and saw my mistake, but was not quick enough with the edit.

But I can confirm, it is L1 inductor that is sometimes replaced by 47R resistor, here is a pic from another amp:

[nickbme07]

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?

[nickbme07]

Sorry, I re-read and saw my mistake, but was not quick enough with the edit.

But I can confirm, it is L1 inductor that is sometimes replaced by 47R resistor, here is a pic from another amp:

Thanks for that! Looks like they installed their resistor and diode flip flopped! Doh

[nickbme07]

If removing the op amps doesn't bring up the +15V supply, try changing the regulator.

What exactly are you referencing with regulator for this circuit so we are clear thanks

[The Dude]

VR1. Probably a 7815.

[nickbme07]

VR1. Probably a 7815.

Can i test that in circuit? Tests ok in circuit anyway, as well as VR2. It's a 78M15C. VR2 is a 79M15C

I get the same voltage reading on the 3rd leg of VR1 as I do on C36 +

[The Dude]

I think you might be assigning pins backwards (assuming it's a 7815). If it is a 7815, pins are 1=in, 2=gnd, 3=out. So pin 1 should be the same as C36 and pin 3 should be the same as C40. If it's not a 7815, then ignore that. As far as testing, you could unsolder just the output leg and lift it from the circuit. Then, carefully measure on the regulator pin to see if the voltage comes up. Here's the problem with that: Sometimes a regulator will work until you load it and then its output drops. Really, the best way to tell if it's bad is to replace it.

[nickbme07]

I think you might be assigning pins backwards (assuming it's a 7815). If it is a 7815, pins are 1=in, 2=gnd, 3=out. So pin 1 should be the same as C36 and pin 3 should be the same as C40. If it's not a 7815, then ignore that. As far as testing, you could unsolder just the output leg and lift it from the circuit. Then, carefully measure on the regulator pin to see if the voltage comes up. Here's the problem with that: Sometimes a regulator will work until you load it and then its output drops. Really, the best way to tell if it's bad is to replace it.

Ok i was just counting left to right, those arent marked on the schematic. None the less thats why i mentioned its the leg as the same with C36. Ok so VR1 pin1 gives the + voltage as expected, also VR2 shows the negative as expected.

[nickbme07]

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

[g1]

Ok i was just counting left to right, those arent marked on the schematic. None the less thats why i mentioned its the leg as the same with C36. Ok so VR1 pin1 gives the + voltage as expected, also VR2 shows the negative as expected.

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.

[The Dude]

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.