Peavey Musician 400G

[nickbme07]

Well now it looks like I'm dealing with an entirely new issue. I am shorted out somewhere. At power up on variac, no load, it does its normal jump to around 1amp draw and then settles, but now when I get to around 10v on mains I can hear the transformer and then the +52v rail bever gets over 14v or so as you watch something pulling it down from main filter cap.

[nickbme07]

Can I use 0.1uF 100v Mylar Film caps in C22,C38,C39,C13,C14,C27,C28?

[The Dude]

Sure, but why? I have a hard time believing that all of those caps are bad.

[nickbme07]

There were a few off and a few basically falling apart so I replaced them all with the ones I had on hand so wanted to check.

[nickbme07]

With the autotransformer disconnected I am able to get to around 70v on mains (limited/variac) but then start hearing a slow chirp pitched sound like a bird. I can't seem to get -52 rail stable now. It seems as if something is pulsing, at the 60v range on mains I can see the wattage draw move from 18w to 25w with the frequency of the audible sound.

Shouldn't I be able to run this without the autotransformer?

[nickbme07]

I'm not feeling good about these Mylar Film caps. I'm only getting volts on one side of a few of them.

[The Dude]

That's because one side goes to ground on most of them. Caps do not pass DC voltage. You have a shorted or near shorted rail. Find the short. Throwing components at this will get you nowhere. You'll have to troubleshoot to find the problem. As I said before, the caps were probably not bad to start with.

[nickbme07]

I missed that being non polarized. You are right. But I had already replaced the caps back when it was half working, I was just making sure the types were OK because the originals were stamped polarized and installed opposites of each other (in the pairs).

I am getting -0.8v at the + of C40 with 50v mains.. Best way to the track short down?

[The Dude]

The + side of C40 is your 15v rail. What is the resistance from that rail to ground? That rail supplies pin 8 of the op amps. If it measures low resistance or shorted, you'll have to start disconnecting things from that rail until the short clears. It could be an op amp. It could be a cap. It could be the regulator itself, etc. What is the voltage feeding the regulator or the + side of C36? Check op amps with your finger. Are any of them getting hot? If so, that's a clue. Unfortunately, there's no fast way to find a shorted rail. It could be anything connected to 15V.

[nickbme07]

For R70, is it OK to use a 10k and 4.7k in series in that position?

[The Dude]

As long as they are both rated at 1W. That resistor is connected to the +and- 52V rails, though. Do you have multiple problems?

[nickbme07]

The original when I got it was reading 4.3k out of circuit, I had replaced it with the two from above while it was half working. Yes 1w each

[The Dude]

Was it marked as 4.3K? It's odd that a 15K would read low. Resistors burn open or become higher resistance. They don't lower in resistance. So, either there was a circuit revision, a previous/incorrect repair, or a measurement error.

[nickbme07]

So there were two resistors I decided to replace. R70 (15k) and R18 (4.7k). I have them and they are coded as a 4.7k, tests 5.4k (not 4.3, sorry) and a coded 1.5k, tests 1.7k.

Does this mean I actually found the resistor that is suppose to be a 15k and was put in as a 1.5k? Its brown,green,red,silver

[nickbme07]

R26 and R61 want to burn up now with transistor modules disconnected - This was not happening when the amp was 'half working' with full mains AC, with or without the modules connected. Something may have shorted out since and now causing additional problems on that rail. Ill have to figure out how to track that down so I can even power up passed 50v on variac..

Also, Is it normal that L1 is a 4.7k resistor and not the 100mh inductor?