Amp experts: Resistors burning in amps?

[Sunbane]

Hey all, I hope someone here can answer this question:

Is it common with resistors burning in an amp?

My reason for asking;
My GF was recently given a small solid state PA system that isn't working. The previous owner told her that something burnt in it during a rehearsal, and after that it went dead. They haven't bothered to look into what wrong with it, so she took it off their hands, and I open it up to see what's wrong.

I found one spot where something has clearly been burning - there's soot on the circuit board and on the inside of the casing. What surprised me was, that sitting in this spot were two ordinary resistors, and I've personally never seen resistors burn - I thought they were almost indestructible (relatively speaking).

I also found a spot on the board where it seems that two diodes have been running pretty hot, and I'm wondering if that may be a problem in case I can get this up and running again?

Pics:

Cover:


Face down, cover opened:


Amp board in the upper right corner:


Resistors 1:


Resistors 2:


Diodes 1:


Diodes 2:

[Infernal Death]

Hi
No, resistors can burn. Actually besides the ohm rating they also have a power rating which they can take. The normal small resistors have 1/4 W of power they can take. But you can also buy resistors with 2W, 5W, 10W or even more. They just are bigger (and normally of different material).
In one of my selfbuild units i also had some resistors smoking. I once shorted out a voltage regulator which stopped working and the resistor behind it was hit with the full voltage and started to smoke. So if something in the psu section went wrong it could be the reason.

Maybe you should try to disconnect the power supply section and measure if your basic supply voltages are still ok. Would be good to know before proceeding.

Flo

[Infernal Death]

Looking again at the pictures it actually is the power supply section where your burned parts are located. So disconnect it from the rest of the circuit so you won't damage anything other.
The first thing i would do is to change the voltage regulators. Those are those transistor looking things in the TO220 housings. Actually one is really near your burned resistors.

Flo

[Sunbane]

Thanks! This is exactly the type of response I was hoping for. :cheers:

So the voltage regulators are those black thingies with 3 legs and the heatsink-looking thing on top?

[DonP]

So the voltage regulators are those black thingies with 3 legs and the heatsink-looking thing on top?

Yep.

Screen grid resistors will go up in smoke as well, usually when the tube takes a shit. It is often recommended to use the same type so that they will burn again in the future, instead of buy resistors with a higher power rating. It's better to replace a cheap resistor than to replace something more substantial upstream if the resistor doesn't blow. In a sense, the resistor is being used as a fuse.

[Sunbane]

Great! Thank you!

Now I'll just have to figure out what value resistors and regulators I need and order the parts. Oh, I plan on replacing the diodes in the last picture as well. I figure that if they've gotten so hot as to make the board brown, they may not have much life left in them, yes?

[jackson1]

Yes, change the diodes as well. Don't bother measuring them. Hopefully you can read what type they are on the barrel shaped body; 1Nxxxx possibly.

Resistor value color charts can be found on the net i'm sure.

[jackson1]

The resistors that has taken damage seems like 1% metal foil 1/4 watt types. The voltage regulators should have a type # stamped on the square'ish body as well. Probably 15 or 12 volts type.

[jackson1]

Btw., lousy pics, JJ!

[Sunbane]

They were taken with my phone, as I don't own a camera.
Anyway, thanks for the tips, Henke!