Looking for help identifying burned up part on Crate PA4 circuit board

[davelang68]

Hi - I was playing an outdoor show near the water last weekend and was given a power line to use that was running 208 volts vs. 120. The organizers of the event had no clue what they were doing re: electricity. So it goes. I determined the voltage using a multimeter after the bass amp and the PA head died. Argh. The bass amp just needed a new fuse the but PA head burned something up.

It's only a cheap 4 channel Crate PA 4 but I like it cause it's through hole design vs. smt and should be fixable for a long time in situations like this.

So I took it apart and found the burned up part which was pretty easy as you can see by the pictures. I was planning on just reading what the part label was, comparing that to the schematic I have, firing up the soldering iron and fixing it myself.

But I've been foiled by the disappearance of the part label. I think the part melted the label when it burned up.

Could anyone help me figure out what part this is? See attached schematic and pic.

I was planning on replacing the two diodes immediately above the burned up part (D1, D12) as well just to be safe since the parts are cheap and it takes a while to disassemble / reassemble the thing.

Any other advice appreciated.

Thanks!

[davelang68]

Here's a picture of the back of the circuit board.

[sofaspud]

It looks to me like it was the part that connects between R20 and base of Q24. That would be R16, a 100Ω resistor. You should verify this as it's somewhat of a guess, made by tracing the PCB.

[davelang68]

Thanks very much - that's what I thought it might be based on looking at the schematic and crossing off all the parts that I could find on the board that were connected to Q4.

Oh wait - did you mean Q4 instead of Q24? I don't think there is a Q24 on the board.

[sofaspud]

My bad. Dunno where I got the "24." I meant Q4, the TIP32 TO-220 transistor at the bottom of the first pic.

[davelang68]

Thanks again - I have a question just out of curiosity - did you look at the schematic or could you identify that transistor just from the blurry pic I posted? I'm not being combative - I'm just impressed if you did.

[sofaspud]

I looked at the pics and looked at the schematic, then used both process of elimination and "sure looks like it connects there." I only knew the transistor type by looking at the schematic, but that was a good clue since the other transistors in that section are in smaller packages.

[Enzo]

And resistors don't burn up on their own, so if that resistor burnt up, something connected to it is also bad. Most likely a semiconductor.


Along with the power amp schematic, I have included the parts layout with parts list.

[davelang68]

Brilliant - thanks for those files, Enzo! They confirm that the part that's burned up is R16.

Question for you - did you read the explanation at the beginning about the problem with the power supply we plugged in to? I was hoping that would explain why the resistor burnt up.

thanks again,
dave

[Enzo]

Yes, I read the entire thread.

That may have been the underlying reason the system failed, but in a repair situation we also must look at the failure itself. Faulty power may have caused such and such a transistor to fail, which in turn burnt up a resistor, or some similar sequence of events. But we are now looking at a resistor. If that resistor burnt up, something else allowed the excess current to flow through that resistor, and we need to find out what that is.

If a college kid gets drunk and falls off the roof, the emergency room may like to know that, but it doesn't really tell them which organs will need surgery.


There is no magic list of things that will fail when 208v is applied instead of 120 to the main plug