[sdiy] Amp / power transistor woes

DTK aurastar at cox.net
Tue Jul 17 02:49:48 CEST 2012

I'm looking at a guitar amp for a bandmate and its driving me crazy...  I 
know its not quite synth related, but I'm hoping some of you folks will let 
me pick your brains anyway :)

Initial symptom, no power/no sound.  I found a blown fuse and a few burned 
up resistors, replaced the fuse and damaged components. Got power, but also 
got smoke pouring off a 10W .33ohm resistor.

Got the schematic and started tracing back, determined that the smoking 
resistor is hanging off the emitter of one of the power transistors in the 
output section.  Pulled out the transistor, tested it with my DVM, its 
shorted from C->E.  Bingo, problem solved, or so I thought...

This amp has 2 channels, identical circuitry, so I had the bright idea to 
swap out the corresponding transistor from the other channel as a quick 
test, before I source a replacement.  On a whim, I decide to test the "good" 
transistor, and I find the same issue - shorted from C->E!

So, now I'm a bit stumped.  I was pretty sure that C->E should NEVER be 
shorted, but I looked it up just to be sure, and that's correct.  So I'm 
either missing something huge, or all the power transistors are fried, with 
exactly the same failure.  I could almost believe that, except I didn't get 
any smoke off of the corresponding .33ohm resistor on the "good" channel.

Not quite sure where to go with this now.  I'm going to step away for the 
day and hope for an epiphany tomorrow, but if anyone has any suggestions in 
the meantime they would be much appreciated!


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