[sdiy] Amp / power transistor woes

Ingo Debus igg.debus at t-online.de
Tue Jul 17 22:10:24 CEST 2012

Am 17.07.2012 um 02:49 schrieb DTK:

> 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.

Something like this happened to me more than once. Each time I swear to myself that next time I'd check ALL semiconductors in a power amp when one (or a resistor) is blown.
In a power amp, often one bad transistor causes others to fail. So you find a blown power tranny, replace it, power the amp up then just to have the new transistor blown as well. Probably this happened to the transistor from the good channel: it was fried after you put it into the bad channel and powered up. Or did you check it before you soldered it into the bad channel?

Anyway, a short between C and E, with the base open (tranny removed from circuit of course) definitely means a blown tranny.


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