[sdiy] Amp / power transistor woes

Oren Leavitt obl64 at ix.netcom.com
Wed Jul 18 23:22:56 CEST 2012

On 7/17/2012 4:31 PM, DTK wrote:
>> 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.
>> Ingo
> Fortunately, I tested the tranny from the "good" channel BEFORE I put it
> back into the amp.  Just a stroke of luck, really... I tested it on a whim.
> Each channel has 2 darlington pairs in a push-pull configuration. I can
> see how a failure in one channel would blow all the trannys in that
> channel, but the fact that I have issues in the other channel seems to
> point to some root cause common to both.  As far as I can see, the only
> thing common to both channels is the power supply, so I suppose that's
> where I'll look next.
> Right after I pull and test all the other transistors, anyway.

I'd definitely replace all transistors in the output stage as a set, 
including the darlington-connected transistors driving the outputs, the 
bias transistor, the current limit sense transistors, etc. Surviving 
transistors may check "good" but they'll likely be weakened or out of 
original spec.
Not an amp guy but I've fixed a couple dozen solid state power amps over 
the past few decades.
- Oren

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