[sdiy] Amp / power transistor woes

[Stewart Pye]

On 7/19/2012 7:22 AM, Oren Leavitt wrote:
>
>
> 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.
> HTH
> - Oren

Without knowing what amp it is I'd have to agree about replacing the 
outputs as a set. With some amps you can just replace the shorted ones, 
do a power test and if it passes you won't have a problem again. On 
others you will be wishing you had replaced the whole lot as a set.

Also check that the bias trim pot is not open circuit.

What model amp is it?

Regards,
Stewart.