[sdiy] Amp / power transistor woes

[DTK]

>The cause may be that there was a short on the output of the amp. (cable
>or speaker). When the user didn't get any sound out of the first channel 
>they plugged it into the next and blew that one too.

I don't think this could have been the case.  The amp was driving a 2 
speaker cabinet, with a speaker on each output, and I think the fuses blew 
before any channel switching could be done.  I suppose both speakers could 
have failed at the exact same time, but that seems very unlikely.


>> I'd definitely replace all transistors in the output stage as a set,

> 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?


Its a Crate GX-130C.

So far, of all the components I've checked, only the 4 output transistors 
are bad.  No other components in the signal chain or power supply have any 
issues, although I'm not done checking everything yet.

I'm leaning toward replacing all the transistors, as that has been advised 
by several people at this point.  Aside from the output trannies, they're a 
few cents each, so I think that's worth it.  However, I'm leery of replacing 
anything until (unless?) I can determine the cause for the initial failure. 
I don't want to needlessly fry components.

The power supply still seems to be a likely culprit, but I haven't been able 
to find a fault so far.  I'd like to do some voltage checks, but the way the 
board is designed its a little tricky to isolate the PS from the rest of the 
circuitry.

Last night I was actually considering just re-assembling everything and 
powering it up without the output transistors in place.  I think I'd gone 
batty lol.  Fortunately I seem to have a clearer head this morning.