[sdiy] Amp / power transistor woes

DTK aurastar at cox.net
Tue Jul 17 23:31:29 CEST 2012

> 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.

More information about the Synth-diy mailing list