[sdiy] Amp / power transistor woes

Stewart Pye stewpye at optusnet.com.au
Wed Jul 18 22:57:34 CEST 2012

On 7/18/2012 7:31 AM, DTK wrote:
> 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.

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.

Depending on how the transistors are soldered to the PCB sometimes it's 
easiest to just desolder two leads (C, E) to test them. You'll also need 
to check the driver transistors and resistor between the drivers and 
output transistors. If in doubt replace the driver trannies. If the 
driver trannies are blown keep going further down the line. Also check 
all emitter resistors (with shorted transistors removed) and all small 
value or fusible resistors and all diodes.

Make sure the transistors are insulated from the heatsink when you 
re-mount them.


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