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-   -   dead output fets? (http://www.diyaudio.com/forums/pass-labs/92751-dead-output-fets.html)

traw 23rd December 2006 05:23 PM

dead output fets?
 
So, got my aleph5 constructed. After testing power supply, hooked up both channels as had made these boards before to test them out and usually meticulous on stuffing.

killed my fuse after starting up, about 1minute after start up. how long does it take for cl40 thermisistor to open up fully?

so, trimmed down to just one channel works fine, minimal to no significant offset. let that run about 45 minutes, heat sinks holding up well.

went back disconnected working channel, connected untested channel.

had tiny tiny puff of smoke, fuse blew about 10 seconds. near the + side first output fet.

they are to3p devices and use alum oxide pads so not suspecting broken insulation like when using silpad etc.

Is it likely to be a damaged output fet? Wondering if damaged it overheading the drain during soldering - seems to soak up more heat than gate or source.

I suppose could replace the mpsa18's while in there to be safe... Can it be powered up with out the irfp240's to test?

chipco3434 23rd December 2006 06:13 PM

how long does it take for cl40 thermisistor to open up fully?
1-2 seconds

i'm too stupid to answer anything else other than the smart guys will ask you to tell them what component got cooked.

Netlist 23rd December 2006 09:47 PM

Chances are great that the output or current source mosfet is broken. Replacing it is the easiest way.
Measure the DC voltage across the gate resistors, it should be 0V. If not, the corresponding mosfet is broken. Use a light bulb and a variac if possible.

/Hugo


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