The most common thing I see in failed FETs is drain to gate
leakage or short circuit.
Some basic checks: Measure resistance from the gate to drain and
then source - both should be very high resistance, be suspicious
of anything under 1Mohm. Short gate to drain and measure drain to
source both ways round with a DVM on diode check. One way should
be open, the other should read 500-700mV, like a power rectifier
diode. If all of these are ok, use the diode range with + to drain
and - to source. Connect a 9V battery between gate and source. +
to gate should read very low resistance, - to gate very high
resistance.
Steve
> Gretings Amps Guru's,
>
> I have a HF PA with a pair of SD2931's and I think they "popped" due to
> a parasitic while tuning at low power. I'm not sure of the failure modes
> for these puppies and I am looking of a short checklist of what to measure
> before trying to rip and replace the finals. Or maybe there a website with
> troubleshooting tips for power FET amps?
>
> Any thoughts, guideance, leads, or suggestions would be appreciated.
>
> Thank you in advance.
>
> 73 Mike K9MK
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