I have in the past found power MOSFETs can partially fail, where they
appear to be working when simple go/no-go tests are applied, but quickly
get hot and fail when put back in the circuit. so it is certainly a good
idea to do the tests Alan and Stewart have suggested. In particular,

pretty

well no gate current should flow, so if you connect a clip lead to the

gate

to bias it on, then remove the lead without touching the gate, it should
stay biased on for several seconds at least, probably minutes or more..

The

"on" resistance of an IRF250 is 0.085ohms maximum (with 10V Vgs bias), so
too small to measure with a multimeter - better to put a known current,

say

1A, through it and measure the voltage drop between source and drain leads
(should be below 85mV).

Also check all the other components in the PA circuit - all the diodes and
zeners, the driver transistor etc. When one component goes, it tends to
take others with it. When you put it back together, with no "HT" supply
check there are reasonably clean 136kHz square waves driving all 4 gates

at

about 15 - 20V pk-pk, symetrical around 0V. Start off running the module

on

a low "HT" voltage, say 5V (but don't reduce the driver supply voltage!!!)
- the current drain should be down in proportion to the reduction in

supply

voltage with a class D amplifier. When you are sure it is working OK at

low

voltage, gradually increase to the full supply voltage, checking that
things don't get hot. Starting off with a low supply greatly reduces the
possibilities for blowing things up again.

Many thanks to Jim, Alan and Stewart for their advice regarding testing
FETs.

From this I find that you can test Decca Tx module FETs (IRF250s) in situ

using a variable PSU with voltage and current control and indication.
Disconnect the transformer wire from the gate but leave the protection
diodes in place (BZX79C10s). Set the PSU voltage to 10 or 12 volts and the
current limiting to (say) 0.5A. Connect the positive and negative leads from
the PSU to the drain and source respectively. Touch the gate with your hand
(best done with the blade of a small screwdriver). This causes an indication
in the current reading. To get full current touch the positive lead
(connected to the drain) with the other hand. This should cause the current
to go to the max limit set on the PSU. Repeat for all other suspect FETs.
Using this method I quickly located the faulty FETs.

Regards,
Peter, G3LDO

e-mail <[email protected]>

Web <http://web.ukonline.co.uk/g3ldo>