DaveC wrote:
In my 400W audio power amp, a power MOSFET died, I think. The speaker output
from one of the channels is 5.5v DC, whereas it's supposed to be zero.


Dave,

I used to work fixing this kind of equipment (many moons ago). Here's
what I suggest.

Disconnet the unit from the AC power - do not simply turn it off!!!!

You need to disconnect at least 2 of the 3 leads on the mosfets and use
and ohm meter to check for leakage. This will find you the bad part.
Be carefull to keep yourself grounded to the unit's chassis or you may
kill one of the mosfets.

Visually determine if each bank is basically just paralleled - source,
gate, and drains. If so, you can just remove one bad one and run the
unit at reduced power.

Here is the important part: Rig a 40 watt 110 volt light bulb in series
with the AC power feed each time you test the unit after replacing
parts. If the light comes on full, you have other problems to go after.
If it comes on bright but settles down to less than full intensity,
you can then run full (AC) power to test it out.

I used to have one of these wired permenantly into my testbench along
with a switch to bypass the light bulb. Anything I worked on was
plugged in there. Low tech, but very useful.

--
Luhan Monat, "LuhanKnows" At 'Yahoo' dot 'Com'
http://members.cox.net/berniekm
"The future is not what it used to be."