Hi,

I'm a bit puzzled by this blown power amp, as the design of these things is
not really my forte.

(I have posted the schematic over on alt.binaries.schematics.electronic)

The problem is around the two Lateral Mosfets Q16 and Q21 (EXC10N20 /
10P20).
R36 from drain to -70v is complete toast, R35 from the N channel to +70v is
OK.
R33 on the P channel gate is also fried, as is R30 on the other gate, but
not quite as drastically.

Now, R36 has been replaced before by someone, but I'm fairly sure they
didn't replace either of the Mosfets or any other components.
Also, none of the SCA / 2SC output devices or their emitter resistors appear
to be faulty, though I have so far just done a cursory check on these and
the usual suspects on a blown output stage.

I have removed both Mosfets, and on a (low voltage) Peak Component Tester,
and they show as both working, which may or may not prove a great deal
really.


Now, I had one of these in a few months ago, and came across a similar
fault, but only R36 AND R33 were burnt to a crisp, indicating I thought, a
blown P Mosfet.
Again this checked out as working on my Peak tester, but I ordered another
from Allen and Heath anyway, which actually took about 3 months to arrive.
Replacing the two resistors and the Mosfet effected a cure, and the unit has
not been brought back in again.


The question is, what failure mode might be causing these symptoms? I don't
fully understand what these Mosfets are actually doing in this circuit, or
whether something else in the circuit is causing these failures. Why is the
gate resistor on the N channel burnt?

Another major problem is actually getting these Mosfets - you can't, as far
as I can tell, and Allen and Heath no longer supply parts, instead having
subbed out parts supply to various "service centres", who can sell you a
complete amplifier PCB, but not individual components.


Sorry if this is a bit long winded.



Cheers,

Gareth