Hello,

I've got a Rauch PB44 power amp that uses the 'classic' J50/K135 paired
Mosfets. One channel is perfect, but the other just gives me a loud mains
hum that does not change with the front volume pots, and doesn't have the
input signal present at all.

I've tested the PSU and all seems to be well (haven't checked it on the
scope yet but DVM says +-70VDC which is the same as the good channel).
I've tested the Mosfets with the DVM using the method at
http://www.4qd.co.uk/serv/mostest.html comparing good and bad channel, and
have come to the conclusion that all four J50s on the bad channel are
duff. I can source replacements at RS but they're about GBP 10 a pop.

I therefore have a few questions:

1. I don't want to waste money on replacement parts if I'm wrong in my
guess. How much damage am I likely to cause if I swap the J50s from the
good channel? I understand they're static sensitive, is it enough to
ground the soldering iron tip and touch the (earthed) chassis frequently?
I have read that solder pumps are bad for static so plan to buy some wick.
If the problem is before the Mosfets, is it possible to kill the good lot
and end up with no amp at all?

2. The Mosfets are Japanese J50/K135, the RS parts are UK 2SJ50/2SJ135.
If I replace the J50s with 2SJ50s, will I need to replace the K135s with
the different parts, or are they drop-in equivalent? I haven't been able
to find this out anywhere. I think 7 new Mosfets (I happen to have 2)
might make the repair uneconomical.

3. Any hints on other debugging I could try? I'm proficient with
solderig iron, DVM and fairly happy with my scope but not experienced with
fixing power amps.

Many thanks in advance. This is a beast of an amp so it would be fun to
have it working...

Lee