"Lee Collier" wrote in message
newsan.2004.07.16.23.54.08.465794@PlSpAcMoHlElRi Eer.net...
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

If you are getting hum on the speakers, disconnect them and check the d.c.
voltage to ground on the output terminals. If the amp is d.c. coupled -
which it very likely will be - there could be up to perhaps 100mV (either
polarity) present. If an output device has failed short circuit then you
will see either full or nearly full rail (+ve or -ve) if one device has
gone, or an offset greater than 100mV if a complimentary pair have failed,
although in this latter case I would have expected it to blow the rail
fuses.

J50 and 2SJ50 are one and the same - it is just the normal way that letters
are omitted to save space: same for K135 and 2SK135. However do make sure
they are the same encapsulation. Originally they were in the TO3
diamond-shaped power transistor cases, but latterly they were repackaged in
a T-shaped flatpack, albeit with some care these could be made to fit the
same heatsink assuming there is enough room.

As said elsewhere unless there is an obvious problem you will have a
difficult job without a circuit diagram. Your DVM will take so little
current that good devices could still appear to be faulty because of
leakage - a common trap into which to fall.


--

Woody