On Sat, 17 Jul 2004 14:16:09 +0000, Woody wrote:

SNIP

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.


The probing I've done so far suggests they've failed open, but I'll give
that a go in a minute.

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.

Good point, although I am comparing good side with bad side - my DVM
is turning on both of each pair on the good side but on the bad side the
N-channel devices are turning on OK but the P-channel are not