On Sat, 17 Jul 2004 02:41:31 -0500, todd wrote:

Hi Lee

Apparently you are not blowing fuses so it would seem your J50 outputs
failed open. Check to see if your Mosfets have source resistors as well as
gate resistors. Check every one, as well as everything else on the board. I
don't have access to a schematic for this amp but I would recommend getting
one. Fixing an amp like this can be a lot harder than it looks without a
schem - even with a schematic for that matter! What you don't want to do is
put new output devices (or the good ones from your working channel) in there
and destroy them due to possible un-detected problems upstream in the
circuit.


Thanks - I wish I had a schematic, but I've spent hours searching to no
avail. It's a Rauch Powerblock 44 from a now-defunct English company.

SNIP
The idea is to determine if the preamp & driver stage is working right
before installing the expensive outputs - which you can about 90% determine
by o'scope probing and following the above method. If you observe a normal
looking amplified version of your input waveform then chances are your
preamp & driver stages are in good shape. Put in the outputs, or the rest of
the outputs and check again. If you have any appreciable DC (more than
about 100mVdc) at the output while the input is shunted - you have problems
somewhere up stream. When you are confident your sine wave looks good - and
you have all the outputs installed, hook up your spkr load and check again.
(Say a blessing prayer first).

If you have a bulb Ilimiter and follow these two rules - you should be able
to fix this thing:
1)do not try to drive a spkr load without the outputs installed.
2)do not install the outputs until you are confident that the preceeding
stages are working correctly.


OK - the above is really useful, thanks very much. I've got 240V mains
here but I can do the sums.

See this link for some very clever advice on replacements for the obsolete
J50/K135 devices:

http://aga.rru.com/TechTips/disp.cgi...owerFETRepl.tt


Yeah, I read that. I can buy the originals for about GBP 8 each, and the
IRFP240/9240 for about GBP 5 each, so I'll have to decide whether I want
the money or the hassle.

BTW, I would recommend replacing all of the outputs.

Good luck
Todd


That's all been really helpful, I'll get on to it next weekend.

Lee.

SNIP