Dave wrote:
wrote in message
ps.com...
Dave wrote:
wrote in message
ps.com...

IIUC all you've lost is your power amps, the rest works. I'm not clear
what you mean by pre and main being integrated, I've not seen anything
like that since the valve days. Same pcb sure, same psu of course, but
circuit intertwined? I cant think how it would be.

I'm not that clear on where the amp begins and where the pre-amp stops in
this case. It's not like an amp with discrete components where everything
after the tone/balance pots is amp and everything before is pre. I'd expect
at least four of the 12 op-amps on the board (2 x dual & 2 x quad) to be a
buffer/integrator for the subwoofer signal... I guess that's the equivilant
of a "pre-amp" for the subwoofer. Maybe four more (or less) to combine the
signal for the center channel output? I really know very little about
multi-channel (I mean more than two) systems and this one is a real deviant
because I think it's basically just some sort of delay circuit to simulate
surround sound from using only two analog R & L inputs.

If the amp is just the drivers and output transistors, and I've replaced the
outputs and a good portion of the drivers, and I've still got DC bias
problems, that would lead me to think that the source of the problem may
well be in the preamp UNLESS there is a coupling cap between the pre-amp and
amp stages. Hmm, if I can figure out where to make the split, maybe I can
add such a cap which would at least tell me on which side of the fence (pre
or main amp) the problem lies... ah, but I was going to dismantle it and use
the components... silly me.

right, it sounds like the problem is one of circuit tracing then. When
you use a capacitor coupled power amp(s), the dc conditions on its
input dont matter. So you can follow the signal back from the (now
blown) output tr pair using a probe thats a single opamp amplifier and
a 2" speaker until you get a nice clean souding signal, and take it
from there. Or, if it uses a pot for volume, you could tap 2 channel
putput straight off the volume pot.

Preamps routinely have dc bias on them, its not an issue.

If you probe around with something very low power, you wont scare your
ears or damage anything, and you should find where its all good. The
signal quality may not be good within some parts of the old dead power
amp, because the dead output trs will have made the feedback loop open
there. Just keep going back till its good and clean.


NT