"Mr. Land" wrote in message
oups.com...

So have you fixed it?

Um, not yet. As you noted below, I also thought about DC getting to the
speakers being the cause of the thumping. That's what started me on my
capacitor-testing adventure in the first place. Which led to my frying the
PCB traces trying to remove the caps and associated solder with solder
braid. So I've got to procure a new pair of 470uF caps (with nice long
leads) that I can solder onto the other side of the board. After (hmm,
maybe at the same time) I procure a solder sucker and some new
decent-quality solder braid and some liquid flux (from other posters) to
clean up the mess I've made. I see that there is quite a bit of technique
involved with this de-soldering stuff... I'll get there.

The IC in question is an LM1036N, combo volume/tone/balance control. It's
available from Digikey for $5, maybe I'll just order one for the price of a
Big Mac and fries, get my caps and assorted tools too. When I get the caps
back in I'll bridge one amp IC's input to the other one (via a 10uF cap
according to another poster perhaps to limit any DC voltage from getting
from the "bad" channel to the "good" and wreaking havoc there). This should
at least tell me if the output stage is working correctly. I guess I should
check input voltages at both op-amp IC's to make sure the bad channel is
being powered first. As I noted I've already replaced the bad channel's
output stage (LM1875T 20W amp TO-220). I've already tested all of the
diodes and resistors and (to the extent that they can be tested in-circuit)
caps with a DMM (not the best test but will show if a cap is open circuit or
shorted).



If not, what's this 10-year-old mixer IC you mention? Do you have a
pinout of it? How do the DC voltages on all the pins of the output IC
compare? If you have a meter-based meter (was that redundant?), can
you see any DC fluctuation on the input or output pins of the output
IC's when you spin the volume control? Check both channels and compare
them.

I didn't think of using an analog meter, that's a good way to roughly
compare outputs... I'll make up a CD with continuous tones to feed a signal
via the unit's RCA inputs and test the signal at various points moving
backwards from the output stage. I have a feeling it's the combo
volume/tone/balance IC but then I had a feeling it was the amp too and it
ain't.... I think I suffer from "jump to unsubstantiated conclusion"
syndrome and I just have to slow down and think things through before
running off to the store to replace another part.

I'll keep folks updated (a retain-my-pride way of saying "I'll likely have
more questions") via this thread.

Thanks for your help so far...

Dave

I've seen speaker "thumping" occur when DC was getting to an input it
shouldn't be.