"Dave" wrote in message
news:AtQzf.93177$AP5.80078@edtnps84...

"Mr. Land" wrote in message
oups.com...
Give up using the braid...that stuff sucks (or, rather, it doesn't).
Get yourself a solder sucker if you're going to do stuff like this.

As I noted before I've used the braid quite a bit in the past and found it
to work well... but will pick up a sucker anyways as obviously I need it.


My general rule is a solder sucker for standard work and braid for smaller
or awkward tasks, especially certain surface mount work. A solder sucker is
far more appropriate for removing components like electrolytic caps from an
amp IMO.

BTW, if I had an audio power amp that didn't work, I think the caps
would be near the end of my list of things to check. Start with the
power (aka output) transistors and work your way backwards. Unless
this is a really esoteric unit, both channels probably share the power
supply, so if one channel is working, you can probably assume that's OK.

I did start at the output transisters and replaced them all on the bad
channel (they're all on an integrated 20W amp chip, NS LM1875). I swapped
the caps in an effort to avoid trying to source a 10-year-old base/treble
IC
which is likely the problem. Other than the output IC's, mixer IC, and
rectifier, there ain't a whole lot on the board except caps and resistors.
Resistors all checked out OK, move on to electrolytics... No?



Do you still have a short? If so that makes things very easy- a short is
probably the easiest fault there is to diagnose. Simply trace the two PCB
tracks which are shorted and find all other components across these tracks.
One of them must be shorted out. Is the capacitor in question on the supply
rail? Check that the new IC you fitted doesn't have any solder bridges, and
if it has a mica heatsink insulator, make sure it's not damaged. I doubt
it's a shorted cap, but it is possible nonetheless.

It's possible the 470uF capacitor is part of the decoupled supply to the
preamp section, if so it may have a zener diode in parallel to give a
regulated supply. Therefore, check for shorted zeners.

The output ICs no doubt are coupled to the speakers via large electrolytic
capacitors, what is the voltage level on the IC side of this cap with
respect to ground? It should be close to half supply potential. Compare it
with the good side, but take care not to short anything out.

Are both output ICs getting a supply? Are they both getting a ground? Have
you measured voltage levels on each pin and compared them with the good
side? Have you downloaded a datasheet for the ICs, found the input pin and
injected a signal there to listen for output? Even touching a finger to the
input pin may give a buzz from the speaker, or a screwdriver may give a
click. There's ways and means, even without test equipment!

Dave