"Dave" wrote in message
news:t1Uzf.110817$km.52961@edtnps89...

default wrote in message
...
On Thu, 19 Jan 2006 15:33:18 GMT, "Dave"
wrote:

Well back in the "old days" we used "eyelets" - didn't have plated
through holes for circuit boards. The eyelets were just miniature one
part hollow rivets - or similar to grommets used in sails or
tarpaulins. That would be an ideal fix if you burnt the plating off.

I think I'll solder to the trace on the other side of the board, just
means
that the cap won't sit flush anymore but that's okay, lots of room in the
enclosure.

Solder Wick for single sided boards - solder sucker for double sided
boards.

Yes, must buy solder sucker. It's on my list now.

Regarding the amp. Have you checked the DC voltages between the
channels?

No. Do you mean between the channel A and channel B signal outputs with
no
signal applied to the inputs? If there WAS a DC voltage differential what
might this tell me? Failed coupling cap? There are no output caps, the
amp
output pin (see below) goes straight to the center pin of the RCA jack on
the back.

Is this a single supply or plus and minus supplies?

Both the amp and bass/treble/volume/balance IC's run off +Vcc.


TO220 -
five lead power op amp? built in thermal, short circuit, and safe
operating area protection? I assume you only have one chip per
channel (or "bridged" with two chips per channel?)

One chip per channel. They're LM1875T's, yes they have built-in thermal,
short circuit, etc. As I noted I have already replaced the one on the bad
channel (they are a TO-220 package).

If you can identify the chip and its input(S), you might try using a
capacitor to bridge signal from the working channel's input into the
input of the dead channel. (observing polarity if using an
electrolytic). Unplug the input to the dead channel while testing if
possible. The cap size will be 10 uf or less for a high impedance
audio stage input. That should localize the problem between the input
or output of the power amp chip.

Does it matter if I use an electrolytic or not? See pinout below, but I'd
connect pin 1 of channel A (working) to pin 1 of channel B (dead). If I
do
use electrolytics, which end is positive, A or B? Why do I need to use a
capacitor to bridge as opposed to a wire?

If it is like the TO220's I'm using, there's a differential input
(inverting and non-inverting) inputs - make sure you drive the correct
one).

Here's the pinout:

1 +IN
2 -IN
3 -V(EE)
4 OUTPUT
5 V(CC)

From what I can see, pin 1 is signal in, pin 2 is used as the negative
feedback loop, being connected to pin 4 via a 20K resistor. There are no
coupling caps on the outputs.

One assumes you switched speakers at some point.
--

Yes, first thing I tried.


All of the advice given so far regarding the fault, is good stuff. As far as
solder wick / solder sucker / desoldering stations go, it's horses for
courses. Contrary to what one of the posters said about solder wick "
sucking " ( or not ... ), in my experience, this is only the case when you
either buy cheap, or use the wrong size for the job. Buying cheap at radio
rallies, will just get you old stock, where the flux has gone off, and then,
it doesn't suck. Using a solder wick with too big a size, with an iron
that's too small tip wise, or power wise, results in insufficient heat
transfer to the joint, and then it doesn't suck.

A solder sucker is very good for medium sized joints, but there is a real
technique to using one well, and if you use it on a ' poor ' quality board,
it will readily suck the print off as well as the solder. There is a real
balance between getting enough heat into the joint to melt the solder to a
point where it will stay molten enough to be sucked cleanly off the board,
and not destroying the bonding between the copper and the substrate.

If you do get yourself a solder sucker, get a good one for which all parts
are available - nozzle, neoprene suction washer, circlip ( you'll lose it
! ) etc. Practice a lot on a scrap board to get the heat / time thing right.
Clean it out regularly, and don't buy cheap !!

Arfa