On Thu, 19 Jan 2006 22:04:09 GMT, "Dave"
wrote:
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.
Sometimes the DC levels can tell you a lot. No signal - since without
a scope it wouldn't do much good. Check input output ground and Vcc -
don't assume it is connected check it at the solder joint or component
leads. Assume nothing. Believe the voltmeter not your eyes when it
comes to connections.
I don't know if I buy the no output caps and single supply . . .
With class AB (which is your amp) the output of the amp running on a
single supply HAS to be one half the supply voltage with no signal
present. That means the speaker would be driven close to its travel
in one direction at all times if the circuit doesn't have a
transformer or cap to take the DC from the speaker.
The choice is a coupling cap (high value like 400 to 4,000 ufd) in
series with the speaker - or two amps per channel with the speaker
between the outputs. Both amps sitting at 1/2 half the total supply
so the speaker has no voltage differential across it - the amps when
driven are working out of phase to one another - one side becomes more
positive the other more negative. (a technique used in car radios)
The other choice is a plus and minus supply with one side of the
speaker to ground.
http://www.national.com/ds/LM/LM1875.pdf See the application
datasheet for the details of dual/single supply operation - note the
cap in the single supply version . . .
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?
It does matter, to the capacitor in particular. Electrolytics don't
tolerate over-voltage or reversed polarity very well. There are
non-polar electrolytic caps, but they aren't common.
The data sheet suggests you use a one or two microfarad cap for input
coupling. A .1 may also work but give some tinny sound quality.
You can "make" a non-polar cap from two polar caps. Take two caps
with the same specs - preferably from the same maker and batch. Tie
either both plus terminals together, or both minus terminals together
- use as you would a normal un polarized cap.
The voltage rating will be the rating of one cap and the capacity will
be 1/2 the value of each cap. (2 X 4 ufd / 25 volts = 1 X 2 ufd / 25
volts)
An electrolytic is usually specified at something like a tolerance of
+100% / -30% of capacity - a loose spec to be sure. The designer
takes this into account and what AC current the cap will have to
transfer at what frequency then allows for tolerance. A point one
microfarad may just sound like there's less bass present.
Likewise if you have a 2,000 ufd speaker coupling cap (as shown in the
data sheet) a working 100 ufd may sound a lot better than a non
working 2,000 . . .
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)
Bridge pin one on one amp to pin one on the other with a non-polar cap
(to be safe). You could just touch the terminals with the cap but
that may cause some output noise and it would be safer to clip the cap
in then apply power.
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.
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