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...
On May 6, 5:38 pm, Sylvia Else wrote:
wrote:
I thought that perhaps I could isolate it by heating or cooling parts
until the unit started or stopped working. I tried using a soldering
iron for heating, and canned air for cooling, but neither had a clear
effect. Is there any other technique I might use for heating or
cooling individual parts?

Holding the can upside down worked a bit better. :-p

So you got a result? What was it?

Sylvia.

Yes. Inverting the can gives a nice shot of cold liquid that rapidly
evaporates. I quickly found that cooling one of the smaller
electrolytics would switch off the power, so I'm fairly confident that
it's the culprit.

Unfortunately it's a low esr cap, and I haven't found a way to get one
to my door for less than 12 times the cost of the cap (due to minimum
shipping costs). It nearly eliminates the advantage of repairing the
supply.

I can get a non-esr replacement locally. Any guess how long that would
last?

The local shop only carries NTE parts. At first glance, I can't find
low esr caps in their catalog.


Depending where in the circuit the cap is, a 'normal' ESR cap will probably
work just fine for a considerable time. There are various things that you
can do to help its life, not the least of which is to make sure that it is
not pressed up close to a heatsink or power resistor. Also, try to get a 105
degree temperature rated cap, rather than an 85 degree. Your local supplier
should carry those, if not low ESR ones. The ESR of 'standard' caps can be
helped by connecting two in parallel, so that might be another thing to try
if you have room to get two in, or one on the front and one on the back of
the board.

A final thing that you could try, if you get desperate, is to try your local
repair shop. They would definitely carry low ESR caps for their own repairs,
and might be persuaded to sell you one. Failing that, they would almost
certainly have a scrap power supply board around from something - TV, VCR,
computer etc - that you could persuade them to part with. A 'replacement'
cap could then be 'culled' from that board. Obvious word of caution. Any
such second-hand cap might be as bad as the one you're replacing, and an ESR
meter would be useful to check, but in my experience, as long as the scrap
power supply that it's coming from is not too old, and the cap is not
located right next to a heat source, chances are it will be ok.

Hope that helps. If you are seriously into repairing this sort of stuff,
your very next investment should be an ESR meter (see Bob Parker's "Blue" at
Anatek Corp's website). This is an excellent instrument and very reasonably
priced. It will save you literally hours of wasted time and cans of freezer.

Arfa