Re: Carbon Comp: Aging Causes Out Of Tol?


"Jim Adney" <jadney@xxxxxxxxxxx> wrote in message
news:updl81581c9s35fhishf8g02lj0ghd2hvi@xxxxxxxxxx
> On Mon, 16 May 2005 18:30:32 -0700 "Watson A.Name - \"Watt Sun, the
> Dark Remover\"" <NOSPAM@xxxxxxxxxxxxxx> wrote:
>
> >Thanks for the info. The four I put on the soldering iron were
probably
> >overheated and won't be salvageable. The rest haven't been exposed
to
> >water or high humidity, generally it's low humidity most of the time
> >here. So I don't see why they could've changed. I'm thinking that I
> >might be able to put them inside a ziplock bag with a dessicant and
> >leave them out in the sun for awhile, and it might help. Can't hurt
> >much since they're already nearly worthless.
>
> I don't think it takes anything extreme to make this moisture
> absorption happen. Try baking them. I think you'll be surprised.
>
> >> I did this test on a batch of 27k, 2W carbon comp resistors a
number
> >> of years ago. They were all high, and most of them were out of
> >> tolerance, this was immediately as purchased from Newark. But they
all
> >> came back into tolerance after heating in my home-made oven for
about
> >> a week.
> >
> >But at what temperature? I've got a bunch of old 1- and 2-watt
> >resistors and I don't think I'll ever use them because I'll never
need a
> >(for example) 27k 2W resistor since it takes a couple hundred volts
to
> >get up to 2W and I don't do tubes anymore.
>
> I wrapped them up in alum foil with a 25W light bulb and stuck them in
> a metal wastebasket for a week. As an experiment, I actually took them
> out once a day and measured R for each one every day and recorded it.
> I suspect the temps were 150-175F. This reduces the RELATIVE humidity
> to something really small. A dessicant would do the same thing, but
> the heat increases the mobility of the absorbed water, so it's much
> more effective.
>
> In a week's time, each resistor was back within tolerance.
>
> As I noted before, however, this will not work with resistors which
> have been heated to soldering temps. Allen-Bradley did not explain the
> mechanism for this and I don't have any idea what it might be.

Well, thanks for the info. It seems that the resistors were made for
tube equipment. The insides of a toob chassis get nice and warm, and
would cook the moisture out of the resistors. Except almost all of the
equipment nowadays is not toob and doesn't get that hot inside.

I found a bag of at least a hundred 56k, 1/2W resistors that are the
same way. I think I'll put the whole bunch into a 'hot box'.
It puzzles me why it should take so long. Maybe the resistors are
coated with something to make them moistureproof, and it's just not
perfect, and lets in the moisture over the years. But when I think
about it, having it take years to get that way, it's really not that bad
having to wait only a week to fix it.

> -


.

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