Carbon Comp: Aging Causes Out Of Tol?

I have fifty 910 ohm 1/4W carbon composition resistors, still in the paper tape, which is labeled:

OHMITE

910 OHM 1/4 W 5% RC07GF911J OC9115

Judging from the last line, they're 14 years old. They've never been used, AKA New Old Stock. Every resistor measures greater than 5%, some are over 1k. Over 14 years, would drift from aging cause them to go that far out of tolerance? I have carbon film resistore much older than these and they still measure within 5% tolerance, usually within 2%.

Thanks.

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Reply to
Watson A.Name - "Watt Sun, th
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Do you know their history? Maybe they were out of tolerence at time of manufacture.

I always heard that carbons change value with humidity. I have many unused Allen Bradley 5% 1/2W carbons which I took from new stock in the early 70's, and checking a few of the BROWN-third-band ones at random show all within tolerence.

Reply to
Michael

been

some

than

of

I don't know their history, only that they are still in the tape that they were in when they came from the factory, with the factory designation stamped on the tape, so they should be NOS. I've seen tapes from some place that have various components customized for a stuffing machine, and this is not one of those.

One would think that it would not be possible for these to be out of tol when they come from the mfgr, unless there was a major malfunction in their testing equipment. However, I've heard of such things as warehouses catching on fire, or goods being shipped in a ship hold next to a steam boiler, where in either case the goods were exposed to excessive temperatures. This could have caused some change. But my experience is that higher temperaturss cause resistors to become lower resistance.

unused

70's, and

tolerence.

I checked another bag of loose carbon comp resistors, 6.8 ohm, 1/4W, 5%. All of them are also greater than 5% out of tolerance, and that is taking into account the 0.4 ohm resistance of the test leads.

Reply to
Watson A.Name - "Watt Sun, th

Maybe your meter needs a new battery? ;-) Long ago I bought a Micronta DVM for the guys to use in testing product (to replace one they killed, and needed *now*). I was surprised to find how nonlinear it was-- a good fraction of 1%, though still within specs-- it had some crap ASIC that was nowhere near the virtually perfect performance of the Intersil inspired chips.

There were also problems about 20-30 years ago, well covered in the trade rags at the time, about out-of-tolerance mil-spec resistors. Don't recall whether they were film or composition types. It was a bit of a scandal-- when you pay 20 or 50 times the price, you don't necessarily expect them to be worse than the standard commercial types (though it makes sense that problems could crop up due to low-volme special production arrangements).

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Yup, carbon composition was notoriously unstable. The phenolic casing was not hermetic and allowed moisture in and out. About the only way they were better than carbon film was in power surge rating. A 1 watt could take 5 watts of overload for a split second without more damage than a small change in value. Carbon films would flash and flameout.

Also, carbon comps have lower self inductance than carbon film, so you'll sometimes see them still used in RF power circuits. Sometimes with coils wound about the body, too.

I remember when Ohmite announced years ago they were ceasing production on their comps and so many engineers wailed 'No!'. Ohmite relented, but did get to raise prices.

You may be able to sell them on ebay to people who need replacement parts for their old gear.

Reply to
_crusty

A client says he switched to them for one application because the extra noise helped an oscillator start reliably. I'm not sure I believe it was the factor (the parts he got looked just like film parts to me, with the telltale bulge of crimped endcaps over a tubular body) but maybe it got the semiconductor maker ********* out of a possibly litigious bind by killing enough time for another wafer run to get out.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

I think this is common. I bought a box of 1000 114 ohm resistors. Stackpole Mfg Date 1980. Unfortunately, they're marked 100 Ohm 5%. Was gonna use 'em in pairs to terminate 50 Ohm transmission lines...oh well... I should try cooking them to see if they change back. mike

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mike

"Watson A.Name - "Watt Sun, the Dark Remover"" wrote in message news: snipped-for-privacy@corp.supernews.com...

I was recently working on an amp from the 1950's; every carbon comp in it was at least 20% higher than its markings indicated.

(Woe be unto the tech who repairs a tube guitar amp and replaces the resistors with new ones whose values are nominally correct! Depending on which resistor it is, it'll change the sound of the amp...)

Reply to
Walter Harley

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5%,

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older

2%.

time

tapes

stuffing

tol

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lower

many

early

within

5%.

I rechecked a bunch of them with a bench DMM and they're all the same:

5% or more higher than marked. The 6.8 ohms measure 7.2 or more. The 910s measure close to 1k. Usually I measure a 1% precision resistor on the same range, just to make sure things are working right.

Someone should take the initiative and include a precision resistor in each DMM, with the two leads sticking out the front. Something similar to the calibration source that I've seen on the front of decent o'scopes. A 1% resistor costs only a penny or two.

Reminds me of the 3 foot cat5 patch cords we ordered. Came bagged 50 to a bag. Supposed to be tested to cat5 specs. I reached in and pulled out a half dozen with one end bare, no connector. Well, if they really _were_ tested, they would've caught that 'minor' problem. :-P

Reply to
Watson A.Name - "Watt Sun, th

been

some

than

Hey, YEAH! Great idea! I'll hold a couple over the soldering iron and see if they stay changed when they cool off. It won't hurt to mess up a few, since they're already messed up to begin with. ;-)

BRB...

Well, that didn't help. I took four of them and layed each on the soldering iron. Each went up several dozen ohme, some between 1050 and

1100. And they stayed at that value when cool. :-(

Just put a 470 ohm resistor in parallel with the two 114 ohms, and you'll have 50.8 ohms, which is just about right.

Long ago, in a galaxy far, far away, I scrounged a dummy load out of a cardboard store display for a CB xcvr. They used a 50 ohm 5W wirewound resistor, and put a capacitor, I think it was 47 pF, in series to series resonate with the wirewound resistor's inductance and cancel it out. Worked just fine.

Reply to
Watson A.Name - "Watt Sun, th

wrote

it

on

For the old toob amps., 20% wasn't all that disastrous. As long as the change wasn't caused by heat. Since those amps get hot, the half watt resistor is probably only capable of dissipating half that, because it's in a hot environment. Could be that the vents were inadequate or were plugged up with dust and spider webs.

Reply to
Watson A.Name - "Watt Sun, th

Don't think that works. Need to use low heat for long term. Put some atop that big CRT monitor for a few weeks. Mine's hot enough to bake bread. ;-)

There's another issue that comes to play on older resistors. Back in the day, they cranked out resistors and marked 'em whatever they measured. 5% resistors were selected out. Then the 10%ers. What was left was 20%. End result was that a 20% resistor was hardly ever within 10%. For each tolerance, you got a bi-modal distribution with a hole where the good stuff was taken out. mike

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Reply to
mike

If you could find an old Allen-Bradley catalog you'd be truly amazed at the amount of detail they lay out on the moisture sensitivity of these resistors.

The gist of it is as follows:

1) Carbon composition resistors will absorb moisture out of the air and go up in resistance. If the resistance is important to the manufacturer, they much keep them in a special dry engvironment, just like flux coated welding rods. 2) If you have carbon composition resistors which have gotten damp, they can be dryed out by heating them to temps below 100C for several days. 2 Watters take longer than 1/4 Watters. Don't try to speed this up by using higher temps. 3) If you solder them when they are "damp" the change in resistance will be locked in, ie no longer reversible.

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.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Reply to
Jim Adney

the

go

2%.

and

up a

and

I've got a LCD panel, doesn't get warm. :-(

they

Yeah, I've seen that with some parts. Sucks big time if you're depending on getting good parts. Thanks.

Reply to
Watson A.Name - "Watt Sun, th

some

than

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.

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.

Reply to
Watson A.Name - "Watt Sun, th

I don't think it takes anything extreme to make this moisture absorption happen. Try baking them. I think you'll be surprised.

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.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Reply to
Jim Adney

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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.

Reply to
Watson A.Name - "Watt Sun, th

These resistors were manufactured to the military specification MIL-R-11, which was cancelled in 1997. I do not have access to the historical document. However, a similar specification for established reliability carbon-comp resistors is available here:

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The moisture, lifetime, aging, etc. for an RCR07 should be very similar to an RC07.

Reply to
tlbs

The tube connection is a very good point. I often think about letting my old tube gear just warm up occasionally to drive off the accumulated moisture, but I'm still not likely to leave any of them on for a week. Of course if I used that gear every day everything would probably be fine.

It can be surprising how long it can take to drive off moisture. In vacuum systems, we will sometimes bake a stainless steel tube to 200C for several hours in order to produce the best possible vacuum. Moisture is not the only thing that's being driven off, but it's the main one, and that's just from a "clean" metal surface.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Reply to
Jim Adney

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