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 Post subject: Using old carbon resistors
PostPosted: Oct Mon 01, 2007 10:39 pm 
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Location: Santa Rosa, CA
I often buy NOS components (New Old Stock) at flea markets. If it hasn't been used, it's got to be good! Right???

Be aware that old carbon comp resistors tend to go up in value; not catastrophically for most tube applications, but they are almost always higher in value than the color code would indicate.

This gentleman did a scholarly study of old resistors and has some interesting data to prove his assertions.

http://www.cliftonlaboratories.com/carbon_composition_resistors.htm

Rich


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 Post subject:
PostPosted: Oct Mon 01, 2007 11:34 pm 
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Rich

The only thing you can be sure of resistors go up in value. Don't think you can depend on any limits. Within a batch value may go up a certain percentage. When tested ar random some will still be within spec, others way up in value and still others open.

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 Post subject:
PostPosted: Oct Tue 02, 2007 12:20 am 
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Rich, I didn't go to the link that you posted, but I think NOS carbon resistors increase in value whether they are unused or not. As cheap as modern resistors are, I don't think they are worth the risk.

I have observed that NOS Allen-Bradley carbon resistors seem to not have increased in value. I might buy NOS A-B resistors if they are clearly marked as A-B. I think perhaps audiophiles go for A-B resistors? Not certain.
Doug


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 Post subject:
PostPosted: Oct Tue 02, 2007 12:29 am 
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As Doug said, old resistors tend to increase in value.

The real problem with them, and the reason that the study must be taken as only a generalization, is that you can't know the storage conditions of the resistors over the intervening 70 years.

Allen-Bradley was definitely the gold standard for carbon comps. Their product was so much better than anyone else's that there was no comparison possible. All of the mil and NASA equipment I worked on had nothing but AB resistors, both fixed and variable.

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 Post subject:
PostPosted: Oct Tue 02, 2007 1:03 am 
Silent Key

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I have to agree with Leigh here. I have wasted my money on buying cigar boxes full of NOS carbon composition resistors at hamfests too many times. Only the A/B ones were within tolerance when checking them with a good ohm meter. But resistors are probably the cheapest part in a set, so why not go with modern resistors and not having to worry about them?
Curt

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 Post subject:
PostPosted: Oct Tue 02, 2007 2:18 am 
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Location: Phoenix, Arizona
I found that if you had 10% resistors they were usually off value near 10% even when they were fresh. If you wanted more accuracy you had to buy 1% parts.

I find except for a few applications you can live with some degree of out of tolerance, manufactures designed to ready made standard values to keep the price down. Sometimes matched parts are needed and they will still use 10% parts and match them up, sometimes the service manual will call them 1%.

The modern resistors appear to be manufactured by cutting a groove in the substrate and making it to a specific exact value. They are so small I can’t read the color bands and they don’t look anything like the original parts.

Jim


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 Post subject:
PostPosted: Oct Tue 02, 2007 3:02 am 
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jimmc wrote:
I found that if you had 10% resistors they were usually off value near 10% even when they were fresh. If you wanted more accuracy you had to buy 1% parts.

Hi Jim,

That's because of the manufacturing process, which was not very precise.

They made a bunch of resistors, then measured each and banded them according to their actual value. If one was within ±5% of nominal it got a gold band. If within ±10% of nominal it got a silver band. The remainder got no band. They charged more for the tighter tolerance parts.

Thus, by definition, a resistor with a silver band will never be within ±5% of nominal.

Allen-Bradley apparently had a different manufacturing process, though I don't know what it was. I can't remember ever finding an A-B resistor that was not within ±1% of nominal.

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 Post subject:
PostPosted: Oct Tue 02, 2007 4:03 am 
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Standard resistor values were decided on as not to have waste. Just about any resistor made falls within a standard value.

Higher value resistor increase value faster than lower values. Worked for a place that bought 1 meg resistors on reels. They exceeded tolerance within a couple years.

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 Post subject:
PostPosted: Jan Sun 06, 2008 4:49 am 
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Location: Shoreview, MN
I had an interesting conversation with a guy at a local swap meet about 2 years ago. He had a 3 Lb coffee can full of carbon comp resistors that had been dragged back from being stationed at the South Pole for several years. He was trying to sell them but he made the comment that nobody wanted 'em. His reason: the resistance on ALL of them were quite below the tolerance listed on them. He made a comment that the low humidity had evaporated any latent moisture that had existed between the carbon particles, causing the readings to drop.
Whatcha think? Should I have asked him for some of what he was smokin'? :wink:


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 Post subject:
PostPosted: Jan Sun 06, 2008 11:48 pm 
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Location: Harviell MO USA 63945 (12 miles S of Poplar Bluff)
The carbon comp resistors don't have to be old to go sour. When Tektronix came out with the 2213 and 2215 in the early 1980s, they used a string of carbon comp 510K ohm, 1/2-watt resistors in the HV CRT circuitry. One-by-one, the focus on these scope would start to shift to the point where the front panel FOCUS control would no longer have enough range to sharply focus the beam. I found that these resistors were drifting in value, sometimes with a value of 600K or 900K or 1.2M or 3M ..... and with them being in series, the shift was compounded. Replacing all the 510K, 1/2-watt resistors in the scope fixed the problem -- and prevented future problems, even if the original component measured 510.0002K at the time. By the way, I replaced these resistors with carbon film types, no problems since.

The four 2213s we had at our school were all affected and I've since have helped out a lot of other folks with the same problem on these two models. The 2213A and 2215A models don't have the problem.

I've never contacted Tek about the problem, but I'll bet that a look through the microfiche will show that there's a reliability modification out for that one!

Dean


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 Post subject:
PostPosted: Jan Fri 11, 2008 6:16 pm 
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Location: Sandwich, IL, USA
Well Kenny, not necessarily from a knowledgeable standpoint but from a logical standpoint, I would say that you ran into a used car sales man, and that he was blowing smoke up the old tailpipe.
I’ve tried many experiments with NOS MicaMold postage stamp caps, which I have thousands of and with NOS carbon resistors, from dogbones up to the newer styles. I’ve tried oven drying, I’ve tried vacuum pumping which would be the absolute best way to remove any moisture from them and in the end learned that moisture content tain’t the problem! Being exposed to humidity over their lifetime no doubt has an effect on both of them, more so on the caps I would think.
With the paper caps it’s the chemical changed in the dielectric and conductive layers from either acidic paper or oxidation and breakdown of the metallic component.
With carbon resistors it is a breakdown of the binder from what I’ve been able to find out. I have for a long time suspected that the junction of the composition and lead wires has something to do with the increase. From what I’ve seen thar taint no stopping that. Some might take longer but in the end all of the carbon comp resistors will change.
If anyone has a more technical explanation or for the umpteen zillionth time another opinion, I’d like to hear it and add it to my reference library, that is if it makes sense.
Denny Graham
Sandwich, IL


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 Post subject:
PostPosted: Nov Sun 22, 2009 12:31 am 
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Location: Jackson, TN
Hi all,

I ressurected this thread due to an issue I found with a 2M pot on an old Zenith chassis.

The carbon circle in the pot has gone from 2M to about 3.6 M. Not surprising as I found several carbon comp resistors in this set which had increased as much as 100%.

A few examples:
220K measured 550K (+150%)
1M measured 1.5M (+50%)
390K measured 500K (+28%)
and so on...

So here's where I'm going. Since this phenomenon is well documented, there's not much point in beating a dead horse.

How do we reverse the degradation?????

Seems to me that if the original binder solution has evaporated or otherwise become gone, why can't the resistance be treated with some similar juice?

Any thoughts on some type of slightly conductive oil?

Tim


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 Post subject:
PostPosted: Nov Sun 22, 2009 12:59 am 
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I always assumed it was humidity? I'm not sure.. maybe you can bake them for a while, and then lacquer them?


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 Post subject:
PostPosted: Nov Sun 22, 2009 1:10 am 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
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Location: Sandpoint, IDAHO 83864
I would think that the ingredients in the filler material simply evaporate over a period of many years and since they are mixed in with the carbon particles would be irreplacable. Heating them would only tend to make them worse, if anything, as that would speed the evaporative process. Interesting question, however.
Curt

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 Post subject:
PostPosted: Nov Sun 22, 2009 7:17 am 
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I was doing some troubleshooting on my Scott 800 and found several resistors out of whack. Two had gone completely open, one was very high, but every other one I've tested has been well within tolerance. I'm not sure who made the resistors, but the ones I replaced had that "fuzzy" look to them, not like the smooth glossy AB resistors I'm used to seeing. Many other resistors in the radio look to be ABs though.

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 Post subject:
PostPosted: Nov Sun 22, 2009 3:05 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
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Location: Sandpoint, IDAHO 83864
The Allen Bradley resistors were the highest quality of carbon composition resistors, even back in their prime. Take a look at the prices on them versus other brands in the older catalogs, say in the 1960's. You paid more for them, but you also got a much better and long lasting product.
Curt

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 Post subject:
PostPosted: Nov Sun 22, 2009 3:52 pm 
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In following this thread and getting ready to start a couple of refurbishes 60 to 80 years in age, I'll ask, do I measure and replace or just wholesale replace the resistors?


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 Post subject:
PostPosted: Nov Sun 22, 2009 3:59 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
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Location: Sandpoint, IDAHO 83864
Most of us frown on shotgunning in replacement parts. No sense in changing out good parts for equal parts. I would measure them first, and make sure that you disconnect one end of it if you are not sure where in the schematic it is used and if it is within tolerance, then leave it.

Most resistors were +/- 10 to 20 percent back then, and that gives you a lot of leeway. Besides, I am lazy and don't like to do more work than I have to. Nobody is paying me for fixing the old sets.
Curt

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 Post subject:
PostPosted: Nov Sun 22, 2009 4:26 pm 
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Location: Mountains of Colorado
Good answer.
Thanks Curt

I'm also wondering if you don't do the opposite with caps??

Neil


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 Post subject:
PostPosted: Nov Sun 22, 2009 4:55 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
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Location: Sandpoint, IDAHO 83864
Simply because most of the paper capacitors WILL be bad in a set. I have seen many sets where the caps were all shot, and all the resistors were just fine.
Curt

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(Connoisseur of the cold 807) CW forever!


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