Reforming resistors?

[Chuck H]

I don't remember ever seeing a CC resistor drift low. It's always been high in my experience. That said...

For the purposes of the thread I think it's the opinion of the participants that heating or otherwise trying to drive moisture out of CC resistors isn't a valid effort. So it's a good thing they're still available if one wishes or needs to replicate exact circuit specs. And it doesn't hurt that they're still really close to the vintage color too. But resurrecting original resistors doesn't seem probable. I'm not a big fan of NOS re-formed electrolytic capacitors either but I know that gets done all the time.

[Helmholtz]

I'm not a big fan of NOS re-formed electrolytic capacitors either but I know that gets done all the time.

Controlled reforming of ecaps can make sense, especially with vintage equipment.
But you need to understand what reforming does and what not.
Ecaps that weren't used/charged for longer periods often show increased leakage current due to a de-forming chemical reaction.
This process often is reversible by reforming and if this brings the leakage current below 1mA it shouldn't be a problem anymore.

But ecaps that have seen many operating hours may have dried out (which is a physical process), resulting in an irreversible loss of capacitance and increase of ESR.
So before re-using an old cap all three properties (leakage current, capacitance, ESR) should be checked.

BTW, with increased leakage current most all DMMs will read increased capacitance, so are not reliable due to measuring method.
A good LCR meter will show the correct value.

[Pixel]

Does anyone here have access to old IEEE publications? I only have from 1988 onwards.

H. Tada, "Aspects Affecting the Reliability of a Carbon Composition Resistor," in IEEE Transactions on Component Parts, vol. 10, no. 2, pp. 67-79, June 1963, doi: 10.1109/TCP.1963.1134810.

Abstract:
The mechanism of resistance drift of a 1-watt carbon composition resistor has been investigated, based on a study of the internal construction. Predominant physical and chemical processes under given environments are postulated. Aging mechanisms, the behavior of resistance drift under various environmental conditions over a period of time and failure modes are considered. Restrictions on application of the part and criteria for part selection are established. Failure analyses have also been performed to verify the postulated behavior mechanisms.

[Chuck H]

Does anyone here have access to old IEEE publications?

I sure don't. But I'll wager Helmholtz is looking into it

[tedmich]

What do you mean with "squeezing by case"?

CCs are high pressure sintered solid "blocks", not having a case.

I didnt say it was a GOOD theory

[tedmich]

supposedly there was an ARRL article on evaluating NOS resistors from the 1960s that drifted almost off scale.
"Carbon Composition Resistors - Not Like Fine Wine"
was at http://www.cliftonlaboratories.com but now long gone.

[g1]

supposedly there was an ARRL article on evaluating NOS resistors from the 1960s that drifted almost off scale.
"Carbon Composition Resistors - Not Like Fine Wine"
was at http://www.cliftonlaboratories.com but now long gone.

found on web: "A greatly expanded version of this web page appears in the American Radio Relay League's Mar/Apr 2008 QEX magazine"

[Mars Amp Repair]

I've notice a tendency for this posting to get rather silly...now we do our best to keep things moving along, but we shouldn't have things getting silly!
Reforming resistors? REALLY??

[Helmholtz]

Like others here I don't tend to post silly stuff.
Yes, the thread title doesn't make sense (though it's kind of catchy), as re-forming refers to an electrochemical process applied to lower excessive leakage current of Ecaps.
From the context of post #1 it gets clear that the actual question was something like "can drifted CC resistors be restored to original value?".
As said, I don't think so.

[mozz]

Who is going to join me in the experiment? Take 100k or 1 meg. I'll put some on top of my hot coal boiler and take before and after measurements, 24h 48h 72h. My stash of carbon comps are at least 50 years old. Besides regular 1/ 2 watt Allen Bradley types, I have others also.

[Helmholtz]

Temperatures of and above 150°C are known to significantly increase resistance.
I wouldn't above 100°C.

[Pixel]

Who is going to join me in the experiment? Take 100k or 1 meg. I'll put some on top of my hot coal boiler and take before and after measurements, 24h 48h 72h. My stash of carbon comps are at least 50 years old. Besides regular 1/ 2 watt Allen Bradley types, I have others also.

We are in luck, I did a trademark search on Audiophool and it is available. We just need a patent search on Mozz' process with the advanced resintering furnace aka hot coal boiler.

Audiophool TM resistors are vintage resistors that have been infused over decades with mojo. This mojo guides the resistor to its true value. Through the Mozz process the mojo is locked in for eternity. Restriping completes the process.

How much do you think they can sell for?

[g1]

Reforming resistors? REALLY??

The first step is getting the resistor to acknowledge it's transgressions.

[Pixel]

6 years ago I worked on a little 1950s tube amp and bought a full set of metal film resistors for all the CC in the chassis. I wound up replacing 9 CC resistors that had drifted out of spec. I bagged the old components along with the unused metal film resistors. I tested all of them today and found 4 of the 9 CC have increased resistance since then by an average of 3%. All of the unused metal films are still in spec. Next time I have that amp open I am going to replace the rest of the CC in there.

[The Dude]

Most of the resistors in vintage amps were more than 3% tolerance anyway. Tolerances weren't that tight even when new. I wouldn't bother changing a resistor that is only 3% off value in a tube amp.