Reforming resistors?

[Axtman]

Carbon comp resistors change value when they age by absorbing moisure. Has anyone tried heating them up to dry them out?

I am just curious. I may do it as an experiement.

[nosaj]

no not for a few cent part. The energy expended exceeds the cost of a replacement.
nosaj

[Chuck H]

no not for a few cent part. The energy expended exceeds the cost of a replacement.
nosaj

But this assumes the parts only value is monetary. If it's the stock part in a classic amp then it has mojo value beyond price for some.

I don't know all the processes involved in the making of vintage CC resistors but I would think the original enamel coating was fairly moisture resistant. I also know that older enamels tend to continue hardening with age until they're actually brittle. And this may be part of the cause if moisture is indeed responsible. So I would consider that if baking corrects the problem then a new coat of something (probably clear) should be applied to prevent a premature recurrence of the problem. In my line of work as a house painter I don't know of any high temp clear finishes but that doesn't mean they aren't out there.

[nosaj]

But this assumes the parts only value is monetary. If it's the stock part in a classic amp then it has mojo value beyond price for some.

Next thing you'll be telling me is you could restuff resistors to keep that vintage look
nosaj

[Jazz P Bass]

I don't see the problem as absorbing moisture.
I see it as an actual breakdown of the resistive element.

[Chuck H]

I don't see the problem as absorbing moisture.
I see it as an actual breakdown of the resistive element.

This was always my thinking as well because of my experiences with the black tinted paper boards. Sometimes it's moisture. But sometimes it's not. I've had amps where the black paper boards formed semi conductive paths after excessive modification and reworking (my own doing) where no attempt to dry them out or clean them cured the problem. It's possible some similar (but opposite) effect is happening to the carbon matrix in those old resistors. One tell is they usually drift high. Whereas if it were a moisture issue I'd expect them to drift to a lower value.

[Helmholtz]

Carbon comp resistors change value when they age by absorbing moisure.

Moisture absorption may be involved. I think normal operating temperatures in an amp should suffice to drive out absorbed moisture.

But CCs also drift at low/normal air humidity.
About 20 years ago I bought an assortment of 5% tolerance CCs. Initially they measured within tolerance.
Resistors were stored in closed plastic bags at room temperature in a dry room.
Meanwhile all of them have drifted upward by around 15%.

.

[Pixel]

Are modern carbon comp resistors also subject to the same drifting as vintage ones? The behaviour of old CC resistors leads me to believe the drift has to do with the binder materials that were used. If so, is the percentage of drift related to the percentage of binder vs carbon in a given resistor?

[Helmholtz]

Are modern carbon comp resistors also subject to the same drifting as vintage ones? The behaviour of old CC resistors leads me to believe the drift has to do with the binder materials that were used. If so, is the percentage of drift related to the percentage of binder vs carbon in a given resistor?

Can't say.
But the performance of CC resistors varies considerably.
I typically find less drift with > 50 year old CCs in vintage Fender amps and Tektronic or HP scopes than with the new ones I bought around 2000.
Now if drift is different I wouldn't epect them to sound the same either as noise and voltage distortion might differ as well.
One would have to find and compare original datasheets.

[tedmich]

Some think its the squeezing of the carbon comp material (by case) which decreases with age, thus their R values go higher, but I've never tried "tightening them up". And some do claim baking them helps, again with no data I can find.

As to CC's intangible "mojo" please remember some audiophools swear by NOS Shinkoh Tantalum Resistors at >$8 each...

[Helmholtz]

Some think its the squeezing of the carbon comp material (by case) which decreases with age, thus their R values go higher, but I've never tried "tightening them up".

What do you mean with "squeezing by case"?

CCs are high pressure sintered solid "blocks", not having a case.
I think that with age and temperature micro-cracks develop, which irreversibly increase resistance.

Absorbed moisture is likely to lower resistance as with conductive boards and PIO caps.

[g1]

I think normal operating temperatures in an amp should suffice to drive out absorbed moisture.

This is what I thought too. If moisture is the cause, and heat would actually help, then CC resistors that are working harder (hotter) should be less prone to drifting up in value. I don't think that is the case, possibly the opposite.

[Helmholtz]

I don't think that is the case, possibly the opposite.

As said, moisture typically lowers resistance.

[g1]

As said, moisture typically lowers resistance.

I meant heat in usage probably more likely to cause drift than correct it.

[Helmholtz]

I meant heat in usage probably more likely to cause drift than correct it.

As always aging is accelerated by temperature and exceeding the temperature limit (of maybe 100C° to 120°C) will speed up damage even more.
Both, driving out moisture as well as structural damage will increase resistance.