Is there any down side to using old carbon resistors? What does aging do to them? As long as they measure to the proper resistance value, are there any reasons not to use them? For instance, do they also retain their power rating?

I guess another related question is: under what circumstances are they preferred to newer types of resistors?

Thanks,
Keith

Old carbon resistors rarely read within tolerance.... and they continue to get worse ( drift high)
So why bother.. use new metalfilm or such.

OK, thanks. I don't want to use them due to cost or "originality". It's just that sometimes it's convenient, because I may have some around and that would save me from having to come up with a "minimum order" to buy just a few.

Keith

They will work just fine.
However... It's rare though now a days... to find any OLD carbon resistors that will still read in ohms what the colors indicate.
After 50 - 70 years or so as they dry out inside, and the carbon granules that were packed in there tightly when new begin to get less tightly packed and the space between the granules causes the values to get higher and higher.... so you'll typically find that they are usually way way high in value... so it's a pain.

Some brands are much better than others. If they've held their values up to now, they'll probably continue to be good.

I usually measure my oldies when needing a replacement, and can almost always find one within tolerance, even after 50+ years.

I use them for the sake of maintaining a consistency in appearance in a chassis. It strikes me as peculiar (and not pleasing) to see a mix of metal film or carbon film and carbon composition resistors in a restoration. I'll use metal films in new builds but if I'm restoring a piece of equipment that has carbon composition resistors I will use carbon composition as replacements. I carefully inspect the replacements for stress cracks before installing them. As long as they don't look stressed and they measure within the tolerance you need then go ahead and use them.

You might find this an interesting read: http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm

All about Carbon Resistor drifting:
http://www.cliftonlaboratories.com/carb ... istors.htm

Resistors in the example posted by Peter are actually fairly good. Some drift 200% or more higher.

Not all old carbon resistors drift higher. I have some, much older, that are within tolerance. Might have to with brands?

The manufacturer and the quality of his raw material supplies would certainly be a consideration.

I also believe the manner in which the set of components were kept, particularly insofar as the climate, has something to do with it.

Sometime back, I bought an un-built Philmore AA5 kit, that had apparently been stored in a damp environment. The chassis had severe rusting, the tube boxes were mildewed, and every single resistor was well beyond 125% tolerance, even though all the parts were still in their individual manila envelopes.

In my limited dealings with AA5's in the same general age bracket, very few, if any resistors normally measure that far out of tolerance.

The question that needs to be answered is; "How will an old aged NOS resistor at its current measured value behave when used near or below its rated wattage?" I use my junk box NOS on a regualr basis and so far have not had any noticeable problems in "consumer entertainment circuits", and quite a few more in ham radio and some test equipment. Sometimes I soak them with a bit of power to see if they can take it and hold value for a crtical go-no-go. I tend to use what I have at hand and if I feel I need brand new stock I order it but then much of the time I see no need to refit. There are just too many variables to contend with and if you have the budget, time and new parts at hand use them otherwise if you're a frugal recycler and have a limited budget but time to fix all the stuff you want to fix... experiment and be surprised with acceptable results most of the time unless you have no luck. Parts value tolerance in consumer circuits can be plus or minus 20% unless it has to do the with a critical circuit. You need to know if the circuit is critical.
Hank

This goes on an on. Old carbon resistors generally drift up in value, and breakdown voltage characteristics goes down. For a nickel each, why take a chance on a 1/4 or 1/2 watt component when also replacing all the caps in a set?

And using wirewound types in any power circuits that generate heat will be good too. The inductance of the wire is of no consequence for DC voltages. Or even lower AC freqs. I have some dog bones that are NOS and some are drifted and some are still well within specs. Probably heat and humidity are what get to them the most. But they can even drift in solid state circuits. And drift setting new in the box. If authenticity is of importance, select ones that are not out of tolerance. And they may drift in the future, but alot of circuits are amazingly tolerant of some drift upwards. It is the leaky caps that kill you.

Ive found that well more than 50% of carbons, even 30's dogbones, are well within their tolerance marking and I leave them alone. The chance of them getting worse is slim to none especially after a recap reduces the current thru many.

Its far from a component cost issue as to me and my customers time is money. Im no different with my own sets.

The set Im working on today, a National NC-300, is from 1956 and only a few values are way out of tolerance and the rest are well within 10%. The prior job, a 1938 Hallicrafters SX-17, was a bit worse but I took the gifts it offered.

I also have a large supply of NOS carbons and have learned to tell the drifters by appearance. The better brands are in the 80-100% range of being well within tolerance.

The cheaper the radio the more likely to have the lowest cost components but I can also name some very high end radios that used floor sweepings.

Carl

Carl
I agree with your pracitice as it is practical. I focus on perfomance and if it doesn't sound right I start checking voltages and if they're off I look for the culprit component. I don't agree with wholesale recapping unless there's a good reason and just becasue they're old and gonna perhaps fail in the future isn't a good reason, however in a ctitical location where a failure would take out a bandswitch segment or a transformer I agree it's cheap insurance, but not to save a resistor. I've yet to see a resistor increasing in value cause a catastrophy but at 71 I'm still learning!
Hank

The reason, at least in my experience, big brother, is that the set performs better once all old caps are replaced, in general. Of course there are always exceptions, but the odds definitely favor newer stock performance.

Well, Im also 71 and I replace ALL paper caps as almost 50 years of experience has proven the alternative to be from foolish to destructive. I get many of the foolish owners whose sets have destructed in here to repair and they are rarely pleased with the cost since their wilted gray matter is stuck in the 50's

But since this thread is about resistors lets keep it that way. The capacitor threads are fairly continuous and can be found in the archives.

Carl

I just don't see the point in replacing an old carbon resistor with another old resistor, used or NOS. They all drift up sooner or later, become more prone to arcing over or humidity/temperature sensitive and become noisy and interfere with proper alignment.

As cheap as they are, compared to transformers, tubes, and unubtainables, do the math for your next restoration. I do not sell resistors, in case anyone is curious.