WTH? NOS CC Resistors don't measure anywhere close to stated values!?!

[bob p]

I've had this big box of NOS 1/2W CC resistors that I've been pushing around for more years than I can remember. Last night I got bored and decided to sort them by value and take a few measurements. The results really surprised me.

While the low-R values (yellow decade band and below) were fine, the high-R values (green and blue decade bands) weren't anywhere close to what I was expecting. For example:

* A batch of 1M resistors, which were all gold or silver banded, had values as low as 440k. Most were in the area of 600-700k, some ranged as high as 700-800k, but not ONE of them fell within the 20% tolerance band that I was expecting.

* A batch of 1M2 resistors, with similar tolerances, and all ended up being binned into the same ranges: 400k+, 500k+, 600k+, 700k+, 800k+. A couple of them actually measured 1M2.

* A batch of 3M3 resistors all measured 500k to 1M.

* Higher resistance ratings >= 8-10M were just as bad -- very few ever measured over 1M.

What struck me as odd about this is that none of these resistors had ever been used in-circuit, they're all NOS, and even though they've never been exposed to current or heat, their values are nowhere near what I expected them to be. Only the high-resistance (megohm range) resistors were out of spec, and wildly so.

I verified my tests using a couple of different meters, though I didn't bother using a 4-wire meter on them.

Is this a normal experience for high-value resistors that are probably as old as the 1940s?

Are they really 1M and below, or will the in-circuit resistance of these old guys actually change when they're used in circuit?

[Leo_Gnardo]

Is this a normal experience for high-value resistors that are probably as old as the 1940s?

I got a stash of old unused half-watt CC resistors also, a couple years ago from a tech who was retiring. Many of the higher value resistors, say half a meg and up, have drifted up in value. Some others, random values, just ain't what they're s'posed to be. That includes all the way down to 47 ohms. I have both brown body types and the older ones that look speckled. If I find many in a given resistance to be off, I don't trust the batch and don't use them.

The trend is upwards in value, but some have drifted down.

I guess I could flog them on ebay as authentic 40-50-60's era carbon comps for a welcome shot of cash. Somehow I don't think buyers would be impressed, and feedback still counts for something. So into the trash they go.

[Justin Thomas]

Aren't CCs known to suck up water/humidity? I can imagine that might make them drift down in value, since impure water is a pretty decent conductor... though I'm sure you're not storing them in the shower...

Justin

[bob p]

Like you, Leo, I was given a box of this stuff. In my case it was when an elderly retired engineer from Motorola decided to clean out his basement. I ended up getting some pretty interesting things, including an HP vacuum tube oscilloscope calibrator. It's so old that it was made when HP still gave devices model numbers that had only 3 digits!

I've seen plenty of CC resistors drift up in value, but all of those have been ones used in-circuit, where they've been bombarded with electrons and heat.
I've not seen CC resistors drift down to such extremes ... especially when they've never been used.

It's kind of disappointing that all of these things might end up being tossed into the garbage. I'd sort of like to use them, if they'll be stable at their metered values. But who knows about that...

One thing about them -- if I used these resistors that aren't anywhere close to measuring the way they're marked, it would sure confuse anyone else who wanted to work on the amp. Maybe that's KF secret to making amps that are so hard to clone. LOL.

[loudthud]

You might try drying them out in an oven. Something around 100C for an hour and re-measure.

[Leo_Gnardo]

It's kind of disappointing that all of these things might end up being tossed into the garbage. I'd sort of like to use them

We could make sculptures like the ones in those old Honeywell ads, what'cha think? I thought those were dam' clever.



I really doubt Ken Fisher was devious enough to use drifty resistors. Now somebody will . . .

[bob p]

You might try drying them out in an oven. Something around 100C for an hour and re-measure.

If I understand you then, you're saying that all CC resistors are hygroscopic and the proposed failure mode has to be long term water absorption from the atmosphere?

If that's the case then baking them should liberate the water, but then putting them in a vacuum bottle and pumping out the atmosphere should accomplish the same thing (without the risks associated with heat). Or I guess I could just put them on top of one of my amps to let the tubes bake them dry.

[Enzo]

Or just think of it as an experiment to see if it has an effect, and if so what. I wouldn;t try to extrapolate without more information.

If you don;t like 100C temps oven, then try lower, like 150F. (too lazy to convert)

If I had to guess, I;d say the ones that rise and the ones that go lower are responding to different processes. Or potentially, different brands had different chemical makeup so one brand tends to climb from chemical breakdown while another brand tends to gather moisture. That sort of thing.

[bob p]

^ Bake them??? Watch out for CC resistors in the "what's for dinner?" thread.

[Tom Phillips]

Are these NOS 1/2W CC resistors that you have Allen Bradly's (I.e. look high quality) or an off brand?

[Steve A.]

Bob, I have a bag of 1/2W CC resistors that I got from Mike Quinn's electronic salvage shop almost 20 years ago... I'll have to dig them up and test them. I was thinking that the ones you got from the retired engineer might have been QC rejects. Any chance of that?

I would think that they would be stable now - how much more moisure could they absorb? If I was to use them I might paint over the color codes...

Steve

[bob p]

I don't think they were rejects -- I don't think anything would have been kept if it were known to be bad.

If I had to guess, I'd guess that they're AB, as that's probably what was used in industry back in the day. These things did come out of an R&D lab. but I'm no good at identifying old resistors. Maybe I can post a pic later on.

WRT being "stable", I doubt that they could absorb much more moisture, but if moisture is the problem then I think the moisture content probably wouldn't get higher but it could probably get lower. If they were to get hot, that might drive the moisture out of them which might make them drift up in value. That's a theory, anyway. It's probably not worth trusting them.

[Leo_Gnardo]

^ Bake them??? Watch out for CC resistors in the "what's for dinner?" thread.

Mmmm, baked resistors, with a sprinkle of shredded Parmesan!

When I was a very young pup just learning about electronics, I used to hang out with another teenager who got his parts by prying them out of old TV's and the like. One day his grandpa came to visit and found a bowl of resistors on the basement bar. He was just about to eat some, thinking they were nuts, when his mistake was pointed out. Close call.

Good brands gone bad? I can say I've bought Allen Bradleys half watt CC's direct from Mouser that were out of tolerance from day one. 82K typically read in the 90's. Not impressed. I think Mouser carries a Japanese carbon comp line now, I've never had a problem with them. Surprisingly I've even noticed 2% metal films I got 15-20 years ago drifting 10% out of spec, just sitting on the shelf.

The collecton of old carbon comps I got originated from a local electronics distributor, all well organized in yellow cardboard slide boxes. Their #1 customer was IBM. Certainly not the production line. More likely development labs & the "model shop," when they needed a handful of parts for a project they'd put in an order at Chief Electronics.

[R.G.]

People don't keep pet rattlesnakes like they have pet cats - the snake will always bite them if it has the chance. The snake is a captive, not a pet. It's the nature of the beast.

Carbon comp resistors are made by pressing a mixture of carbon granules in a "binder" (glue of some kind) into a phenolic resin tube and then pressing plugs/caps on that hold the leads and then curing the glue. Hmmm - phenolic; glue; carbon granules? What could possibly soak up water and change over time there?

Carbon comp resistors' resistance is set by the size of the carbon granules and how they're packed because the actual resistance is the number and size of the tiny places where the carbon granules touch each other. Variation in the resistance of carbon granules touching each other was the basis of the microphone in all telephones before the 70s or 80s.

The wonder is that they're stable enough to use for electronics, not that they drift.

Does this post make my opinion look fat?

[bob p]

RG, I'm totally missing your snake and cat analogy. But that may be because I'm a dog guy.

Sure, CC resistors drift. that's to be expected ... but I've never seen them drift down; I've never even heard anyone report that they drift down. I've only seen them drift up, and I've never heard anyone report that they've drifted below 1/3 of their stated value. That seems to be pretty extreme.

I'm not one of those guys who is a big fan of CC resistors, in spite of the theoretical mojo. When I remove CC resistors I typically replace them with something else.

I'm not even a big fan of CF resistors either. For large values I prefer MF and I won't buy CF resistor unless I can't find an MF that fits the application.

I've had these CC resistors gathering dust for so long that I thought I might do some experimenting with them in an old Fender, but these antiques that I have in-hand are a definite no-go. Now it's a question of whether I should pay through the nose to buy a selected range of CC for plate resistors, or just use one of the thousands of MF that I have on hand. I'm beginning to think CC is just not worth it.