Using old carbon resistors

gadget73 · **Posted:** Nov Sun 22, 2009 8:45 pm

I'm with Curt. I don't even unsolder one end, I just check that they read in tolerance or a little low, and leave it alone. If they read way low or something out of the realm of possibility, I unsolder one end and check again before replacing. Its just too much work to shotgun all the resistors, especially when many are likely just fine.

Burnt Fingers · **Posted:** Nov Tue 24, 2009 9:35 pm

These days everything is suspect. Mica caps are failing at an increasing rate; early disc ceramics (especially early 50's Zeniths) are as leaky as paper; IF cans have silver on ceramic substrate migration, and silver on mica sheet punctures.

Riveted grounds have developed corrosion.

Some tubes develop leakage just from sitting.

The fun never ends.

Carl

Dale Saukerson · **Posted:** Nov Tue 24, 2009 11:45 pm

The beauty of an old resistor is even if the value has gone waaaay high, it still is acting only like a resistor and the radio usually plays. Old caps don't cooperate, they start acting like resistors becoming a nuisance if not a risk to the radio operation.

sofaslug · **Posted:** Nov Wed 25, 2009 12:20 am

I've noticed little or no difference in how a radio sounds and performs after replacing some or all of the out of tolerance resistors. Who knows, with today's higher line voltages, maybe upward creep of the resistance is a good thing.

Caps are another story. I replace all of the electrolytic and wax/paper caps.

Bob

Peter Bertini · **Posted:** Nov Wed 25, 2009 4:23 pm

I've seen comments in other non-ARF forums that screen dropping resistors (carbon comps) will go high in value by virtue of the high amount of DC voltage that is dropped across them... can anyone confirm this effect and if it is true or just coincidental age related failures?

Pete

Burnt Fingers · **Posted:** Nov Wed 25, 2009 5:12 pm

It is rare to find a screen dropper that hasnt increased in value, even in high end comm gear. I have found several 22K-47K that have increased even up to 1-2 meg with many others in the 100-400K region.

I can theorize about this and attribute it to leaky screen bypass caps as the primary cause. They both increase the screen current, adding more dissipation to an already marginal rating in many radios, and allow some degree of RF to flow into them. RF is well known to increase carbon composition values.

When I test the droppers in later radios that use disc caps for bypassing the value changes are minimal and I leave them alone. Going from a 22K to a 27K for instance is fine with me and the very slight gain reduction wont be noticed.

Carl

amptramp · **Posted:** Nov Thu 26, 2009 5:59 pm

Carbon composition and carbon film resistors disappeared from the preferred parts lists of most military manufacturers many decades ago, to be replaced by metal film resistors. If you want the wisdom of the ages about resistor applications, download this:

http://www.dscc.dla.mil/Downloads/MilSp ... dbk199.pdf

Many resistors are non-critical and can be left as is. But if you have low noise stages or high reliability requirements for any resistor, this handbook is a must-read and it is only 50 pages. It's free! Force yourself!

KyMike · **Posted:** Nov Fri 27, 2009 3:04 am

>>> 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? <<<

The resistive surface wears away with use in carbon pots, so it makes sense that the resistance would go up. Its rare to find a cheaper model tv or radio more than a few years old that doesn't have a noisy volume control, unless it has had very little use. Just throwing out ideas here, but it should be possible to make a conductive grease by adding carbon dust or graphite powder to a small amount of heatsink grease. A tiny dab of this on the carbon surface might give the pot a new lease on life if the resistance is not too critical.

Mike

Curt Reed · **Posted:** Nov Fri 27, 2009 7:53 am

Problem is, is that if you had all the stuff and the time to do something like that which "might" work, or it might not work, you would have more invested in it than a replacement part would cost, especially if you value your time.
Curt

KyMike · **Posted:** Nov Fri 27, 2009 10:11 am

True enough, and even if it did work it probably would be only a temporary fix, but it might get you by until a replacement could be located.

Mike

Tom Herman · **Posted:** Nov Sat 28, 2009 6:24 pm

Burnt Fingers wrote:

These days everything is suspect. Mica caps are failing at an increasing rate; early disc ceramics (especially early 50's Zeniths) are as leaky as paper; IF cans have silver on ceramic substrate migration, and silver on mica sheet punctures.

Riveted grounds have developed corrosion.

Some tubes develop leakage just from sitting.

The fun never ends.

Carl

Hi Carl!

You're a real font of optimism today!
But you are correct...

-Tom

Tom Herman · **Posted:** Nov Sat 28, 2009 6:25 pm

amptramp wrote:

If you want the wisdom of the ages about resistor applications, download this:

http://www.dscc.dla.mil/Downloads/MilSp ... dbk199.pdf

Thanks for the "heads up" on this! It's downloaded and in my reference library.

-Tom

AB1JX · **Joined:** Jun Wed 08, 2011 6:03 am **Posts:** 7

I found a slightly different version of this post by Dean a few months ago, ordered resistors, opened it up, and found mine is different. They made a change around serial B020100 to using a preregulator A18 board, and changed the divider to 1 meg 1/2 watt film resistors. Mine are OK. Unless there are 2 dividers anyway.

For anyone considering or owning a 2213 Google '"Tektronix 2213" dim'. Some of them ran too much filament voltage on the CRT leading to premature aging. I have to turn off the room lights to see mine sometimes. It should be about 6 volts, not 7 on the filaments. The fix is to take 1/2 turn off the filament winding but for many it's too late.

In defense of carbon comp resistors, I used to work for a guy who was too cheap to buy 1% resistors for attenuator pads. He'd buy the next lower value, then sit around and whittle them to raise the value while hooked to a dmm and seal them up with nail polish. I don't think that would work with any other type resistor.

I'm new to the forum, just registered to comment on the 510k resistors. I'm into early solid state communications receivers like the Realistic DX-150A.

Alan, AB1JX

Dean Huster wrote:

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

codefox · **Joined:** Nov Sat 27, 2010 6:15 pm **Posts:** 3676

Gotta open up my C3 Hammond and tone cabinet soon, I'll need a couple of hands to do this, and then some. This is not your AA5. I would have to be a complete idiot not to replace all the coupling capacitors, stuff the electrolytics, and sure there are 50 or so discrete resistors. I'm not going to pass judgment on them one by one, that would truly be insane. Disconnect, dress and solder one end unless there's one to go, do the other, and go on.

In the words of a master, "Wlways make a mechanical connection before considering how solder will preserve it.

The tone wheel with it's inscruitale capacitor array will have to wait for another day.

cokekid · **Posted:** Jul Fri 01, 2011 4:15 am

Just curious, as I have never seen a Allen Bradley resistor. How does one know if it's made by this company.?

Gerry

wazz · **Posted:** Jul Sun 17, 2011 2:05 pm

I hoarded a bunch of NOS dog bones here. Partly because they just look neat. And many are within a 20% value window, which is probably just fine for general radio use or restoration. At work they had red plastic bins full of all the values of the quarter watt "Little Devils" carbon comps and picking thru them I found quite alot of them had gone above their tolerance, just setting in a good environment. Still, carbon comps of common usage are starting to attract interest and selling well, but presumably you dont know what you are getting besides the labeled values. Might buy a lot and find them to be half drifted off. And still they are not as pretty as the painted dog bones! Even if your 300k drifted to 330, you could use it as a 330, as how many people know how to read dog bones? Or bother to try, if the set works?

codefox · **Joined:** Nov Sat 27, 2010 6:15 pm **Posts:** 3676

Problem is today you get a radio up and running, make it look great and sell it or give it away (as I mostly do.) AB's were often good,as gold, but I'll not use an old one anymore.

Using old carbon resistors

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