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Out of tolerance resistors - Printable Version

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Out of tolerance resistors - tab10672 - 05-01-2014

Can a drifted resistor still be used? Say for originality, I need a 70k dog bone and have a 50k that has drifted to 70k, could that resistor be used as a replacement with reliability?

Todd

RE: Out of tolerance resistors - morzh - 05-01-2014

It depends. I usually do not use drifted up resistors as one simply cannot predict what the further drift will be like.
Also if the resistor is subject to larger power dissipation this will affect the way the resistive element contacts the lugs and the resistance simply will be very different from the cold state.
And even if the resistor does not dissipate a lot, it still can be heated by nearby elements - it can get quite warm inside the chassis of a tube radio.

Won't necessarily destroy anything but might make for problems and if used in critical places may complicate things more.
Unless I see a resistor that is within 10% or so which means to me it has not drifted in 70-plus years and so is not likely to move any time soon, I replace them.

RE: Out of tolerance resistors - Phlogiston - 05-01-2014

+1 only I go with the rated 20%

By the time you heat it up getting it out and in, it will probably drift up some more.

But I do replace dogbone resistors with NEW dogbone resistors.

RE: Out of tolerance resistors - exray - 05-01-2014

Hit it with a BIC lighter. That will give you an idea of how much the value may wander with heat. Some old dogbones go nuts with a tad of heat.

RE: Out of tolerance resistors - codefox1 - 05-02-2014

Do not use old out of range resistors. Certainly more trougle than they are worth. Many now replace all little old carbon resistors while doing a recap/restoration because it is easily done, cheap, and can make a world of difference for long term reliability and safety.

RE: Out of tolerance resistors - Jamie - 05-02-2014

I agree with Codefox. Why risk it? I can understand the originality thing, but really, how original are you keeping it by using out of spec resistors? Anyone working on it in the future isn't going to know what you did. They will only see the colorings on the resistor and think you used the wrong ones.

RE: Out of tolerance resistors - Mondial - 05-02-2014

Well, I think it depends a lot on what the plans for the radio are. If it is going to be used on a daily basis or sold to a non technical buyer then you are probably better off replacing off tolerance resistors with modern replacements.

On the other hand, if the radio is otherwise very original and will only tend to be played occasionally, them I see no problem repainting the replacement drifted resistor to indicate its current correct value. I have done this myself on occasion and have never had a problem. Another technique I have used is to parallel the drifted upward resistor with a modern resistor hidden underneath so the combined value is within tolerance.

I believe there is something to be said for preserving the original style components whenever possible.

RE: Out of tolerance resistors - Phlogiston - 05-02-2014

Preservation is the reason, only reason, that I don't change them all out. I want to keep as much of the original radio as I can. So anything that I think is serviceable for the next ten years or so stays. That which is replaced is replaced with a fabricated original-looking replacement of better value, meaning higher voltage rating or much higher wattage rating in the case of resistors.

A few of you will think "ten years". Yes, that is why I avoid using new electrolytic capacitors whenever possible - 10 years! Instead I use polypropylene film caps. So, See, it goes both ways.

RE: Out of tolerance resistors - TA Forbes - 05-02-2014

Must agree with Codefox. NOT a good idea to use old resistors that have drifted. They will continue to drift especially when hot.

RE: Out of tolerance resistors - DeckApe - 05-04-2014

Agree with Codefox and TA Forbes... I'm a sufficient amateur that I don't need to deliberately lay myself any booby traps... I'm quite good at doing it unintentionally.

RE: Out of tolerance resistors - Arran - 05-05-2014

I would not try to re-use a resistor pulled from a radio, especially a BED coded dogbone style resistor with radial leads. Even if it tested good the weak point in those resistors is the connection between the carbon composition rod that makes up the body of the resistor and the leads. The leads are just wrapped around the rod and over time the wrap can loose contact, removing the resistor by moving the leads around makes this even more likely. I believe that this is the reason so many of those dogbone resistors drift so much in value so much worse then then molded types that succeeded them.
If you like the look of the old style resistors I would try duplicating them through Steve Davis's method, modern resistor, plastic tube, solid buss wire, and paint.
Regards
Arran

RE: Out of tolerance resistors - Chuck Schwark - 05-05-2014

I have a box of out of tol. resistors pulled from radios
holding down one corner of my basement floor. 8)

They make great ballast (no pun intended...)

Chuck

RE: Out of tolerance resistors - tom.seeger - 05-05-2014

I always change resistors that have drifted more than 20 to 25% with new 2W carbon film resistors. I use 2W only because they look more authentic. My logic is, if we don't change then now, they will probably continue to drift and resistors with leads might be really hard to find in 20 to 30 years.

RE: Out of tolerance resistors - Arran - 05-05-2014

I don't think that resistors with leads are going to become rare anytime soon, as long as there is a demand for them. The surface mount craze among engineers a few years ago has undergone somewhat of a reversal in favor of through-the-hole construction again. Part of the reason may be the tooling costs but also surface mount design has become synonymous with premature failure, especially in power supplies.
Regards
Arran

RE: Out of tolerance resistors - morzh - 05-05-2014

Arran

As an engineer who designed many T-holes and then many SMTs I beg to differ about the craze and the failures.
T-hole has its place but if anything SMT power parts are excelent today, and if we do not dwell on power - SMT is the only reasonable way to design todays digital devices.