Testing old resistors

There is no failure mode for resistors to drift low. The core conductor becomes compromised by a number of possible mechanisms and this never results in increased conductivity, only reduced conductivity (there are some materials which increase conductivity under some of scenarios but they are not used for making resistors) and hence increased resistivity.

I occasionally come across a resistor that tests low in-circuit, so you have to lift a leg to disconnect any parallel resistance if you want to make sure it is not really low. I have very occasionally come across resistors that have been slightly out of tolerance on the low side when tested out of circuit. I have always assumed they were low when they rolled out of the factory, or maybe mis-marked, which is effectively the same thing.
 
I occasionally come across a resistor that tests low in-circuit, so you have to lift a leg to disconnect any parallel resistance if you want to make sure it is not really low. I have very occasionally come across resistors that have been slightly out of tolerance on the low side when tested out of circuit. I have always assumed they were low when they rolled out of the factory, or maybe mis-marked, which is effectively the same thing.

Yes, low-value out of factory is common/normal and very different (manufacture/spec) issue, they never drift low with use/failure-mode.

Regarding the parallel resistors, again, a different issue.
 
Parallel resistance, not necessarily parallel resistors. That's why you lift a leg if you want to be sure.

Yes, I understood that you referred to resistors connected in parallel; yes, sometimes resistors look low value if measured incorrectly. but this is just a measurement/understanding failing (but certainly one everyone can be reminded of... thank you). Still, this is not to be confused with the actual resistor value being low and does not counteract the fact that a resistor can not drift low.
 
There is no failure mode for resistors to drift low. The core conductor becomes compromised by a number of possible mechanisms and this never results in increased conductivity, only reduced conductivity (there are some materials which increase conductivity under some of scenarios but they are not used for making resistors) and hence increased resistivity.

Nope. Despite the fact that the internet repeats this, it is not true.

Here are two mechanisms.

(1) Carbon-composite resistors decline in value with applied temperature. When the temperature returns to normal the resistor's value is permanently impaired. Over time the reduction in impedance lead leads to increased current, which, in turn, leads to a further heating, a further decrease in value, and eventual failure from over-current.

(2) Carbon-composite resistors decline in value with applied voltage, but this is non-uniform across the grains. Preferential paths exist, which become permanently altered to have a lower impedance. Overtime these paths expand, and the resistor can short out. This is why voltage ratings for resistors matter.
 
Nope. Despite the fact that the internet repeats this, it is not true.

Here are two mechanisms.

(1) Carbon-composite resistors decline in value with applied temperature. When the temperature returns to normal the resistor's value is permanently impaired. Over time the reduction in impedance lead leads to increased current, which, in turn, leads to a further heating, a further decrease in value, and eventual failure from over-current.

(2) Carbon-composite resistors decline in value with applied voltage, but this is non-uniform across the grains. Preferential paths exist, which become permanently altered to have a lower impedance. Overtime these paths expand, and the resistor can short out. This is why voltage ratings for resistors matter.

Yes, these are correct. I just have never seen them outside of a high-power lab setting (phd physicist here). I will edit my post since it is not correct that there are 'no mechanisms' for decreasing resistance, and modify to say they are not that common. Have you seen the above (1) or (2) you mention; I have never but it would be interesting. I have seen resistors go short via external carbon tracking, particularly in tube amps when resistors were used that had a small physical distance between conducting legs compared to the voltage applied.... over time carbon can track between and cause problems. I didn't include the earlier this in my earlier statement because it doesn't present as a creeping drift, just more of a sudden short when the track nears completion and it is general component failure mode, not just resistor.

Again, I was incorrect in the 'completeness' of my claim that there are no mechanisms to decrease resistance and will edit my above post and note that I am not rigorously correct. Thanks (it's unusual of me to be careless less that and I thank you for pointing it out).
 
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I never measure carbon-composite resistors to determine which are in spec and which are not. I regard the entirety of them as uniformly defective by way of the physical construction (Vcr, Tcr, Pcr, drift high/low, noise, etc.) and accordingly completely replace every unit. These issues are well-known in reliability circles. I know others who have measured units which were low enough to cause improper functioning.

In terms of consumer electronics, tube televisions were known to fail because excessive heating caused resistors to lower in value as a runaway process, and radios have lower values when current through a resistor causes excessive heating. The magic value for this is STOL (Short-Time Overload Condition) and the manufacturer gives a limit. Exceed that and the resistor is damaged. Heating starts at about 50% of the rated wattage, btw, so chronic conditions which spike the heat can cause damage over time.

I agree that the most common failure mode is increased resistance because most resistors are not used in power situations which cause heating; instead, the failure mode is the phenolic jacket permitting ingress of atmospheric moisture and the clay (a composite resistor is a combination of carbon black and clay) is hygroscopic. The resulting swelling spreads the carbon-to-carbon gaps and thereby increases the resistance.

None of this is secret lore. The resistor manufacturers explain how excessive heat can alter properties.

If one avoids carbon-composite resistors all of the problems go away. These components were the only inexpensive options before about 1980; we now have low-cost alternatives (metal film) which offer superior properties. No need exists to use carbon-composite or carbon-film. Wire wound would be superior because of even lower noise, but those remain more expensive and not suitable for every single resistor. Next amplifier I rebuild will get wire wounds for the noise-critical resistors.
 
Interesting discussion. Thanks OP for posting and thanks to all who are discussing.

Where do carbon film resistors fit into the discussion? Not as bad as CC? Almost as bad? OK?

Cheers,
James
 
Carbon film are also bad resistors. I've elsewhere discussed this, but the thin film of carbon has many of the same undesirable properties as carbon composite, including noise.

The best resistors to use are metal film for general purpose and wire wound where higher power or lower-noise are specified.
 
I never measure carbon-composite resistors to determine which are in spec and which are not. I regard the entirety of them as uniformly defective by way of the physical construction (Vcr, Tcr, Pcr, drift high/low, noise, etc.) and accordingly completely replace every unit.

While I agree with this completely for my own equipment - it can be a different situation with some customers equipment.

I recently repaired a commercial stereo tube amplifier that was manufactured in 1960, It was all point-to-point connections and the chassis was not well planed or laid out. There were 86 carbon-composition throughout the chassis and the customers budget did not allow for them all to be changed.

Each resistor was measured and only those which were out of specification were replaced, along with any that could easily be exchange while or other sections of the amplifier were repaired. All of the new resistors were metal film.

For this application, this was the best choice - even knowing that the complete replacement of all of the resistors would result in a cleaner sounding amplifier. For the owner, and his application for the amplifier - it didn't matter that much to him - Chris
 
. . . I recently repaired a commercial stereo tube amplifier that was manufactured in 1960, . . . There were 86 carbon-composition throughout the chassis . . . and only those which were out of specification were replaced . . .

How many out of 86 were out of spec?
 
There were 9 out of specification and an additional 7 were replaced during the course of the repair - Chris
Wow, that's just amazing. 58 year old resistors that are still working as they should. I'm even more surprised when I see 80 and 90 year old radios that still work in 2018. I doubt modern metal film resistors will last longer than 20 years. It's kind of funny to see people in 2018 disparage a working resistor with so much proof of their longevity all around us.
 
Wow, that's just amazing. 58 year old resistors that are still working as they should. I'm even more surprised when I see 80 and 90 year old radios that still work in 2018. I doubt modern metal film resistors will last longer than 20 years. It's kind of funny to see people in 2018 disparage a working resistor with so much proof of their longevity all around us.

No. You cannot draw that conclusion as it is totally unfounded.

This has been elsewhere beaten to death, but carbon composite resistors are a horrible technology. The resistance considerable varies with applied voltage and temperature, for example. Those resistors are functioning as best could be accomplished at the required price point. I refer you to my writeup here:
http://audiokarma.org/forums/index....osition-resistors.601998/page-2#post-10276507
http://audiokarma.org/forums/index....osition-resistors.601998/page-2#post-10276509

What Chris has written is that 10% were replaced for being too out of spec to use. One in ten had utterly failed. That suggests a serious reliability problem. He did not describe what the tolerance was for the others. So no conclusion can be drawn. Neither did he specify what the reliability was after the replacement. How many failed a year later? We don't know.

We also do not know how much better the unit would have functioned with a full replacement to metal film.

Radios and guitar amplifiers are very forgiving of poor-quality circuits and poor-tolerance components. That doesn't mean the components are wonderful, it just means the radio still works. Radios work using a rusty razor blade as a detector. Doesn't mean this is the best detector around.

Carbon composite resistors are not used for new construction for a reason and should be removed during rebuilds whenever possible.
 
Wow, that's just amazing. 58 year old resistors that are still working as they should. I'm even more surprised when I see 80 and 90 year old radios that still work in 2018. I doubt modern metal film resistors will last longer than 20 years. It's kind of funny to see people in 2018 disparage a working resistor with so much proof of their longevity all around us.

The point is not that most have lasted a long time, but that people are using them now, and after 40, 50, 60, 70 years, they are failing at a high rate. Why do you say metal films won't last as long. Who knows? You may be right, but when you make a claim, the burden of persuasion is on you. I myself am betting on modern materials science.
 
The point is not that most have lasted a long time, but that people are using them now, and after 40, 50, 60, 70 years, they are failing at a high rate. Why do you say metal films won't last as long. Who knows? You may be right, but when you make a claim, the burden of persuasion is on you. I myself am betting on modern materials science.

Ummm, no. You have no data from which to conclude that the resistors still function.

Brand new carbon composite resistors are poor resistors and poorly function. Vcr, Tcr, Pcr, etc. All awful. The properties are non-linear and cause odd-order harmonic distortion. (High on one side, low on the other, that's odd-order distortion.) The resistors were highly out of specification, but radios will works with poor tolerance components. A radio is very robust and, as I've noted, can be built with a rusty razor blade. One could also use a homemade galena crystal. Tubes which would shows as failures on a tester will work in a radio, even though those tubes would not work in an audio amplifier.

You should study the construction of a carbon composite resistor and metal film resistor before equating the lifespan. Metal film contains no hygroscopic clay, does not have internal rectification effects between the grains, does not have non-linear properties, and has no degradation. This is the superior resistor and if it were available for roughly the same price in the fifties and sixties, it would have been used in the better (non-Muntzed) equipment.
 
Ummm, no. You have no data from which to conclude that the resistors still function.

Brand new carbon composite resistors are poor resistors and poorly function. Vcr, Tcr, Pcr, etc. All awful. The properties are non-linear and cause odd-order harmonic distortion. (High on one side, low on the other, that's odd-order distortion.) The resistors were highly out of specification, but radios will works with poor tolerance components. A radio is very robust and, as I've noted, can be built with a rusty razor blade. One could also use a homemade galena crystal. Tubes which would shows as failures on a tester will work in a radio, even though those tubes would not work in an audio amplifier.

You should study the construction of a carbon composite resistor and metal film resistor before equating the lifespan. Metal film contains no hygroscopic clay, does not have internal rectification effects between the grains, does not have non-linear properties, and has no degradation. This is the superior resistor and if it were available for roughly the same price in the fifties and sixties, it would have been used in the better (non-Muntzed) equipment.

You misunderstood me. That's exactly the point I was making. Metal films are better for a number of reasons.
 
Well, you set forth, "The point is not that most have lasted a long time, but that people are using them now, and after 40, 50, 60, 70 years, they are failing at a high rate."

The implication being that only now are the resistors failing, when in fact, carbon composites were failing the day they rolled off the factory because the inherent problems by way of construction from clay and carbon black.

Metal film is superior for every application that is not in the GHz, and even at that point the parasitic inductance and capacitance of PCB traces, component leads, and wiring matters.
 
The point is not that most have lasted a long time, but that people are using them now, and after 40, 50, 60, 70 years, they are failing at a high rate. Why do you say metal films won't last as long. Who knows? You may be right, but when you make a claim, the burden of persuasion is on you. I myself am betting on modern materials science.
It's still amazing that they are in use and working after 70 years. Nothing made in 2018 will be working in 2098. Materials science has nothing to do with what is being manufactured. Make it cheap is the new mantra and nothing else matters.

No one here has address the fact that metal films have a higher shorting failure rate than other types of resistors (probably why carbon comps and carbon films are still made).
 
No one here has address the fact that metal films have a higher shorting failure rate than other types of resistors (probably why carbon comps and carbon films are still made).

Not true at all.

At high voltage, as in many kiloVolts, the adjacent regions in a spiral-cut metal film resistor can arc if the resistor is not rated for that voltage.

A carbon composite resistor will be destroyed by internal arcing long before the metal film. No benefit. CC resistors, in fact, are more vulnerable to pulse-current overloads as any overheating of a CC will damage the grain structure and the phenolic coating. The CC resistor has no mojo, no benefit, no reason to use.

Metal film is a precision technology, highly reliable (more than CC or CF), low noise, linear, no aging defects, and inexpensive.

I do not understand why people promote old technology out of misplaced nostalgia. CC resistors were resistors when new and were used only because wire wounds were ten times the price and carbon film, another bad technology which is slightly better than CC, were three times the price.
 
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