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Testing old resistors

Retrovert said:
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.
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Why not? If you were right in your assessment, we, who own older than 40 year equipment, would be replacing resistors and noting their failure. That's not the case. The same can be said for old tube gear like 1930s radios, that still work. For one in ten resistors that are 58 years old to fail, drifted, is amazing and you should reassess your conclusions.
 
fernarias said:
Why not? If you were right in your assessment, we, who own older than 40 year equipment, would be replacing resistors and noting their failure. That's not the case. The same can be said for old tube gear like 1930s radios, that still work. For one in ten resistors that are 58 years old to fail, drifted, is amazing and you should reassess your conclusions.
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Say, what?

You are using one reported experience about a guitar amplifier from 1960. You don't know how it was stored, you don't know how many hours it had on it, and you don't know anything about it. The fact that 10% of the components were so out of spec as to need replacement is impressive. We have the rebuilders statement that he did not have the ability to perform a complete replacement for budgetary, not technical, reasons.

But you still haven't addressed the performance issues I explained in the links set forth above. Vcr, Tcr, Pcr, noise, etc. These problem existed when the resistors were new. You can't overcome a lack of understanding why these properties matter using misplaced nostalgia.

The CC does, as you claim, not have a longer lifespan, it is not invulnerable to over-voltage conditions, and it has no desirable properties. All it has is wonderful nostalgia for you. Ok, I can't measure nostalgia on a meter, but I can measure instability, drift, incorrect voltages, harmonic distortion, and poor performance. I'll stick to minimizing those and keep the nostalgia for the exterior knobs and case.

This is pointless. I'm going back to watching Hammer Films from the 1950s. Far more interesting. At least I know that Frankenstein, Dracula, and the Werewolf aren't real. The horror of carbon-composite resistors? Yeah, that's real and it needs the silver bullet of metal film to stop it.
 
As for CC, I cannot see why in this day and age, anyone would choose to use them in a new device.

Restoring ancient vintage radios or collectible guitar amplifiers that have considerable intrinsic value when functionally original, that would be compromised by modern substitution, one may be able to make a case if in-spec NOS components were available.

I like to believe the modern MF will have a lifespan equivalent or better than CF. I guess it all comes down to the longterm stability of the device and it's ability to remain hermetically sealed as the decades roll by. I've already seen plenty of MF resistors where epoxy has cracked and end caps detached, but there is also a phenomenal amount of fake MFs in the marketplace which are simply CFs 5% in a MF 1% coating.
 
According to datasheets I've read, carbon composites can have better transient power handling capacities than metal films, but I would think this is not a significant factor in most audio equipment, tube or solid state.
 
fernarias said:
Why not? If you were right in your assessment, we, who own older than 40 year equipment, would be replacing resistors and noting their failure. That's not the case. The same can be said for old tube gear like 1930s radios, that still work. For one in ten resistors that are 58 years old to fail, drifted, is amazing and you should reassess your conclusions.
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No. You are cherry picking from a single comment posted, and the poster even said it would be his choice to replace every single resistor with metal films, but the customer's budget didn't allow it. That's a ridiculously small sample size. The antique radio forums are full of comments about drifted carbon-composite resistors. The hygroscopic nature of the clay and the permeability of the phenolic jacket are the problems.

You obviously have not replaced radios which will work when horribly out of spec.

If CC resistors were so wonderful they'd be used in modern equipment. The fact that they are not should tell you something.

Your nostalgia for components made when Eisenhower was president is misplaced.
 
All of this is losing sight of the core issue.

What audio gear needs is a need for resistors, and capacitors, which are in-spec, create well-defined circuits with specific RC timing constants and filtering properties, are low noise, and provide a resistance (or capacitance) which does not vary with voltage, applied power, or temperature. Any variations in linearity, whether with voltage or time, creates distortion, and distortion is bad, mmmmkay?

Guitar amplifiers thrive on distortion, but not the audio amplifiers which reproduce that recorded distortion.

Everyone who rips out tone controls screaming, "my ears!, my ears!", but who then insists upon carbon-composite resistors which add noise and odd-order harmonics (aka resistor distortion as R. G. Keen puts it) among other problems, clearly has an utter inability to understand what clean, pure sound is.
 
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restorer-john said:
I like to believe the modern MF will have a lifespan equivalent or better than CF. I guess it all comes down to the longterm stability of the device and it's ability to remain hermetically sealed as the decades roll by. I've already seen plenty of MF resistors where epoxy has cracked and end caps detached, but there is also a phenomenal amount of fake MFs in the marketplace which are simply CFs 5% in a MF 1% coating.
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The fake issue is an excellent point.

Counterfeit resistors are often painted instead of epoxy coated.

The world is full of thieves. Talk to one of the capacitor companies about fake labels and datasheets. I have. Reputable companies receive complaints about failures and during investigation it turns out the part is a counterfeit. The reliability studies on actual metal film resistors show very high reliability, which is why the aren't failing in the field. But when one buys components on eBay one is asking for trouble. I have seen so many analyses of the fake semiconductors daily sold on eBay with fake numbers or rejects that I'd never buy there.
 
restorer-john said:
As for CC, I cannot see why in this day and age, anyone would choose to use them in a new device. Restoring ancient vintage radios or collectible guitar amplifiers that have considerable intrinsic value when functionally original, that would be compromised by modern substitution, one may be able to make a case if in-spec NOS components were available.
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Exactly the point.

The value depends upon nostalgia for properties which aren't real, so having original-looking components is a factor in the price. Circa 2008 Gibson put out fake Bumblebee capacitors as "historic capacitors"; these magickal devices contained modern polypropylene film capacitors in a striped jacket which looked like a 1950s Bumblebee capacitor. The users paid a bundle and raved about the wondrous properties. Until someone cut one open and discovered it was all a lie.

I could easily defend and support someone slicing open wax capacitors to make a 1930s radio chassis look original. Same goes for slicing open phenolic-jacket resistors , assuming the thermal dissipation was not compromised. If one wants an original-looking device for cosmetic purposes there's nothing wrong with that. But using poor components to maintain a cosmetic look will not and cannot yield the best functionality.

I have an AA5 AM clock radio (built five miles from me back in the days when Queens still had manufacturing) which needs to be restored. Will I restuff wax capacitors and hollow out CC resistors. Nope, because that's not necessary for this purpose. The bakelite case will be cleaned and waxed with a hard petroleum wax to restore its lustre, and the innards will be completely rebuilt, along with a heater delay and an isolation transformer. It should last another sixty years without a rebuild other than replacing the electrolytics in thirty years. Can't do anything about that.
 
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