And now a bad resistor

[beans-on-toast]

A failed component needs to be replaced. Nobody is saying anything different.

I was saying that there is a way to restore some resistors to spec. When a resistor is in service, it gets heated regularly and moisture doesn't have to an issue. In some cases where a CC resistor has sat for a long time on the shelf, it can absorb moisture and drift upward in value. Baking can drive out the moisture in a way that will preserve the component. Putting it in a circuit where it heats quickly can damage it resulting in a noisier resistor.

Let's put things in perspective, this is a higher wattage resistor probably part of a power supply. As I said, in this case it seem to far gone and baking probably will not restore it to spec. So it needs to be changed. Even some manufacturers recommend baking to pre-condition their resistors. RCD manufacturers vintage style carbon composition resistors. Below is a FAQ with their recommendation on baking. I have test equipment with ovens built into them to stabilize components. Same sort of idea.

If someone has a vintage museum quality amp and they want to maintain 100% original parts, it is their choice. They take pride in having their amps all original. It is better to have a working amp than one that can't be used. An original part doesn't have to be what was in the amp when it was built, but it does mean sourcing a NOS component to replace the out of spec or failed one with. That shouldn't be an issue if that is what they want. I use my amps as testbeds and want to improve them. That isn't for everyone.

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[Mr. Foxen]

Do you have in mind that anyone was talking about restoring a completely failed component?

That's what a resistor that doesn't work is. The fact it didn't even start off good isn't essential to the need to replace it, but its even more reason to do so when it has failed.

Would you also deny that the market for "collector restoration" with only the possibility of very occasional powering is not a valid market.

Not a valid market to me. I fix amps.

 

[Slevage]

I recently had one fail like that in my SVT. Original value was 15k and it metered out at over 1M. It was the plate resistor for the PI tube and I suspect it had been drifting for some time before it got to the point where it dropped the plate current so much it just stopped conducting. Man, did the amp sound better after I replaced it.

1M or open circuit ---- almost the same thing in this case

 

[twinjet]

There's a trick you can try with carbon composition resistors. They can absorb moisture which causes their value to increase.

For resistors that have been in storage for a while, a solution that sometimes works is to slowly bake them at a low temperature. Put them on a try in the oven set to a low temperature, around 175-200 degrees F for at least eight hours. The required baking time varies with wattage. If you try to bake them at higher temperatures they will crack and get noisy. Baking to solidify the composite material was part of the original manufacturing process. Seasonal changes in humidity can affect these resistors a bit.

Bill's resistor is probably too far gone to be reconditioned by baking. It would be an interesting experiment to see what can be done in this case.

I'm learning basic electronics. It's all so interesting, then I read this! What a cool hack.

 

[Handyman]

Some of the metal oxides have a brown exterior, so they look correct in a vintage amp. Bright Orange Drop caps kinda stand out though....

One advantage of upsizing the resistor's power rating is that it can reduce the resistor's noise. At least in theory.

If you're swapping a carbon comp resistor with a metal film, I doubt you'd notice the the difference between a .25w vs 2w when compared to those awful carbon noise factories.

 

[Mr. Foxen]

If you're swapping a carbon comp resistor with a metal film, I doubt you'd notice the the difference between a .25w vs 2w when compared to those awful carbon noise factories.

2w ones are more like the same size as old carbon comps so fit in neater.

 

[Handyman]

As an aside, absorbing moisture isn't just a problem with old resistors. Many manufacturers of ICs in plastic packages specify that the parts should be baked before soldering to drive out any absorbed moisture.

Burr Brown used to put warnings about this all over their data sheets.

Though I think the concern in those situations is some sort of catostrophic failure due to moisture + the very high heat from soldering.

 

[B-string]

If you're swapping a carbon comp resistor with a metal film, I doubt you'd notice the the difference between a .25w vs 2w when compared to those awful carbon noise factories.

It is the voltage rating of the higher wattage that may be needed. As Mr. Foxen said the 2 watt CF is about the same size as a 1/2 watt CC.

 

[Handyman]

It is the voltage rating of the higher wattage that may be needed. As Mr. Foxen said the 2 watt CF is about the same size as a 1/2 watt CC.

Yes, it's important to look at the max voltage specs especially when messing with tube equipment.

Fender quite famously ignored the voltage ratings for the carbon film resistors used as the Hot Rod Deluxe and Deville plate resistors. As a result these parts self destruct all the time keeping amp techs employeed.

They shipped them like this for decades. I can't imagine it having anything to do with the resistors they used being a few cents cheaper than properly spec-Ed ones.