Welcome • User Guide
ToS • Privacy • Canary
Donate • Bugs • License

©2025 Poal.co

I've got this old tube device on my bench. It's not really of a whole lot of use - it can be used, but I have other things that do what it does. This one is interesting to me because it looks like it was used at a telephone central office as a tone generator. Thing probably was turned on for 60 years before it was taken out of service. It has some nice decals on the side, and you can see where there was a strap to hold the controls stationary while in use.

It has the standard old-device-of-this-type issues. I was poking around on it last week before I had to travel, and was making some progress - then it just quit. No idea why, but no time to mess with it so I put it aside and said I'll work with it later. Had some time tonight, so I started poking at it again. Chased some red herrings around until I got to the power supply.

The picture shows a 10kΩ resistor that goes across two terminals on the power supply filter capacitor. This has the effect of dropping the voltage a little and providing some ripple rejection for the next stage.

To read this part, you use the standard color code: Brown, Black, Orange - 1 0 000 with a tolerance band of silver (10%) - pretty typical for old carbon composite resistors.

Two meters both agree that this part is now 295Ω instead of 10k. I can't say I've ever had a carbon comp part short like this, but that could certainly be a problem when the devices it's powering are expecting a different voltage. I have some replacements, I just need to dig them out and see if anything else has gone bad.

So where are the missing ohms? They probably leaked out and puddled up on the capacitor below, which is why it's leaky. It's now full of ohms instead of microfarads (joke!)

I've got this old tube device on my bench. It's not really of a whole lot of use - it can be used, but I have other things that do what it does. This one is interesting to me because it looks like it was used at a telephone central office as a tone generator. Thing probably was turned on for 60 years before it was taken out of service. It has some nice decals on the side, and you can see where there was a strap to hold the controls stationary while in use. It has the standard old-device-of-this-type issues. I was poking around on it last week before I had to travel, and was making some progress - then it just quit. No idea why, but no time to mess with it so I put it aside and said I'll work with it later. Had some time tonight, so I started poking at it again. Chased some red herrings around until I got to the power supply. The picture shows a 10kΩ resistor that goes across two terminals on the power supply filter capacitor. This has the effect of dropping the voltage a little and providing some ripple rejection for the next stage. To read this part, you use the standard color code: Brown, Black, Orange - 1 0 000 with a tolerance band of silver (10%) - pretty typical for old carbon composite resistors. Two meters both agree that this part is now 295Ω instead of 10k. I can't say I've ever had a carbon comp part short like this, but that could certainly be a problem when the devices it's powering are expecting a different voltage. I have some replacements, I just need to dig them out and see if anything else has gone bad. So where are the missing ohms? They probably leaked out and puddled up on the capacitor below, which is why it's leaky. It's now full of ohms instead of microfarads (joke!)

(post is archived)

You are currently inside a comment thread.
Click here to see all the comments (18).
[–] • 1 pt

Talked with a 'grey-beard' colleague about this phenomenon and he thinks it's probably a carbon composition resistor. These like to change their value permanently from a number of environmental and process conditions. Their advantages: Low parasitic inductance and high resistance to impulse overload.

https://en.wikipedia.org/wiki/Resistor Carbon composition Carbon composition resistors (CCR) consist of a solid cylindrical resistive element with embedded wire leads or metal end caps to which the lead wires are attached. The body of the resistor is protected with paint or plastic. Early 20th-century carbon composition resistors had uninsulated bodies; the lead wires were wrapped around the ends of the resistance element rod and soldered. The completed resistor was painted for color-coding of its value.

The resistive element in carbon composition resistors is made from a mixture of finely powdered carbon and an insulating material, usually ceramic. A resin holds the mixture together. The resistance is determined by the ratio of the fill material (the powdered ceramic) to the carbon. Higher concentrations of carbon, which is a good conductor, result in lower resistances. Carbon composition resistors were commonly used in the 1960s and earlier, but are not popular for general use now as other types have better specifications, such as tolerance, voltage dependence, and stress. Carbon composition resistors change value when stressed with over-voltages. Moreover, if internal moisture content, such as from exposure for some length of time to a humid environment, is significant, soldering heat creates a non-reversible change in resistance value. Carbon composition resistors have poor stability with time and were consequently factory sorted to, at best, only 5% tolerance. These resistors are non-inductive, which provides benefits when used in voltage pulse reduction and surge protection applications. Carbon composition resistors have higher capability to withstand overload relative to the component's size.

Carbon composition resistors are still available, but relatively expensive. Values ranged from fractions of an ohm to 22 megohms. Due to their high price, these resistors are no longer used in most applications. However, they are used in power supplies and welding controls. They are also in demand for repair of vintage electronic equipment where authenticity is a factor.

German kikepedia has a special article with some nice pics: https://de.wikipedia.org/wiki/Kohlemassewiderstand

[–] • 1 pt

Yes, it's a carbon comp resistor. They generally increase in value because the ceramic grains get damp and/or lose adhesion. Some will decrease in value, but not like this one did. This is practically a short circuit.

We don't use them anymore because they're noisy (electrons bouncing from grain to grain like an old phone mike) and they just kind of generally suck in light of newer film technologies. This device is from the late 40s, that's all they had that was in cost reach of this type of device.