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Carbon comp resistors are sensitive to moisture as well as heat. The resistance to moisture depends a lot on the quality of the housing. About the best of the CC resistors is either Allen-Bradly or Ohmite. They are the same, the resistors were made by AB and marketed by both companies and are recognizable by having the values in both color stripes and printed in ink on the body.
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CC resistors were made by a great many companies, including IRC and Stackpole. In general resistors with seams in the molded bodies will be found to have drifted significantly. CC resistors were never made in tolerances closer than 5% because just the heat of soldering them can most them that far. While they are supposed to have some advantage for handling pulses I don't think that makes a difference for most applications. For them most part they should be avoided as either replacements or new work. CC resistors are made from a slurry of granulated carbon in a carrier of an insulating material with a binder. Drift depends on absorption of moisture and heat. It is not moisture alone. Heat can change the way the carbon particles contact each other in a non reversible way. I was never involved directly in making them so don't know the details. In the industry they were called "mud" resistors. While they are supposed by some to be superior for RF applications it is a myth as measurements will demonstrate. On 2/9/2025 4:39 AM, Steve Wedge, W1ES/4 via groups.io wrote:
I have an extensive collection of parts that I¡¯ve owned, off and on, since the 1980s (it¡¯s a long story). In this stockpile, I have lots of resistors, including some carbon comps that go back as far as 70 years old. --
Richard Knoppow Los Angeles WB6KBL SKCC 19998 |
