Quote:
Originally Posted by bandersen
What was a little surprising is that all the larger valued resistors (10K and up) were within spec. It's the smaller (< 500 ohm) resistors that were way off. I've seen the same in some early Admiral sets.
|
The failure mode of most carbon composition resistors is moisture absorption - the carbon particles absorb moisture, causing the particles to separate, and forming a higher resistance bond. It's noticed more greatly in small, sub 1K values, as the shift is tens or hundreds of ohms, a large percentage of the original value. This is typical whether on the shelf, or installed in equipment.
No manufacturer was ever able to eliminate moisture as a problem. IRC tried, and got a patent for their "molded" carbon composition resistor process that sealed the outside of the resistor and thus reduced moisture intrusion, but aging and lead flex allowed the moisture intrusion.
If it wasn't for economy and pulse/surge capability, carbon composition resistors would have been dead long ago, as most film types suck at handling surges.
And no, you can't bake the moisture out - DoD and NASA both tried, but found that the heating process shifted resistance greater than the moisture did.
Cheers,