[Boatanchors] Carbon Comp Resistors: The Darkness Gathers?

[Rob Atkinson]

Gary is correct.  This is what I was getting to when I asked the
original poster to tell us what he meant by "high voltage."  Low value
carbons rated for dissipation on 750 v. modulator plates are fine.
Ditto for parasitic suppressors where the coil is taking the current
and in low voltage RF and audio stages (receivers, audio pre-amps...).
   I use carbons for the most part (measuring them first) because I
can get them NOS and they work okay for ham radio.  That's because as
hams, we don't typically run our gear night and day nonstop.  If
they're in the current path I tend to go overboard on the power
dissipation.

73

Rob
K5UJ

On Mon, Jul 2, 2018 at 9:10 PM, Gary Peterson <kzerocx at rap.midco.net> wrote:
> It all depends on the application.  There are RF circuits where carbon composition resistors work and some carbon film and metal film ones don’t perform as well.
>
> I have read that some carbon film and metal film resistors are trimmed to value by cutting a spiral in the film.  I have measured and found some of these film-types to be inductive.  No big deal on 160 meters, but a potential problem at VHF.  Carbon composition resistors, that I have measured, tend to be very slightly capacitive.
>
> I have hooked  older carbon composition resistors up to a power supply and applied enough voltage to cause the part to be hot to the touch and most returned to within their original tolerance within an hour, probably due to baking out absorbed moisture.  I would think that equipment that is used regularly would not suffer value drift anywhere near as much as the ones that have gathered dust.  Old radios, engines and human muscles and brains need to be used regularly or they atrophy.
>
> Once, I measured a huge pile of 10% carbon composition resistors.  Not a single one was 5% or better.  I had to assume that as these came off the assembly line, the better ones got the 5% gold bands and the ones between 5% and 10% got the silver bands.  In many cases, the tolerance isn’t all that critical.
>
> Gary, K0CX
>