"Is there any procedure to test them, if no datasheet is available"
Some years ago, I needed some resistors that were good at low uhf. I tested
some 2 watt carbon composition resistors. When I ran them at 2 watts, then
cycled them down, they kept changing resistance. I thought they would
stabilize, but they never did. So, I ran them at 1.5 watts and they still
did the same thing. I abandoned their use and used some glass film
resistors. Unfortunately, the type I used are no longer available so I need
to try them all again.
Some years later, I learned that carbon composition are hygroscopic (absorb
water). And, as Ian pointed out, they change resistance just sitting on the
shelf.
Years ago I watched the ALC zero point keep varying on my Collins KWM-2. I
figured it was a capacitor and kept changing them. Finally, out of
desperation, I started changing resistors and that was it.
I think most of the resistors today are stable - except the old carbon
composition, so they all should work well in a power supply - if they meet
the voltage rating.
73, Colin K7FM
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