Jim Yanik wrote:

"N_Cook" wrote in
:


JW wrote in message
. ..

Here's one I haven't seen before. Bought an EDC (now Krohn-hite) 521
DC calibrator on the surplus market that was blowing fuses. There
were two power supplies that seemed to be causing the problem +-150V.
Turns out that there were two 22K 2W carbon comp resistors that are
used as bleeders across the two 470uF filter caps. Both had changed
resistance - one was about 2K, the other was 150 ohms! I've seen
carbon comps drift, but never saw ones that drifted *that* far. Also,
in circuits like this where they are dissipating over a watt, they
usually drift high; at least in my experience...

Well, after replacement it seems to be working well. May need a
calibration though.


If you still have them , try a neat 0.5mm thick Dremmelgrinding disc
cut across the middle and see if there is a gradation of resistance
developed across the material , and so along the length. Just under
the surface coating you may find the high conductivity path.
I keep a "black museum" of such oddities , don't know if anyone else
does







I used to see 2W carbon comp resistors change values drastically all the
time in TEK 520 and 520A vectorscopes. I believe it's heat-related.
(it also might depend on how much V drop across them)

They used to char the PCB even with a 1/2" standoff spacing and some even
dropped off the PCB.



Electronics isn't fun unless you have some flames, forehead contact with
debris, and escaping blue smoke!

Jamie