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.

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

"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.


--
Jim Yanik
jyanik
at
localnet
dot com

You might do a dissection, as Nigel suggests, just to see what's in there..
Eyes On can sometimes clear up a lot of mystery.

I have seen old resistors that were a combination of resistance wire and
carbon, but your example could be something else that's interesting.

--
Cheers,
WB
..............


"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.

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

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.

Sounds as if they used a bunch of recycled SWTPC Tigersaurus power
amplifier PCBs to make those vectorscopes (or, possibly, a couple of
recycled SWTPC designers).

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:

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.

Sounds as if they used a bunch of recycled SWTPC Tigersaurus power
amplifier PCBs to make those vectorscopes (or, possibly, a couple of
recycled SWTPC designers).


Bull****. Those vectorscopes were used at TV stations, where most
were on 24/7. The TV stations I've seen didn't have enough cooling for
the equipment racks. SWTPC crap would have burnt up in a week, or less.

--
You can't fix stupid. You can't even put a band-aid on it, because it's
Teflon coated.

JW wrote:
Here's one I haven't seen before.

See what comes from living right, kids? 8-)

there were two 22K 2W carbon comp resistors[...]used as bleeders
[...]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.

Count yourself lucky up to that point.

[...]they usually drift high; at least in my experience...

Wirewounds and films age upwards;
carbon comps are a crapshoot.

If you had a dissipation task with outrageous peaks,
carbon comps were a useful option;
they took abuse[1] and mostly just smiled when others would fail.
Otherwise, as this shows, they were not a great choice.
..
..
[1] All that comes close these days is those thick film jobs.