EDC 521 DC calibrator blowing fuses
 

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.

EDC 521 DC calibrator blowing fuses
 

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

EDC 521 DC calibrator blowing fuses
 

"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

EDC 521 DC calibrator blowing fuses
 

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.

EDC 521 DC calibrator blowing fuses
 

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

EDC 521 DC calibrator blowing fuses
 

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!

EDC 521 DC calibrator blowing fuses
 

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.

EDC 521 DC calibrator blowing fuses
 

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.