EDC 521 DC calibrator blowing fuses

[JW]

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.

 

[N_Cook]

JW <none@dev.null> wrote in message
news:35upl6lr8suad9btcu6jskug49oel94vod@4ax.com...

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

 

[Jim Yanik]

"N_Cook" <diverse@tcp.co.uk> wrote in
news:ijj5kg$3bn$1@news.eternal-september.org:

JW <none@dev.null> wrote in message
news:35upl6lr8suad9btcu6jskug49oel94vod@4ax.com...
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

 

[Wild_Bill]

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" <none@dev.null> wrote in message
news:35upl6lr8suad9btcu6jskug49oel94vod@4ax.com...

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.

 

[Jamie]

Jim Yanik wrote:

"N_Cook" <diverse@tcp.co.uk> wrote in
news:ijj5kg$3bn$1@news.eternal-september.org:


JW <none@dev.null> wrote in message
news:35upl6lr8suad9btcu6jskug49oel94vod@4ax.com...

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

 

[Dave Platt]

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 <dplatt@radagast.org> 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!

 

[Michael A. Terrell]

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).

Bullshit. 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.

 

[JeffM]

JW wrote:

Here's one I haven't seen before.

See what comes from living right, kids?

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.