EDC 521 DC calibrator blowing fuses
JW
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
news:35upl6lr8suad9btc...@4ax.com...
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
news:ijj5kg$3bn$1...@news.eternal-september.org:
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
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" <no...@dev.null> wrote in message
news:35upl6lr8suad9btc...@4ax.com...
Jamie
Electronics isn't fun unless you have some flames, forehead contact with
debris, and escaping blue smoke!
Jamie
Dave Platt
> 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 <dpl...@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
>Here's one I haven't seen before.
>
>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.