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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.

Reply to
JW
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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

Reply to
N_Cook

"N_Cook" wrote in news:ijj5kg$3bn$ snipped-for-privacy@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
Reply to
Jim Yanik

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 .............

Reply to
Wild_Bill

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

Jamie

Reply to
Jamie

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
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Reply to
Dave Platt

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.
Reply to
Michael A. Terrell

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

Count yourself lucky up to that point.

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.

Reply to
JeffM

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