EDC 521 DC calibrator blowing fuses

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 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.

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

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

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).

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.

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.