[Amps] How to know when filter caps begin to fail

[Mike Andrews]

On Wed, Mar 28, 2012 at 05:37:58PM -0700, Jim Barber wrote:
> 
> Many moons ago my boss at the time was probing about in a live chassis 
> when an aluminum cap blew. His glasses kept the stuff out of his eyes, 
> but when he lifted his head I saw he had a chunk of the aluminum can 
> sticking out of his forehead. When I told him, he calmly reached up and 
> pulled it loose, then put me to work cleaning up all the paper and gunk 
> that had showered the shop.
> 
> Since then, I've been a bit shy about working directly above a string of 
> electrolytics...

Back in 1969, I had a cap skyrocket on me while I had the power supply
drawer open for tweaking; this was on some military gear. It was attached
by a big nut below the chassis that screwed onto a big threaded plastic
protrusion with the wires sticking out of it. The wires and plastic stayed
in place, but the can went through the acoustic tiles overhead and into the
air space above the tiles. It cleared my ear by about 1/4", and a damn good
thing it did.

I heard a *POW*, then a _thwap_, and one of the HV meters changed its
reading. It took me about 15 seconds to figure out what had happened.

A check showed that *every* *one* of the power supplies (about 100) had
bulging caps. We reported the incident to next-higher, with notes about
all the other bulgy caps, and next thing we knew, there was a DOD-wide
mandatory change of all the electrolytics in every PS of that type.

Funny thing was that I didn't really have to clean the PS out at all, just
flip the power switch, hotstick the remaining filters, replace the failed
cap, finish the PM, and put it back on-line.

And I've been damn shy of working directly over filter cans ever since,
too.

-- 
Mike Andrews, W5EGO
mikea at mikea.ath.cx
Tired old sysadmin