[Boatanchors] Fun with old electrolytic caps

[Charles]

O'Toole's Commentary On Murphy's Law: Murphy was an optimist.

Luckily the choke didn't fail - not because I couldn't have found 
something similar, but because it's mounted in a *really* inaccessible 
part of the chassis. The capacitor bank comes out as a unit by removing 
six 6-32 screws on the bottom plate!

On 3/20/22 18:56, Nick England wrote:
> Murphy’s Law #37(B)
> The unique hard-to-find choke will fail in order to protect the 
> garden-variety cap.
>
> On Sun, Mar 20, 2022 at 7:46 PM Charles via Boatanchors 
> <boatanchors at mailman.qth.net> wrote:
>
>     An unexpected adventure this afternoon. I realized it had been a few
>     years since I'd powered up my PP-7482/G (1 KVA 115v 400 Hz power
>     supply)
>     that I repaired 11 years ago, so I turned on the power strip and
>     flipped
>     its front panel breaker. A couple of seconds of 400 Hz transformer
>     "singing" and the green NORMAL output light on, then the power
>     strip's
>     15 amp breaker popped! Reset it, immediate trip. Thought it might
>     be the
>     cheap pushbutton breaker, so I plugged it directly into the 20 amp
>     wall
>     outlet. The front panel breaker didn't pop, but something started
>     to let
>     smelly smoke out so I quickly shut it off.
>
>     It didn't take long to determine that the DC supply was a dead short.
>     That consists of the non-isolated AC line entering through various LC
>     filters (not Rifa caps), four hefty stud-mount rectifiers that
>     share the
>     H-bridge heat sink, and six paralleled 1100 uf, 200V Sprague 32D
>     caps,
>     which are short & fat compared to the more common 36D computer-grade
>     caps. I quickly isolated the problem to the capacitor bank itself.
>     After
>     removing four caps and testing them for shorts, opens and leakage,
>     finally the fifth was a hard short. The bottom /might/ be slightly
>     rounded compared to the others, but the rubber plug vent had not
>     ruptured. My nose tells me the smell was actually coming from one
>     of the
>     potted input chokes at the rear entrance. Everything else checked out
>     since I hadn't kept trying it long enough to burn up the chokes or
>     rectifiers. The caps were made in 1979 but all except the failed
>     one are
>     working perfectly. Looks like they either didn't need reforming, or
>     survived the sudden application of 160 volts after years of lying
>     dormant.
>
>     Just for fun, I decided to see if I could blow the shorted cap clear
>     with my 28 volt BFPS (on a variac, not wide-open!). I applied DC
>     current
>     up to 90 amps and it was still a very very low resistance short. I
>     quit
>     before it could rupture ;) It just seems like an odd failure
>     mechanism,
>     more like a transistor "punch-through" than an electrolytic that more
>     typically draws excessive leakage current, gets hot, blows the
>     vent, etc.
>
>     Anyhow, the supply now runs fine with five instead of six caps (given
>     electrolytic tolerances of the day, it's probably still in spec) so I
>     put it back together and ran it for a while with a couple of light
>     bulbs
>     for a load. No further magic smoke escaping. I can get a
>     replacement cap
>     for $9 plus postage at Surplus Sales if I feel the need.
>
>     -Charles
>
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> -- 
> Nick England K4NYW
> www.navy-radio.com <http://www.navy-radio.com>