[Boatanchors] Fun with old electrolytic caps

[Charles]

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