[sdiy] Cap shorted in only one direction

[Michael Bacich]

Hello DIY folk,

I have come across a fairly large electrolytic cap (1000uF -- 50  
volts) in an ARP 2600 power supply that has failed.  When I test it  
with my ohmmeter, it appears to be internally shorted -- but only  
when I test it with my meter's plus lead on the cap's minus  
terminal.  If I reverse the test leads, I get a reading that  
indicates a certain amount of resistance (not as much resistance as  
with a healthy cap, but a lot more than when the leads are the other  
way, which is a dead short, reading Zero Ohms).

Is this kind of reading to be expected?  It seems to me that every  
other blown/shorted cap I have encountered has always measured as  
shorted no matter how I attached the test leads (particularly on the  
numerous blown Tantalum caps I have come across).  What's going on  
with this one?  Is this a standard failure mode that I've just missed  
until now?  I'll admit that I don't know much about what's actually  
inside these caps, or even much about how they physically work -- I  
just solder 'em in and go!  BTW, this is a standard polarized  
electrolytic cap, not one of those non-polarized or bi-polarized types.

Michael B.