quick esr meter
 

doing some trouble shooting on an old astro sonic I found an open 5uf cap on the preamp board. thinking wow, it would be nice to have an esr meter, I decided to try something. I hooked up my hickok osc set to 100khz and a DMM set to ac uA in series, and sure enough the bad cap would flow only 3uA while the good one (on the other channel that was working) was more like 80uA a dead short was 100uA. Any way it was a fun test, I check the rest of the cap and the seemed ok. Of course this does not check for leakage at rated voltage but for modern stuff where the issue seems to be high ESR maybe this would work. Of course a real ESR meter would be the way to go but I am so cheap it may be a while.

I am pretty sure this would only work on a cap that was nearly open.