Hafler DH-500 PS electrolytic ESR

[lhquam]

I am not sure if this is the most appropriate forum for this post, but...

While "tuning up" my 26 year old Hafler DH-500, I discovered that there was .6 volts of power supply ripple, mostly due to high ESR of the ageing 100 volt, 20,000 uF electrolytic capacitors. While probing other parts of the amplifier, I accidently shorted the +90 volt supply to ground, causing one of the electrolytics to discharge quickly (making a very loud pop). Putting a scope probe on the +90 supply showed that portion of the ripple due ESR dropped from about .3 volts to about .05 volts.

Since I was considering replacing both electrolytics anyway, I intentionally shorted the other electrolytic which resulted in the same reduction on ripple due to ESR on the -90 volt supply.

Can anyone explain this effect? Are these capacitors likely to retain this reduced ESR for a useful length of time, or should I replace them?

 

[singa]

Hi, Shorting the cap to ground probably passed a heavy current through
and the arcing may may cleaned up the terminals giving better
conductivity,same with the electrodes of the cap.

Normally you will get about 200mV of ripple but if it is now very
much lower than leave it at that.Money don't grow on trees so if it ain't
broke,don't fix it.Unless you are a perfectionist or have excess cash on hand.

singa

 

[lhquam]

Singa:

You are probably correct. I removed all of the screws and carefully cleaned (by scraping with a knife and using emory paper) the terminals and solder lugs. The ESR was reduced a little bit more. It will be interesting to see how long this improvement lasts.

 

[stratus46]

Singa:

You are probably correct. I removed all of the screws and carefully cleaned (by scraping with a knife and using emory paper) the terminals and solder lugs. The ESR was reduced a little bit more. It will be interesting to see how long this improvement lasts.

You're lucky you didn't blow any internal conductors in the caps. I assume you welded your screwdriver to the terminals. My guess is you didn't improve the capacitors with the arcs but you did improve the screw connections.

G²

 

[eclectic2k]

interesting...

I personally doubt the effect was due to the screw contacts. Maybe in part though. I'd be inclined to guess the effect was internal to the capacitor electrolyte or oxide layer... (something akin to reforming them?)

 

[stratus46]

I wouldn't expect any electrolyte reforming in just a few microseconds but I could be wrong. But, we all know about aluminum terminals oxidizing very quickly and the oxide is a good insulator.

G²