Disc Ceramic Caps failing, Doghouse HV...

[jeyurkon]

Originally Posted by disco

I wonder if it is possible to place a leaky (not shorted) paper wax capacitor in a sealed container of molten wax and apply a high vacuum for a length of time to extract the embedded moisture. Fred.

I took a 0.1uF capacitor that had a leakage of 300uA and tested at 0.14UF . The end was pretty much open so I didn't put it in molten wax in a vacuum. That would have been difficult. I kept it in vacuum at 2x10-5 Torr for four hours. I then dipped it in a Cenco vacuum wax that is very much like the wax used to coat the caps. When I got it back home and tested it, it still was 0.14uF but the leakage was now 500uA.

I then heated it with a heat gun to pull it out of the paper tube. Besides the outer wax coating there was a much harder and higher temperature wax on the ends. I melted one end off and kept it hot for some time to drive the water out. Testing it right away gave 10 mA of leakiage. It was very temperature sensitive, which is what you would expect if there was acid acting as an ionic conductor. Once it reached ambient temperature again it read 800uA. Seems like anything you do only makes it worse.

wa2ise is correct about acid products. It is mostly from the paper. Heating the cap only accelerates the damage it does and causes the paper to break down more releasing more acid. The acid doesn't do the metal foil much good either.

I've only tried one. I have no reason to suspect any others would behave differently though.

Hopefully this will discourage the guitar amp enthusiasts who like the sound of caps like bumble bees from trying to restore old paper caps.

John