Disc Ceramic Caps failing, Doghouse HV...

[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 imbeded moisture. Fred.

[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 imbeded moisture. Fred.

If the idea is to maintain the original look, I'd gut it and stuff it with a film capacitor.

You could get rid of the moisture this way, but I don't think I'd want to trust it. I do have the facilities to try this with little effort, so I will out of curiousity to see what the results are. I won't reuse the cap though.

John

[wa2ise]

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.

Problem is that the moisture has reacted with the acid in the paper, and that has reacted with the metal foil inside the cap, and that causes the leakage. Basically, it's too late. Just replace the wax cap, and throw the wax cap out.

[kvflyer]

Originally Posted by wa2ise

Problem is that the moisture has reacted with the acid in the paper, and that has reacted with the metal foil inside the cap, and that causes the leakage. Basically, it's too late. Just replace the wax cap, and throw the wax cap out.

Agree. Only exception would be that you could not throw the cap out and re-stuff it if you really want to get anal. But if you do, please put a note under the chassis to tell the next repairman that the caps are restored too!

[Phil Nelson]

Re electrolytics, I have had poor luck with reforming electrolytics and don't even bother trying it any more.

I don't like crossing my fingers every time I turn on a set, so I don't leave any decades-old electrolytics in place. If you burn up an "unobtanium" power transformer, as I have, you'll see why I just replace the danged things now.

They can easily be restuffed, and it's cheap insurance.

Phil Nelson

[jeyurkon]

Originally Posted by Phil Nelson

Re electrolytics, I have had poor luck with reforming electrolytics and don't even bother trying it any more.

I don't like crossing my fingers every time I turn on a set, so I don't leave any decades-old electrolytics in place. If you burn up an "unobtanium" power transformer, as I have, you'll see why I just replace the danged things now.

They can easily be restuffed, and it's cheap insurance.

Phil Nelson

I admit the power transformer issue worries me. This model has numerous fuses. There two additional places that I might fuse to protect the transformer.

It's difficult to explain why I chose to go this way. There are people, few, who want everything original and don't care if the TV works. There are those who restuff all the paper and electolytics to keep it looking the same. There are those who feel that repairs are part of the history and that they don't need to be hidden. There is at least one poster who said he replaces all of the resistors in addition to the caps. The extreme would be buying an old set from Harry Poster that has had a modern TV installed into the cabinet.

I'm a combination of some set of the above. I enjoy doing repairs as long as it doesn't involve unobtainium, so having a reason to haul the chassis out again doesn't daunt me. In fact, I look forward to it.

John

[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