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Paper Capacitors

What happens to waxy paper capacitors over time. I have heard that these capacitors were sealed in wax to keep out moisture and prevent the capacitor from deteriorating that way. I have also heard that the paper wasn’t acid free. So the acid in the paper would create holes in it allowing the foil plates to touch. As these holes become larger the resistance of the capacitor goes down until finally it’s just like a wire jumper.

There was another type of paper capacitor called a “ black beauty” or “Bumble Bee” where the paper capacitor has a plastic case over it that’s filled with oil for the same reason, presumably, as the wax on the other type. Both the waxy type of paper capacitor and the “bumble bee” fail the same way.

In some of the very old radios from the 1930s these paper capacitors were potted in wax. The main filter capacitors were very large and were also of the paper type. These were usually located in a very large box mounted on top of the chassis, and captive wires ran from the box to where they connected in the circuitry. The bypass “bypass” capacitor cans were smaller and usually connected to the inside of one of the sides of the chassis. A short time ago I received an chassis where some of the filter capacitors were faulty and overheated. This made a huge pile of wax on the inside of the bottom chassis cover. If these hadn’t been replaced, this could have damaged the power transformer, output tubes, or rectifier tube. New film capacitors are much smaller and easily fit under the chassis.

The film capacitors I use as replacements don’t suffer from these problems because there’s no paper in them at all. But the construction seems to be similar internally. In the waxy paper type, from what I remember, two strips of foil (aluminum?) sandwich a strip of paper, where one foil strip sticks out of one side of the strip of paper and the other foil strip sticks out of the other side of the paper strip. This is rolled up, pushed into a tube and each side of the foil is flattened so that a loop of wire can be pushed against it. These form the capacitor leads (wires). The wires are held in place by something that looks like thick wax to me. So some paper capacitors fail because the lead wire isn’t touching the plate anymore.

A film capacitor has the metal plated on both sides of the film (a plastic material. If I’m incorrect, please send me an eMail message or private Facebook message about it. That will show someone is reading this.


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