Buy the print and get FREE access to our online edition!

Tech Forum

August 2017

Oil Filled Caps

I’m trying to bring an old tube transmitter back to life. The power supply uses oil filled capacitors, which don't seem to have handy replacements. Assuming that I can change the oil to reduce the leakage current, what kind of oil is used?

Will motor oil work, for example?

William Helm
Eugene, OR


I would not try it. Many old oil filled capacitors used oil that contained PCB (polychlorinated Biphenyl). This is classified as Hazardous Waste by the EPA.

I have replaced electrolytic capacitors in several old radios in the past few years. The problem with compatibility is that new capacitors are a lot smaller than the old ones were. That's a nice problem to have but mounting them can be a bit difficult if you are wanting to use the same holes. Find modern dry replacements at places such as Digi-Key or Mouser Electronics with the same capacity in pico Farads or micro Farads and the same or higher voltage rating and you should be OK.

Don Pitchford
Springfield, IL

I don’t know why you are concerned with leakage in a filter capacitor. If the capacitor is doing its job of smoothing out the DC and is not overheating, it should be fine.  If the cap is buzzing or sizziling, throw it away. Good luck on your project, and be careful; voltages in a transmitter can be quite high.

Russ Kincaid
Milford, NH

OK, I'll bite. No, you can't take your caps to a garage for an oil change. grin

There are a number of vendors (for instance Antique Electronic Supply) who sell modern replacement parts. Most capacitors (other than Mica and Ceramics) are subject to suspicion, and if you have no way to test them, replacement would be prudent. Resistors can also drift in value, especially those that get hot. Restoring tube equipment is a lot of work, but very satisfying.

Bill van Dijk
Ottawa, Canada

SAFETY WARNING: If the cap doesn't clearly say "NO PCBs" LEAVE IT ALONE. It may contain oil contaminated with dioxin, a potent poison. With that said, what kind of transmitter are you working on anyway?? I have never heard of changeable oil in a cap smaller than used by an electric utility.

If you really want to make the change, you want non-PCB transformer oil. The second-best alternative is Johnson's Baby Oil.

Chip Veres
Miami, FL

Changing the oil is hazardous (most oil capacitors used toxic polychlorinated biphenyls) and useless, as leakage is most likely due to damage to the paper separator immersed in the oil. Better just look for an equivalent value capacitor.

If it's a high-voltage power supply cap, you can use modern electrolytic capacitors. If you need to put two or more (identical!) capacitors in series for higher voltage rating, put large equalizing resistors across each (e.g. 500 kilohm or 1 megohm ) and remember that capacitance decreases according to the number in series (e.g. two 20 microfarad caps in series are 10 microfarads).

via Internet

I’ve never heard of refilling that size capacitor, but I do have some experience with much larger ones.

We had a number of very large capacitors that our lab received from a national laboratory as part of a research project. They originally had transformer oil as a dielectric but the outer cases were cracked and it had leaked out. Because of the problems at that time with transformer oil containing high levels of PCBs, we used castor oil as a substitute with good results. I have no idea if motor oil would work but I wouldn’t recommend it.

Jeffrey Tuttle

Please do not attempt this. Many oil filled capacitors used oil that contained PCBs — a carcinogen. They should be disposed of as VERY hazardous waste. PCBs were banned sometime in the 70s.

A product that I worked with contained oil filled capacitors and we had to change over to new types. It is almost certainly NOT motor oil. There seem to be a fair number of antique radio restoration enthusiasts, so you might try a web search. Also, just search for the model and manufacturer of your transmitter. Any markings on the capacitor will also be useful.

You might also look up references to the American Radio Relay League (ARRL). They have been the technical center for amateur radio for a very long time. If your transmitter is home made (home brew), it might be one that they described in one of their handbooks or a QST maazine. You can probably find a few ARRL Amateur Radio handbooks (annual publications) at your local library.


Please do not open or handle oil filled capacitors. The oil in most oil filled caps is poisonous. PCB’s are a known carcinogen. Not all capacitors containing PCB’s are marked. Many of the older capacitors from high voltage power supplies are suspect. Our standard operating procedure with oil filled caps that were leaking or bulging was to put in a metal can, then fill the container with cat litter and then handle as if it was Hazmat.

Steven K Ashcraft
Mineral Wells, WV