Be Careful With Old Dummy Loads

A ham friend recently sent me the following e-mail:

> I have an old MFJ oil-filled dummy load. I wondered how to dispose of the
> transformer oil, which MFJ says does NOT contain any PCBs. So I did some
> research and found the following info regarding PCBs on military bases in
> Japan.
> “…under the current rules for Japan, military officials say, anything
> tested for PCBs is assumed to be contaminated, even if the test equipment
> cannot detect any PCBs.”
> So I guess you’re better off just to say, “Nope, it doesn’t contain any
> PCBs, so we won’t bother to test it.”
> MFJ told me the oil is not hazardous and can be disposed as “trash.” I
> think I’ll check with my recycle center just the same.

I replied:

Back in the 70s, I bought a Heathkit Cantenna dummy load kit. You may remember these things. Basically, they were just a paint can into which you dunked a non-inductive resistor. What you were supposed to fill the can with was “transformer oil.”

My father somehow procured a two- or three-gallon can of the stuff, which I’m now guessing had PCBs in it. Of course, we didn’t think about these things then, and the Cantenna was far from hermetically sealed. In fact, it had a pressure relief valve on the top that leaked, so there was always a film of transformer oil on top of the Cantenna.

Also, since the Cantenna only required one gallon of transformer oil, the rest of the oil sat in my father’s garage for who knows how many years. I have no idea of how he disposed of it, either. In retrospect, I still feel a little guilty about this.

About 20 years ago–maybe more, actually–I purchased a Drake 300W dummy load. The resistive element looks remarkably similar to the one in the Cantenna. The resistor in the Drake dummy load is inside a perforated metal enclosure and is air-cooled. This leads me to believe that I probably could have done without the transformer oil altogether back in the 70s. I never used a linear amplifier, and therefore probably never got up to the power levels where the cooling oil was needed.


  1. Transformer oil——-Hoo-Yah! My 85 year old neighbor used to work for the Power Company. I asked him who made the composition shingles on his 1946-vintage house, they looked so good. He replied proudly that they were the original wood shingles – he kept them looking good over the years w/”transformer oil” in a pump sprayer. I probably live next to a SuperFund site but that roof looks great.

    PS: I wonder if motor oil for high temp applications would work.

    Cheers and Good DX!
    Jim Falls
    Eureka, CA

  2. Bob Hand, W8WQS says:

    I need to replacement oil for my cantenna. I recall seeing an article comparing the uses of different oild but can’t recall where. Do you know where I can find some information. I was told that 10 weight non detergent motor oil will work fine up to abour 50-0 watts. Any comments? Bob

  3. Bob Hand, W8WQS says:

    In my previous comment I meant 500 watts not 50-0 watts. Bob

  4. John 2E0CDQ says:

    Hey Bob you can use mineral oil, the type used for massage just get the unscented one. Its perfect or buy a tin of transformer oil from the recycler.

Speak Your Mind