Refurbishing the MFJ-264 Dummy Load
At a recent fleamarket I purchased an MFJ-264 dummy load for what I thought was a good price. Caveat Emptor! It measured near 50 ohms alright but when I got home I opened it up and I found a big surprise. The huge resistor inside was bridged by a well-cooked 270 ohm 2 watt resistor! When I removed this resistor, the original dummy load measured 67 ohms! I don't know if it was 67 ohms from the factory or because of abuse, but I did know that I didn't like it at all. When you think about it, 1500 watts with no fan or cooling oil for more than a few seconds would certainly leave a dummy load resistor "well done".
I wondered if a 50 ohm, 100 (or so) watt non-inductive resistor was available and how much it cost. A search on DigiKey did not turn up anything like the original, but I did find a series of high-power non-inductive resistors such as the Caddock Electronics MP9000 series power film resistors. I chose to use two MP9100-25.0-1% units in series. These are 25 ohm, 100 watt, 1% non-inductive resistors in TO-247 plastic packages. They have a ceramic block exposed on the back for better contact to a heat sink. Here is the result:
I mounted both resistors on heatsinks which I conveniently found in my junkbox. In testing it works well into the 6 meter band but not as well at 2 meters.(Perhaps a single 50 ohm unit would be better for 2 meters, or a better parts layout. For more ideas about this see PA0FRI's page.) What's really nice is that the resistance is almost exactly 50 ohms.
My station runs 100 watts maximum, and a minute or two of tune up does not produce excess heat. I would be really cautious using this kind of unit or even the original at higher power for more than a few seconds (if at all).
Another thing, the SO-239 had only 2 screws and they didn't even use lock washers. I would strongly advise anyone with a unit like this to replace the screws with longer ones and use lockwashers. My modified unit does not use the case as a conductor anymore.
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