I went to my local Otter Co-Op farm-store, and bought a brand-new paint can, to re-home the guts of my MFJ-250. The paint-can is internally epoxy-coated. I made a taller vent-tube, by drilling the lid and soldering some copper-coated brake-line. A loose-fitting stainless sheet-metal screw drops into the tubing, allowing for thermal expansion of the oil and air-movement in and out.
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I wanted to address the pervasive oil-leakage out of the original. The epoxy internal coating was the first step; soldering the vent-tube was the second step (MFJ originally used a short, stamped brass vent with plastic cap). The top mounting-surface of the original resistor-bracket received an alcohol-cleaning and thin coating of Permatex Grey RTV, intended to dissuade leakage up and around the SO-239 connector. The underside of the four washers also received a thin RTV coating. Lastly, I cut and fitted 3mm (1/8-inch) silicone foam rope into the top groove of the paint can. When the lid is installed, this rope gasket is somewhat compressed, in an effort to prevent oil-creep here too.
Amazon provided the 8-inch silicone cake-pan, as a final guard against oil leakage. And red is cool :-)
Otter Co-Op also provided the mineral-oil fill for this dummy load, at a reasonable price. Intended as a farm-animal laxative :-)
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I found a few aspects of the MFJ-250 specifications that were "aspirational". Comparing the power-ratings of the Heath HN-31 and HN-31a to those on the MFJ Versaload, I decided to label mine as 200 Watts "Continuous Commercial Service". Good for exciters. And although I had applied 1kW previously to my MFJ, I felt better calling it 800W for 3min after considering all three loads' curves.
(Speaking of "aspirational", we can't forget Ameritron's ADL-1500! Claimed 1500W for 5min, but only using their transformer-oil! You can't get either the ADL-1500 nor their oil ... both now Unobtanium. Their notes warn that exceeding 30s may result in oil expulsion :-O Doubles as a science-fair volcano, I guess :-O )
The DC resistance of my MFJ resistor: 55 ohms. This ought to provide about 1.10 VSWR, despite being at the outer fringe of tolerance. Perfectly serviceable.
Using my nanoVNA, I also found the frequency / bandwidth spec's to be "aspirational" again. This shows clearly when comparing the Heath frequency range (to 2-metres/144MHz) with that of the MFJ (to 400MHz).
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Summary: The MFJ-250 makes a great 3.5MHz - 30MHz dummy load, for the usual 100W ham transceiver.
NOTE: There are no longer any Amateur oil-filled dummy loads on the market, following the 2025 demise of MFJ. However, if you own one of these, it will serve for many decades so long as it's limitations are observed. Or, there are the 4-figure BirdRF Termaline products :-)
NOTE2: The Heathkit Cantenna (both HN-31 and HN-31a) had a cylindrical sleeve ("shield tube") surrounding the resistor. I have to wonder if this is better than my MFJ's three-sided square trough?? I've seen pictures of later models (esp. Ameritron ADL-1500) that added a fourth side to the square trough, and also featured some holes around the top to allow better oil convective-circulation. It would seem my MFJ-250 is among the earliest iterations, with the fewest "improvements". Very Special
I filled it, per the Bird manual, to within 3 inches of the vent seat. So there is a couple of inches of air atop the oil. Roughly, my 8890 took 6 litres of Mineral Oil. I really, really doubt I'll ever heat the oil significantly, and I don't worry about oil-expansion expulsion.
I emailed Bird, and this dummy load predated my correspondent's knowledge, and his ability to dig up any information. 50 ohms, 2500W CCS was all that was confirmed, so I dug out my nanoVNA and sorted out the usable frequency range ... draw your own conclusions, based on your own needs and circumstances:
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And, just for fun, let's sweep right up to 2.4GHz (where many modern Bird 8890's are spec'd):
