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MFJ-260B HF Dummy Load – Impedance Nudging

If you happen to own an MFJ-260B dummy load and it’s giving you weird SWR values, take the cover off and roll the power resistor in its mounting clips:

MFJ-260B HF Dummy Load - power resistor

MFJ-260B HF Dummy Load – power resistor

My buddy Aitch discovered that oxide / corrosion / dirt buildup between the resistor and the clips can produce absolutely baffling results, even while passing enough current to warm up the element, far more power than you’d think would burn away any crud.

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  1. #1 by rkward on 2013-07-29 - 08:46

    Just like batteries with poor contact I suppose. What about the use of silicone grease that can be used on lights bulbs for cars?

    • #2 by Ed on 2013-07-29 - 10:50

      I’m leary of putting a tenacious insulator into electrical joints, but in hostile environments it might prevent more corrosion than it impedes good contact.

      Maybe something like DeoxIT would help, although I don’t know how it would interact with the porous resistor terminals.

  2. #3 by Raj on 2013-07-29 - 23:07

    Take care with those resisters. I found in a box of them I have that a lot of them had the end plating starting to peel away. Some of them had drifted away in value.

    • #4 by Ed on 2013-07-30 - 07:33

      the end plating starting to peel away

      Eeeewww!

      They look porous, so I thought they were sintered metal. Maybe they’re an organic substance glued together, sintered to drive out the volatiles, then coated with a stable epoxy? Beats me…