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MFJ 260CN dummy load needs RF to work it?

Discussion in 'General Technical Questions and Answers' started by 2E0LPL, Oct 14, 2019.

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  1. GM3ZMA

    GM3ZMA Ham Member QRZ Page

    I do not know how the MFJ dummy load is constructed, but a couple of years ago I had some very strange results with a Narda 50 Watt attenuator.
    It would read high resistance at DC from a DMM, but would be good when low power RF was put in to it.

    The fix was to unsolder the body of the attenuator and resolder the attenuator element to the body, It was make poor contact.


    Look here to read the whole story, with pictures.
    https://forums.qrz.com/index.php?threads/how-to-disassemble-narda-attenuator.542731/#post-4019931

    Jim
    GM3ZMA
     
  2. AG6QR

    AG6QR Premium Subscriber QRZ Page

    Some meters use low voltage, well under 0.5V, for checking resistance. The idea is that such a low voltage won't cause silicon semiconductor junctions to conduct, so many resistors may be checked in-circuit.

    If there's a diode or two in series with your dummy load, it could register as open circuit on such a meter. If there is a cracked solder joint, a similar effect might be observed, even with a meter that uses a few volts for testing.
     
  3. KP4SX

    KP4SX XML Subscriber QRZ Page

    Surprise, surprise. :rolleyes:
     
  4. KA2IRQ

    KA2IRQ Ham Member QRZ Page

    Well... we had to ask those questions, so I hope you didn't take it personally.

    Honestly, I would pop the dummy load open to make sure you don't have a bad solder joint or other poor connection.

    53-ohms is actually 6% off from the nominal 50-ohms, so it is possible that a poor connection would account for an additional ohm or two, and also explain your symptoms. Like I said, worth a look under the hood. 6% is well within a normal 10% tolerance, but often they're still much closer.
     
  5. WA9SVD

    WA9SVD Ham Member QRZ Page

     
  6. KA5IPF

    KA5IPF Ham Member QRZ Page

    You might check the solder job on the back of the connector, maybe have solder maybe not.
     
  7. 2E0LPL

    2E0LPL Ham Member QRZ Page

    Had the lid of the dummy load...the only solder joint looks fine...been ok since....just one of these things.
     
  8. KP4SX

    KP4SX XML Subscriber QRZ Page

    Maybe time for some new DVM leads?
     

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