Mfj antenna tuner help

Discussion in 'Survival Communications' started by sdr, Jan 14, 2018.

  1. sdr

    sdr Monkey+

    Going to get my station back up and running. I remember from years ago some trouble I was having with one of my tuners. Decided to open it today. Would like to get some input before I start doing repairs on it. Looks like I probably forgot to change the power setting while tuning up a rig and fried a few components.

    According to the schematic it's only the metering circuit. So worse case is I'll connect a swr/power meter inline and not worry about it. However, I'll attempt the repairs.

    Couple questions I have.

    That is a potentiometer right? Where do I find one?
    Should I replace the diodes? One looks black inside.
    Should I do a continuity test on the inductor? Close inspection shows it doesn't look too bad.

    How do I calibrate the pot after I replace it?

    Resistor is easy enough to find.

    Figure I better get some advice before I start. dbe57c4d-8772-4ea6-b8b2-f69f510c1191. 5d6deb3b-1d9c-4c29-a2c1-401501ffc4db. 4b8ea1ca-4be0-4256-a2a7-f843915da251. 1379f429-054e-450d-976e-c0f8b482626e. af9b0181-0f52-4d9f-8be1-8a53b32a2787.
    Idahoser likes this.
  2. BTPost

    BTPost Stumpy Old Fart Snow Monkey Moderator

    Replace the Diodes, the resister,.. The check the Variable Cap to make sure it is not shorted with a Carbon Track... If you can find one replace the Cap....
  3. sdr

    sdr Monkey+

    I don't see a variable capacitor in the schematic. At least not on the metering circuit. That's why I thought it was a pot.
  4. ghrit

    ghrit Bad company Administrator Founding Member

    It isn't obvious to me, but it looks like those two diodes are the 1N34 on the schematic. If so, that pot may not be a pot, but a simple variable resistor in place of the diagrammed 1/2 watt. Of course, a pot can fill in for a variable resistor, but the only way you are going to be able to tell is look at the underside and see how many leads are connected, and what they are connected to. (It is not uncommon for factories to make substitutions during fabrication for any of several reasons, including a stock shortage.) It should not have been affected by the power selector, it comes in ahead of that circuit. The power selector affects nothing but the meter circuit.

    To get calibration data if that is indeed a variable resistor, a call to the service tech is your only hope. I'm betting it is a substitute for the called out fixed resistor. Also, ringing out components takes a bit of thinking, especially when the circuit are looped as those are. You may need to make some cuts and be prepared to reconnect.

    Now bear in mind that I'm a pumpist not an electronist so question everything I say.
    Last edited: Jan 15, 2018
  5. techsar

    techsar Monkey+++

    Just call FMJ and get the parts you need. I've never had an issue with them. Tech support has been helpful in the past, even for obsolete gear.

    ...and yes, ohm out the inductor...but carbon tracking could be a hidden issue, especially at higher power levels.
    AD1 and sec_monkey like this.
  6. azrancher

    azrancher Monkey +++

    That's a Variable Cap, used many of that type in my early days, it's the XMIT or ANT Match part, just because of it's placement on the board doesn't mean it's part of the metering circuit.

    sec_monkey likes this.
  7. ghrit

    ghrit Bad company Administrator Founding Member

    OK, a bit more looking. Your schematic does not match your hardware. I have a 941E vs. your D, and a variable cap is shown on my schematic, but not in electrical proximity to the diodes. Seems time for a call to a techie.
    Last edited: Jan 15, 2018
  8. sdr

    sdr Monkey+

    Thanks for all the replies. I'll call mfj today
  9. sec_monkey

    sec_monkey SM Security Administrator

    ya can order 1N34 Germanium Diodes on Amazon

    these might work YMMV

    the resistor looks like it might be a 1/2 watt??, ya should be able to order it online

    the variable cap is gonna be potentially difficult to find :cautious: :cautious:

    found the variable caps on a website they aint cheap, they range from about $15 to about $25 each :( :(
    Last edited: Jan 15, 2018
  10. sdr

    sdr Monkey+

    I have Stan the repairman at mfj working on locating the parts.
    techsar, Dunerunner and ghrit like this.
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