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pF Trimmer Caps.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K2WPL, Jan 10, 2020.

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  1. K2WPL

    K2WPL Ham Member QRZ Page

    Can I use a 6-70pf Ceramic Trimmer Cap. to replace the 3-10pf Ceramic Trimmer My MFJ-949D? My thinking, (which is probably wrong, lol), is that I can....can anyone clarify Please???
  2. W7UUU

    W7UUU Super Moderator Lifetime Member 133 Administrator Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    It should work - "sort of" - but the required tuning range will be TIGHT - and if the device ever wants to see the 3, 4, 5 pF you'll be out of luck.

    Why not just source the proper cap? MFJ sells replacement parts and it shouldn't be expensive to get the right one

    KI4ZUQ, NH7RO and KP4SX like this.
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    If you put a 12pF fixed cap in series with the 6-70pF variable, the net capacitance will be adjustable from 4 to 10.26pF.

    If 4pf is too big at the min cap end, then reduce the 12pF cap to something smaller, like 8.2 or 10pF.
    Last edited: Jan 10, 2020
    VE3CGA and NH7RO like this.
  4. N7EKU

    N7EKU Ham Member QRZ Page


    It's way out of range. Do you have some small NP0 caps? Put one around 10-12 pF in series with it and then the range will be appropriate. You could just cut a trace and solder it across the trace on the underside.


  5. AB2YC

    AB2YC Premium Subscriber QRZ Page

    There are some 3-10 pf trimmer caps listed on ebay as well if you can't get it from MFJ
    NH7RO likes this.
  6. KI4ZUQ

    KI4ZUQ Ham Member QRZ Page

    So what happened to the original? I just finished the 941EK and know what the cap is and its use! Probably the best part in the whole kit! Wha' hoppen?
  7. K2WPL

    K2WPL Ham Member QRZ Page

    Forgot to tune on low power and put 100 Watts into it for an extended period of time. when I noticed that there was some funky power ratings as compared to my dummy load I opened it up and noticed a black chard marking on the Cap. when I touched it with my O-Scope probes it fell apart....Literally....
    So what is its purpose? I am still using the tuner just with an external SWR/WATT Meter.
  8. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Its part of the coupling circuitry to the metering circuit.
    KI4ZUQ likes this.
  9. K2WPL

    K2WPL Ham Member QRZ Page

    Oh then I'm okay using the tuner with an External SWR/WATT meter then...Little more work to tune but If I can use this way until I can afford to get it fixed, I'm good with that.
    KI4ZUQ likes this.
  10. N7EKU

    N7EKU Ham Member QRZ Page


    If you look at the schematic for the metering part, you can see that the trimmer is part of a capacitive voltage divider at the input. The input voltage is opposite to the voltage generated by the reflective voltage side of the current sensing toroid transformer. There will be a certain voltage generated by the transformer circuitry when a 50 ohm load is connected. The trimmer at the input make the capacitive voltage divider adustable so that the voltage can be made equal so the two cancel each other out and you get zero voltage input to the meter (and so a zero SWR reading).

    Google "Bruene bridge theory" for more information.

    You don't have to replace the trimmer with another one. If you have an assortment of small value capacitors you can try a few variations and see if you can get the reading down close enough to zero for you.



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