Issues with MFJ 994BRT tuner - I'm stumped!

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AA4TI, Aug 9, 2020.

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  1. AA4TI

    AA4TI Ham Member QRZ Page

    I have a 20 foot flagpole antenna fed with about 100 feet of RG8 coax. There are plenty of ground radials as well as grounding rods. Grounds are all bonded with house electrical system.

    I was using an LDG 600 RT remote tuner and it worked very well to tune the antenna, however, I had a number of heat related failures so I decided to try out the MFJ-994BRT tuner. The tuner actually tunes the antenna quite nicely, however, at higher powers (over 75 watts) I am getting a high SWR spike momentarily when keying the transmitter. It is intermittent, but causes my ALS600S amp to fault. Even without the amp, I've noticed that the radio folds back power momentarily when these spikes occur.

    I've tried several things including using a current balun between the tuner and the vertical. I've tried using the "random wire terminal" on the tuner. I've tried all HF bands including 10MHZ where the antenna is most resonant.

    Any ideas or suggestions would be welcome.
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Try setting the "Uh Oh, the SWR is too high, I am going to re-tune" threshold a bit higher.
  3. AA4TI

    AA4TI Ham Member QRZ Page

    Hmmm. Interesting idea. SWR solution has been excellent (typically well under 1.5:1) but your idea definitely has merit. I will try that. Too bad that it is such a pain to do. MFJ makes this change a major undertaking. It is quite difficult to remove the cover and adjust the set points without actually removing the tuner from service. The tuner control buttons are located on a circuit board that is mounted upside down inside the tuner.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Don't I know it. Mine is 55ft in the air....
  5. AA4TI

    AA4TI Ham Member QRZ Page

    Resetting SWR Target didn't work. Also tried a full factory joy. Tried using semi automatic mode... nope. Will call MFJ tomorrow but I don't expect much help.
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Aren't there two separate settable parameters? One is how close it needs to get during tuning. Other is how much the SWR can deviate before it tries to retune?

    Is it possible that something in your antenna system is heating up as you transmit, making the tuner decide to re-tune mid transmission?
  7. AA4TI

    AA4TI Ham Member QRZ Page

    Only one setable parameters which is the target SWR. The deviation is factory set and is not specified. Not likely a heating problem either. Anything over about 40 - 50 watts on any frequency causes the occasional spikes. If I send a series of "V's" I can see the spikes show up randomly. This happens regardless of whether or not the amp is in line. SWR goes infinite for a split second then settles back to near perfect match.

    I did a factory reset on the tuner. No change

    I did a Target SWR change to 2.0:1… No difference

    I rewired the antenna using the tuner/balun combination. No difference

    I fiddled with the SWR set on amp. No difference

    I checked and tightened all grounds. No difference

    I put the tuner in semi-automatic mode so that it will only tune once when power is turned and and RF is keyed. No joy here either.

    I tried with and without a loading coil for 80 meters. 80 meters is a fairly close match at 3.5 mHz without the tuner. Amp still faults when the tuner is in line.

    I tried using the random wire antenna stub on the tuner without any balun. Nope.

    I can tune any frequency just fine but depending on the band I get an occasional SWR spike over 40 watts +/- that will trip the amp fault.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    I do not see any spikes on a SWR meter. I am running the '998RT at about 750W from an AL-80b with a pi-net (no auto shut down). My antenna is well behaved, so the tuner is not being pushed...

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