New Antenna Tuner Smoked

Discussion in 'Ham Radio Discussions' started by KB3ZGV, Nov 13, 2012.

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  1. KB3ZGV

    KB3ZGV Ham Member QRZ Page

    Just great, brand new MFJ 969 and I'm working off the twin lead on 10 meters. I'm just telling somebody how good this thing tunes up. 5 minutes later I swear I heard a little tick, and my meter falls to zero, then I smell it :mad:

    Now it tunes on the coax but not on the balanced line. :(

    I bought it off of HRO in Delaware. Who do you think I'm better off dealing with, HRO or MFJ?
  2. WH2HAO

    WH2HAO QRZ Moderator Platinum Subscriber QRZ Page

    Sorry to hear that. Were you using an amp ? The problem is a replacement MFJ will keep you nervous that it will happen again. See if you can get a different brand from HRO. Dealing with MFJ will probably be filled with frustration, I could be wrong.
  3. KC8VWM

    KC8VWM Premium Subscriber QRZ Page

    Get a Palstar.
  4. K5FH

    K5FH Premium Subscriber QRZ Page

    Looking at the schematic, I suspect you overstressed the balun. Tuners, for some reason, are still rated under the old system. That is, a 300W tuner means it's rated for a transmitter that runs 300W input to the final, meaning that its output is about 150W (assuming 50% efficiency in the final amplifier). So, you shouldn't exceed 150W going into the tuner.

    Another problem is the balun at the tuner's output. Baluns don't handle high reactances very well. What I have seen of MFJ products is that the baluns used in their tuners are pretty light, and should be used with near-resonant antennas (that is, relatively low reactance). You didn't specify what kind of antenna and feedline you were using but I suspect it was highly reactive at the frequency you were using.

    Probably all you need to do is replace the balun. I would recommend using a heftier balun, that is, something wound with larger wire on a larger-diameter core (i.e., a T200-2 core wound with #12 solid). Overkill, yes, but it'll save you from burning it up again.

    Call MFJ. They have a really good warranty policy.
  5. KB3ZGV

    KB3ZGV Ham Member QRZ Page

    Running a 10 meter dipole fed with 300 ohm twin. and only pushing 100 watts.

    I took the cover off and the Balun looks ok. although it does look a bit on the light side. Nothing obviously burned, but there is a slight "stink" . No flash over marks that I could see.

    BTW, I just tested it single wire, and it doesn't tune. I guess that eliminates the balun
  6. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Just saw your new posting that states you cannot tune a single wire after having written a great dissertation on what to do with a cracked balun. As for tuning the wire that would depend on the length of the wire. Do you have a receive signal through the tuner?
    Switching to the dummy load and using the tuner into it can you get the tuner to see and tune the dummy load?
    Check all solder joints. There may be some that are cold or not done at all. MFJ is known for that. You can fiddle around all you want and it won't affect your warranty one bit. MFJ will still honor it.
    Hope this helps
    Last edited: Nov 13, 2012
  7. K5FH

    K5FH Premium Subscriber QRZ Page

    I still think it's the balun. The only way to know for sure is to remove it and test it with an ohmmeter. I'm pretty sure it's a lighweight voltage balun, which isn't the best choice for the application.

    Did you remove the external shorting strap between the balun and the single-wire port? If you leave it in and try to load up a random wire, it's essentially a dead short to ground.

    If it still won't tune after removing the strap, then something is wrong with the multi-position antenna switch - probably a burned or shorted contact.
  8. KB3ZGV

    KB3ZGV Ham Member QRZ Page

    Not much to this thing, but looking at the schematic, and being that the coax works, it's got to be the switch.
  9. KB3ZGV

    KB3ZGV Ham Member QRZ Page

    you beat me to it :)
  10. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Can you tune into the internal dummy load?
    That's a slightly different switch setting as are the coax SO-239 connectors. If you use the center connection it's just like the single wire post except for the switch position.
    The single wire post uses the same switch contact as if you were going to the balun.
    Gheez...I gotta learn how to read. Yup it's the switch. Just contact MFJ and tell them while running 100 watts to the tuner the switch popped. They usually will just send you one and if you're in the mood to put it in then your good to go. If not then you'll need to send it in.
    Hope this helps
    Last edited: Nov 13, 2012
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