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Thread: Resonate on one band only?

  1. #1
    Master Navigator
    Join Date
    Mar 2009

    Resonate on one band only?

    This is a good starting point, my horizontally (40' above ground) mounted OCF Dipole (balanced feed input) suddenly, only works on one band, 20 meters.

    I use a Dentron Super Tuner Plus with a Yaesu YC-60 watt-meter to read reflected watts between transceiver.

    I go to the settings I have transcribed for each band and, the reflected watts shows that the Tuners is not transforming the load to the radio; the SWR meter on the radio shows a "bad match".

    I hooked my MFJ-259 Analyzer in place of the radio and tried the settings for the different frequencies; tuner doesn't tune balanced load.

    Quote Originally Posted by kb2vxa View Post
    One last thing, I have a bad feeling about this, the last time I heard a tuner "singing" it was internal arcing, the internal balun screaming in pain. This was surprising at first, an MFJ "3KW" tuner arcing with only 100W of AM, modulation struck the arc followed by the hiss of carrier arcing inside the balun. It was more the antenna system than the tuner, a 160M sloping open delta fed with open wire ladder line. It turned out the line was a half wave on the 75M AM Gangsta band sending the high RF voltage at the antenna feed point down to the tuner, baluns like current feed, voltage feed kills them. This is where the old Johnson Matchbox shines over today's tuners, it has a balanced output and one leg can be shorted to ground for unbalanced output, it laughs at high voltage.
    There is no arcing or singing from my tuner, running digital power levels; nothing over 25 watts so, how would you diagnose a bad balun in a 2 Kw antenna tuner?


  2. #2
    'Grumpy old bastid' kb2vxa's Avatar
    Join Date
    Aug 2007
    Lakewood, NJ
    That involves equipment few if any hams have like a HVDC high scale ohmmeter aka an insulation resistance checker or megger. A shorted turn will only turn up in an Ix (excitation current) test, trouble is I have no idea what frequency to use on an RF inductor. That leaves component substitution, the guts to any old current balun will do as long as it fits properly, no crowbars allowed. A crowbar circuit belongs in a power supply, not a tuner. (;->)

    You asked a specific question so I gave you a specific answer, however a previously working balun subject to digital low power, typically QRP level should never break down. I suspect an open circuit somewhere, a cold solder joint opened up, a THOROUGH visual inspection is in order. If that fails to turn up anything a continuity test with an analog ohmmeter and wiggly fingers is in order. Put the meter across every joint and switch contact in turn, wiggle the wires, rockers and knobs, and watch the pointer for any twitching you'll never see with a digital meter, they don't react fast enough and tend to bobble. If push comes to shove re-solder every connection. You can find bad joints in any hippie... uh, even top of the line equipment, once upon a time I found a terminal lug in a Collins transmitter with not a drop of solder on it. That wasn't the problem but I soldered it anyway, (;->) a fried screen resistor and shorted tube in one of the IPA stages killed grid current and subsequently power output.
    "New Jersey, the most American of all states. It has everything from wilderness to the Mafia. All the great things and all the worst, like Route 22."
    Jean Shepherd K2ORS (SK) & WOR radio personality

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  3. #3
    The quote from K2ORS was great.
    73 DE AA1LL, ex W2OPB

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