10 Meter Input Problem with AL-80B

Discussion in 'Amateur Radio Amplifiers' started by K4PVA, Apr 5, 2012.

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  1. K4PVA

    K4PVA Ham Member QRZ Page

    I have an Ameritron AL-80B that I bought second hand. I knew that it had an
    arcing problem when I bought it so I sent it to Ameritron for repair. They
    reparied it and fixed the problem (replaced the loading capacitor) and returned
    it to me.

    I have been adjusting the input coils using an MFJ-259B antenna analyzer and plastic hex tools.
    I was able to get 1.2:1 and most of the time a 1.1:1 swr throughout 160, 80, 40, 20 and 15
    meters with no problem using this technique. Was easy with repeatable readings.
    Amp works great on these bands.

    When I tried 10 meters on the "AUX" position it was a different matter.
    Ameritron assured me that the 10 meter mod had been made. However, tuning the
    slug with the plastic hex tool makes NO difference in the swr. It remains at
    2.6:1 no matter how much I turn the slug in any direction.

    I tuned the MFJ-259B down frequency to see where the 10 meter coil might be
    resonating and the lowest swr was at 21.090 mHz (!). No amount of adjustment
    would change that.

    I think that either the mod was NOT made or I have a bum 10 meter input coil to
    replace. Or maybe a wrong coil was installed in the first place. Any ideas or
    anyone have the same problem? Greg, K4PVA.
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If the "mod" is not made, the input SWR should be nearly infinity because the "stock" amp has the input circuit shorted to ground as shipped when switched to 10/AUX.

    The "mod" is to open that connection.

    It kind of sounds like the input switch is stuck on 15m even when you switch it to 10m. Maybe a loose shaft coupling inside the amp?

    One way to test that would be to leave it set exactly as you have it (switched to 10/AUX) and then turn the 15m input slug and see if that changes anything.
     
  3. K7MH

    K7MH Ham Member QRZ Page

    First thing I would do I'd make sure that the 10 meter mod was actually done with visual confirmation using my own eyes.
     
  4. W1BR

    W1BR Ham Member QRZ Page

    An analyzer is a good way to tune the amp, but are you placing a 110 ohm resistor from the filament to ground and setting the tuning
    with the amp "cold", or are you tuning with the amp running and the antenna energized?

    Pete
     
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