Amp Supply LK500-ZB

Discussion in 'Amateur Radio Amplifiers' started by AB9VN, Mar 4, 2011.

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  1. AB9VN

    AB9VN Ham Member QRZ Page

    Looking for advice from anyone who may have encountered a similar problem. I lost 40 meters, but all other bands work fine. I suspect the reason I lost 40 meters, my own stupidity, was I tuned the amp with a 150 watt carrier. I haven't had much time to poke around and troubleshoot, I suspected initially that the relay on the tuned input board failed. I swapped relays from 80 meters to 40 meters, and there was no change. RF output is 0 watts unless in bypass, where it is 100 watts from the transceiver.
    I looked the amp over visually, and nothing is amiss. No melted wires or solder joints, no burnt or exploded components. No smoke was let out when the failure occurred.
    I'm somewhat limited as far as test equipment, I have on hand a fluke 73, a capacitor tester (I suspect I will be using), and a soldering iron or two.
    One more thing, I am aware of the lethal voltages present, thanks in advance for the concern there.

  2. W9GB

    W9GB Ham Member QRZ Page

    Rob -

    GET the Manual and Schematic to troubleshoot this HF amplifier.

    1. Visually check (with good lighting) that band switch's contacts for the 40 meter position.
    2. IF the relays work properly -- then check the components (schematic) that energize the relay's coil as well as are "in line" with switched contacts.

  3. KM1H

    KM1H Ham Member QRZ Page

    Since that amp uses a heavy duty bandswitch I doubt if that is the problem, that amp and the tubes should easily survive 150W input.

    Id unbolt the input board again and disable the AC HV from the PS board if you are not well experienced working around HV.

    Then switch thru the bands while checking for path continuity. Also check for a fried mica cap or a bad solder joint that is now open. Thats a very easy amp to work on, Ive had a ZC model since 1986.

  4. AB9VN

    AB9VN Ham Member QRZ Page

    Bandswitch looks fine. Its only 40 meters that's affected, I used it on 20 meters over the weekend during the DX contest without issue. Didn't see any open solder joints, and I looked it over pretty good.

    I'm in the middle of a basement remodel, so my bench is in the garage for a few days or weeks. Once I get things back together downstairs I will get the amp on the bench and start pulling and testing. I'm really leaning towards those caps on the input board.

    Thanks for the suggestions guys.
  5. AB9VN

    AB9VN Ham Member QRZ Page

    As expected, it was a bad trimmer cap. Replaced it and tuned for 1:1 SWR on the input. Everything works great again.

    But this leads me into another question. This amp does not have the 10 meter input installed. Since I'm running short on things to fix or modify, I think its time to install it. The mod is very simple, and its well documented in the manual, the only unknown is what toroid and how many turns. From what I gather from various online sources, a T50-6 with 8 turns of #20 should satisfy the requirements. Does this sound about right?
    Last edited: Apr 10, 2011
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