ZM-2 not working; hints?

Discussion in 'General Technical Questions and Answers' started by KB7PWJ, Jun 12, 2018.

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  1. KB7PWJ

    KB7PWJ Ham Member QRZ Page

    Hi all,

    I recently built a ZM-2 ATU, and am having difficulty getting a match. Perhaps someone can help me figure out why.

    First I connected it between my big rig (QRP'd down to 5W) and a random wire strung in the garage. With that I was able to get full light's-out on 30m, a so-so match on 20m, and no match at all on 40m.

    I replaced the random wire with a 50-ohm dummy load and eventually got good matches on 30m and 20m. (It took some time to find the null on 20m, even with the dummy, so I might have matched the wire on 20m if I'd futzed a little longer.)

    But 40m remains problematic, even after endless futzing. I do get some dimming on the dummy, but with a perfect 50-ohm load the LED should go full out, right?

    Also, I have to max out the righthand polycap even to get that. If I could dial it to 13 or 14, I might get a perfect match, but of course that's not possible. (12 is as far as it goes. No Spinal Tap version available, that I'm aware of.)

    Popping in the 250pf cap gives the exact same results if I drop the lefthand knob two notches. The 500pf cap does nothing.

    Because I'm new at building, I ordered prewound toroids, but the main one arrived with just two colours on it - the short, tapped winding is green, but the two long-leaded ones are both black. (No red.) This made it difficult to follow the directions. I took pains to ID which wires were which, but I might have messed up. Would this sort of matching problem be consistent with that?

    I should also say that the SWR meter on my big rig likes even the maxed-out 40m match, but unfortunately it's not a QRP meter. At 5W I trust it to warn me I'm 5:1, but a 1:1 reading at that power isn't necessarily reliable.

    At any rate, since this ATU enjoys a strong reputation for matching anything, I suspect I've screwed something up. Hints would be much appreciated.

    Thanks for your time,

    Rob
    KB7PWJ
     

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