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Kenwood TM-742 Query

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by N3EAQ, Feb 13, 2015.

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  1. N3EAQ

    N3EAQ Ham Member QRZ Page

    Hello Folks,

    One of my K-Wood 742's just developed an interesting issue.
    As soon as I power it up it immediately keys up in Xmit.

    If I unplug the Mic. it does not do this, however it is NOT the Mic. causing the problem!
    I've replaced the Mic with 2 other stby. units, and it always does the same. It's something in the radio.

    I've been told, and have read that there are some caps that go bad in the 742 that cause this over
    time, but no one seems to know which caps these are.

    Do any of YOU know which caps these are? I've got the unit tore down right now cuz I needed to
    replace the Bkup batt anyway.. So, while I have it apart, I'd like to also repair this issue.

    Anyone familiar with this?

    PLS! Contact me direct @ N3EAQ@N3EAQ.NET since I don't regularly monitor this site!
    Your assistance is appreciated.

    Mike - N3EAQ
  2. K8JD

    K8JD Ham Member QRZ Page

    It sounds like something at the inside of the mic socket is shorted and some non PTT fucnction in the mic (DTMF pad or control buttons) are forming an unintended path to ground the PTT line. Check out the mic and radio connector schematrics.
  3. N3EAQ

    N3EAQ Ham Member QRZ Page

    Found the problem

    Thanks to those of you who contacted me.

    I had already checked all the obvious items like shorted mic pins, something stuck in the connector, etc., etc.
    If it had been something easy, I wouldn't have posted it. Anyway.....

    After a little troubleshooting it turned out to be a bad 8v regulator on the control board (IC102 - L7808CV).
    I've replaced it with a new unit from Mouser, and all is now working fine!

    Just wanted to follow up and let everyone know I'd found the problem and fixed it, plus leave a trail for anyone
    who had this issue in the future!

    Thanks, and 73!

    Mike - N3EAQ
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