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TM-741 Repair - No Power On

Discussion in 'Radio Circuits, Repair & Performance' started by KE8CPD, May 10, 2020.

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  1. KE8CPD

    KE8CPD XML Subscriber QRZ Page

    Its a 742 NOT 741 can not edit title.

    Little back story first.
    Radio came to my bench with no power on, so i replaced the coin cell battery, and inspected the traces and IC around and found that IC 7 (HD74HC165FP) VDD pin had been jumpered to another IC VDD Pin to provide the 5V. So After tracing where the 5V comes from i have traced it to the IC11 in the 144 RF module (VCC PLL Chip). I measured that 5v line and its at 0.2 volts so basically nothing. And all the ICs that are powered by that 5v line are at the 0.2 volts as well.

    So here is my questions.
    From my understanding of the circuit that 5v line should be at 5v all the time as long as the radio has supply voltage. Can anyone confirm that?

    Lets add a twist.
    Every once in a while, i will get the radio to boot and act normally. Any when i check that 5v line, its at 5 volts. So im not sure why it would do that, any one have this issue or have an idea why its doing that.
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    A good place to look / clean would be the connectors that connect the Detachable display and control panel.

    Good Luck.
  3. KA5IPF

    KA5IPF Ham Member QRZ Page

    Replace the 22UF SM cap in the head and clean up the mess.
    W9GB likes this.
  4. KB2JSG

    KB2JSG Ham Member QRZ Page

    Check F101 control unit & C101 & C103 (display unit)
  5. W9GB

    W9GB Ham Member QRZ Page

    Follow Clif, KA5IPF advice, clean up mess (from last tech) and do proper repair.
    Welcome to the Real World of Electronics Bench Repair.
    That is referred to as an Intermittent ... sometimes works, sometimes does NOT.
    Kenwood TM-742A/E Service Manual
    Exploded Diagrams of Radio : Pages 116-119

    Parts List
    General Parts of TM-742 : Pages 72-74
    Control Unit (X53-3460-XX) : Pages 76-81
    Display Unit (X54-3130-XX) : Pages 81-82
    Last edited: May 29, 2020

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