Yaesu FT840 xmit problems

Discussion in 'Radio Circuits, Repair & Performance' started by KI4ABA, Jun 15, 2019.

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  1. KI4ABA

    KI4ABA Ham Member QRZ Page

    My Yaesu FT840 has developed a transmit problem while it has been moved several times and been in storage for the last several years. When I press the mic switch to transmit, the radio clicks rapidly in and out of TX mode and it also seems to have intermittent SSB audio transmit. I know it's an old radio, but I'd prefer to not have to spend the money on a new rig. The antenna is a DX-CC at about 50ft inverted-V. I made a choke basin to ensure the problem wasn't stray RF, but no luck.

  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    If the power supply voltage is good at the back of the radio, The mic may be suspect.

    Testing into a dummy load would be nice also. Does it do that on all bands ?

    Good Luck.
    Last edited: Jun 15, 2019
  3. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Do you have a scope? I'd check the power supply for excessive ripple and correct voltage for starters, at the radio.
  4. KI4ABA

    KI4ABA Ham Member QRZ Page

    I cobbled together a momentary switch for the PTT circuit and wired up a regular vocal mic to the input plug. That seems to have fixed it, or at the very least made it usable. I have a desk mic on order now. Thanks for the help. My 2m radio shows 13.7v as does my multimeter.
  5. PY2RAF

    PY2RAF XML Subscriber QRZ Page

    Hey John

    Happened the very same thing with my good FT-840. It was a cold solder issue. Just rest your soldering iron at the solder joints of the Mic port (man, that is HORRIBLY bad positioned and hard to remove, try to do it onsite without disssembling, careful to not burn the nearby wires) and you will be good (and PROUD)!

    Hope that helps!!!

    P.S.: Do a MOX pressed test in CW, if this is flawless, it might very well help to confirm the $ABOVE.
  6. KI4ABA

    KI4ABA Ham Member QRZ Page

    Thanks. I will definitely check the mic port for cold solder joints. The radio works fine in MOX at full power on CW into my dummy load.
  7. KI4ABA

    KI4ABA Ham Member QRZ Page

    I found no cold solder joints in the mic connection. The new microphone worked perfectly for Field Day. Thanks for the suggestions.

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