iCom IC 751 Stuck In Transmit

Discussion in 'Radio Circuits, Repair & Performance' started by KJ7LYB, Nov 14, 2020.

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  1. KJ7LYB

    KJ7LYB Ham Member QRZ Page

    Hi all, so I have an iCom 751 (non A version) which is stuck in transmit mode, when switched to receive mode it does not change over. The problem can be seen at the linked video (since this was filmed, we've gotten the frequency display working)

    What is confirmed working:

    1) RIT and XIT circuits work
    2) Audio and Tone circuits work
    3) Band switching works
    4) speech circuit works
    5) xmit monitor mode works
    6) memory mode works
    7) notch, filter & pass band work
    8) the circuits that change what the display shows

    Does anyone here know which circuits would have failed to cause it to get stuck in transmit mode? Thank you.

    Attached Files:

  2. K6LPM

    K6LPM Ham Member QRZ Page

    VOX maybe?
  3. KJ7LYB

    KJ7LYB Ham Member QRZ Page

    I don't think so as it is a physical switch not voice controlled, thank you.
  4. KC3RN

    KC3RN Ham Member QRZ Page

    I would suspect a stuck relay...
  5. VE3CGA

    VE3CGA XML Subscriber QRZ Page

    do you have the service manual? its here https://www.opweb.de/english/company/Icom/IC-751

    probably (if you havent already) going to have to crack it open and meter the send, ptt, t/r change over switch.
    Pin 3 on the acc socket. according to the manual, if thats at 0v then the rig is in xmit. if thats the case then you'll have to
    trace that back to determine whats causing the pin to be low

    The block dwg (page 5-1) shows some areas to check out for the send and t/r blocks with some part ID's for reference

    Theres a fairly detailed description in the transmit section on what drives what t/r & send electronics

    I would start by measuring the acc socket pin3, if thats stuck low then trace it back on the block dwg & schematic, if thats
    working then its something after that circuitry, possibly driven by those parts that is stuck.

    The rig may have some capacitors that have failed as shorts or partial shorts, I had to replace several in my IC751A for a different fault
    Also check the regulator voltages, if the send circuitry isnt getting proper voltage, send circuitry might think the send is in xmit

    not much definate help I'm afraid
    73 Bob VE3CGA
  6. KJ7LYB

    KJ7LYB Ham Member QRZ Page

    Thank you very much. My apologies, I only just saw this post. I will try it.
  7. KD0RCB

    KD0RCB Ham Member QRZ Page

    I believe you said it was working fine before the fuse blew....just curious ,what caused the fuse to blow in the 1st place? There may be a correlation between these issues...keep us posted
  8. WO1X

    WO1X Platinum Subscriber Platinum Subscriber QRZ Page

    At the risk of a stupid question, there’s a switch just above the microphone connector on the left side of the rig that controls transmit and receive..... you don’t have that in the wrong position do you?
    Grey plastic as I recall.
    Edit.... I was not able to view the video before my earlier remarks.... but just make sure that the xmit recv switch is pushed down before you power it up
    Last edited: Nov 21, 2020

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