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IC 746 2m failure

Discussion in 'Ham Equipment' started by HomerBB, Mar 24, 2018.

  1. HomerBB

    HomerBB Sr. Member

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    Simple question.
    My IC 746 has started to power off when I key the mic on 2m band. Why?
    I researched CMC potential. Revisited shack and equipment grounding, coax, and antenna switch outs. No other band does this...
    Not running external amp, nor any other rf generating equipment in the shack. I switched out 25 amp PS for 35 amp PS. Also put a deep cycle 665 CCA battery in auxiliary to PS and it still occurs...


     

  2. Road Squawker

    Road Squawker Sr. Member

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    check the dc voltage under load... not at the PS but after any fuses in the power cord.

    does the 746 use that terrible ICOM molex connector on the rig rear panel? if so, check the voltage AFTER the connector.[​IMG]
     
  3. HomerBB

    HomerBB Sr. Member

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    I'll check it. It does use that plug.
     
  4. HomerBB

    HomerBB Sr. Member

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    Screenshot_20180324-010135.png
    Looks to be the 6 pin version.
     
  5. tba02

    tba02 WOOF

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    and check the fuse area too. They have a tendency to develop poor connectivity.
     
    Road Squawker likes this.
  6. HomerBB

    HomerBB Sr. Member

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    Everything comes on. All other bands are fully operational. Only 2m band shuts the radio down when the mic is keyed.
     
  7. sp5it

    sp5it Master of puppets

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    Short in 2m RF amp?
    Mike
     
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  8. HomerBB

    HomerBB Sr. Member

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    Maybe.
    Guess it's take it to the shop time.
    I still hope someone may have encountered this issue and knows why it is happening.
     
    #8 HomerBB, Mar 24, 2018
    Last edited: Mar 24, 2018
  9. Road Squawker

    Road Squawker Sr. Member

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    the number of pins on the connector is immaterial, only 4 of the pins are connected (I think).
    have you checked the voltage after the connector under load with a VOM?

    When you just key the mic in SSB, there will be very little increased current draw.

    Are you keying 2 meter SSB or FM,there will be a big difference in FM amperage draw.

    Almost every ICOM will develop voltage drop issues sooner or later due to the connector and/or resistive fuses.

    good luck
     
  10. HomerBB

    HomerBB Sr. Member

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    When I get home I will check it out. I was keying FM. Been using an alternative radio for 2m, but that one lacks SSB capability. I want to have that option available on 2m. Must have the 746 fully operational.
     
  11. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    Icom Service Center - Michigan
    1792 Nash Drive
    St. Joseph, MI 49085
    Phone (269) 429-2334...MATT

    The 746's(+pro's) did have some issues with driver/final circuits both HF/VHF-UHF...I have not heard of too many that at some point did not have an issue...Some early versions went back to service centers 2-3 times before the issue was solved. This stems from how the driver/finals were mounted to boards and chassis.
    Matt is about the best I have heard for ICOM rigs...very well respected. So I would suggest him before any others. I would let him go completely thru it, and that way he will check for other issues and make any service updates that have come along since production.
    The 746's have had all parts purged by ICOM(Japan)...they no longer support that rig with factory parts stock...so it is best to send it to some centers that have good access to after market replacements. Be glad it does not have display issues as new displays or parts for repair are almost on the unobtainable list.
    Good Luck
    All the Best
    Gary

    Homer: after asking around...seems some had issues with the VSWR/protection diodes on VHF/UHF...might be your issue???

    https://forums.qrz.com/index.php?threads/icom-ic-746-2m-low-power-output.372025/
    http://www.kb6nu.com/OLD406/
    http://www.qsl.net/k5lxp/projects/746Pro/746ProNoTX.html
    http://www.rudius.net/oz2m/ic746pro/index.htm
     
    #11 BJ radionut, Mar 25, 2018
    Last edited: Mar 25, 2018
  12. HomerBB

    HomerBB Sr. Member

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    Thanks, B.J.r. I first suspected antenna line issues. There were coax moisture leakage issues with the original coax when it first manifested. Changing all of the antenna system out did not resolve things so I moved on from that. I am going to hook up the dummy load and see what happens. Damage may already be done
    Good info..
     
  13. Road Squawker

    Road Squawker Sr. Member

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    Agree about Matt, He repaired my IC7K when it failed one day before the warranty expired.(y)

    FWIW, I don't see anything in any of those links that would cause the rig to turn off as post 1 stated.

    If the PA was shorted, you would think it would blow the inline fuse.

    The only things I can think of that will shut the rig off (and not blow a fuse) are RF "confusing" the CPU and low voltage.

    Homer, have you done a master CPU reset?
     
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  14. HomerBB

    HomerBB Sr. Member

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    I have not done a master CPU reset.
    New to Amateur rigs. Guess I can read how in the manual.
    If I release the Mic key quickly enough the radio will recover power. If I dont it has stayed shut off.
    I quit keying on 2m preferring not to have the radio do that.
     
  15. HomerBB

    HomerBB Sr. Member

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    Went out to the shack and did the CPU reset. Checked the key up at full power through a dummy load. All good. Put the new coax and antenna on it with less than 1.5:1 SWR and key again. Still good. Went to call freq 146.520 and called. Had a convo with a nearby ham for a few minutes with multiple keys and no problems.
    I think I'm good now.
    Thanks!
     
    #15 HomerBB, Mar 25, 2018
    Last edited: Mar 26, 2018

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