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Thread: Icom 751A Issue

  1. #1
    SK Member 05/26/2022 WX7P's Avatar
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    Icom 751A Issue

    I recently got an Icom IC-751A through Craigslist dumpster diving.

    The receive works great. Even with a BS antenna, I can hear qso's on 20 and all wwv and Hawai'i condx permitting. All of the other functions work, IF shift, the filters, everything. AM and USB are FB. All the memories are there also.

    Here's the problem:

    There is absolutely NO receive on LSB or CW no matter what band. Completely dead.

    I did see one item on an Icom sheet referring to to a potential bad PBT Oscillator. In their scenario, the only receive function that works is FM. I have AM and USB working, so I'm not sure their fix is applicable.

    Symptom:No RX audio in any mode except FM. No TX. Unit is not unlocked.

    Probable Cause:PBT oscillator is not running.

    Cure:Verify absence of 9.4665 signal at W86 on main unit in 751A, or W134 in the IC-751. Replace faulty X7 on main unit in IC-751A, or X5 in the IC-751.
    The scope is coming out of storage this week. I posted this to see if anyone had experienced this issue before.
    Last edited by WX7P; 06-29-2015 at 09:27 PM.
    http://i138.photobucket.com/albums/q275/nx6d/ham%20radio/SANY1260.jpg

  2. #2
    Administrator N8YX's Avatar
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    Check with the service manual - particularly, examine the PLL schematics.

    There is a set of switching logic which determines the mode(s). One of the lines from the controller isn't setting the state properly. While I haven't experienced this with either of my '751As, I did run across a couple of threads about it on the various radio forums and user groups. Let me dig into my schematics later today and I'll tell you what to look for - and where.
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

  3. #3
    Administrator N8YX's Avatar
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    Look here:

    http://www.download.n7tgb.net/Icom_M...Schematic1.pdf

    What voltages do you have on J1 of the "Main" board? Look at each with a scope as you toggle through the various modes.
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

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