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Thread: Icom IC-451A frequency range modified, how to fix?

  1. #1

    Default Icom IC-451A frequency range modified, how to fix?

    I inherited an Icom IC-451A transceiver. Frequency range SHOULD BE 430-440MHz BUT IS 440-450MHz and seems to operate properly in that range! How do I mod it back to original 430-440MHz range?
    Thanks,
    Skeeter

  2. #2

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    It's a long time back, but it's a diode or a soldered pad in that radio isn't it? Has somebody fiddled? The 251 was the 2m version and I seem to remember that a fast switch off and switch on widened the two metre band to a wopping 4MHz, up to 148!

  3. #3

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    Thanks for the info Paul. Fast on/off changed nothing. Don't know if previous owner modified it and just looking at the electronics maze I don't see obvious tinkering. Do you have link to the schematics?

  4. #4

    Join Date
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    Quote Originally Posted by Skeeter View Post
    Thanks for the info Paul. Fast on/off changed nothing. Don't know if previous owner modified it and just looking at the electronics maze I don't see obvious tinkering. Do you have link to the schematics?
    Hello Skeeter,

    Go to the following website https://www.mods.dk/manual.php Now Scroll down to ICOM and Click on that. Scroll down to IC-451A and download the PDF to your computer and Save. This method will allow you to look at the information off-line. I'm guessing whomever did the modification, did it in the Logic IC section.

    Good Luck,

    Dan
    WA9WVX

  5. #5

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    Thanks Dan. Got the schematics and still trying to figure out where the mod was made. Not much rf knowledge in this brain and signal labels in the schematic are almost non-existent.
    Skeeter

  6. #6

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    Quote Originally Posted by Skeeter View Post
    Thanks Dan. Got the schematics and still trying to figure out where the mod was made. Not much rf knowledge in this brain and signal labels in the schematic are almost non-existent.
    Skeeter
    Hello Skeeter,

    As I wrote before, most likely the modification was made in the Synthesizer / Logic Circuits where the Frequency Bandwidth is generated ... long before the signal turns into RF. I think that transceiver was manufactured in 1980 and the Solid State technology was very new at the time. "If" that transceiver employed Crystals which I'm only taking a guess that could be the problem but I don't have a Schematic to refer too. If you belong to an Amateur Radio Club, try to locate a ham that's been licensed for 40 years or more and ask for his assistance on your transceiver. You'll need a Oscilloscope with a 250 MHz Probe, a Frequency Counter, a DVM and whole lot of patience troubleshooting the problem. If you keep running into brick walls, get up and away from the transceiver for a couple of hours or a day or two. Good Luck

    Dan
    WA9WVX

  7. #7

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    Page 7 of the schematic to the left of IC12 and just above - a small diode matrix that sets the version IC-451E (europe 144-146) and not the A for america.

    These will need to be located - D52,56,57 are needed for the E version - looks like your 56 might be missing or perhaps just come unsoldered if it was 'tagged on' and vibration over the years has failed the joint. Could be a simple dab of the soldering iron.

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