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Looking To Build An Expo S Channel Kit Looking For Diagram Please

Discussion in 'Home Brew' started by blake25537, Dec 29, 2020.

  1. blake25537

    blake25537 Active Member

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    I learned how to make the Expo A Kit, but looking for a diagram for the Expo S Kit for a few Cobra 25's n Unidens I have. Could someone please post a diagram on building one?

    Thank You....


     

  2. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Is this what you're looking for?
    Part 1
     

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  3. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Part II
     

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  4. nomadradio

    nomadradio Analog Retentive

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    The S kit came in two versions. A "plus" and "minus" version.

    The reason you can't get one with both upper and lower channels has to do with the dirty trick behind the kit. Since you can't change this PLL chip's coverage, you change the first IF frequency of the receiver. Requires that you jump around the 10.695 first-IF filter in the receiver. Stretching this stage of the receiver to cover that wide a range of frequencies is pretty much the limit. Stretching it far enough to get both upper and lower channels just doesn't work well.

    I was not impressed by the way a radio's receiver worked after this kit was installed. Didn't sell very many of them.

    73
     
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  5. blake25537

    blake25537 Active Member

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    Thanks for the info, would you happen to have the diagram for the S kit I like to try to build one. I built an A kit n tried to make it work, but it didn't lol. If you could please post pics of a diagram n part numbers of the S kit

    Thanks

     
  6. nomadradio

    nomadradio Analog Retentive

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    It's the same circuit, really. Just with a single crystal instead of a switch the selects one of two crystals.

    An A kit really should work as long as it's oscillating The instructions for that one stink. The old instructions involve taking loose the capacitor that feeds the original 15.36 MHz into the PLL and adding a second tiny disc cap in series with it. The white wire from the kit connects where the two caps are joined. Any instructions that tell you to hook the white wire directly to one pin of a slug-tuned coil is doomed to be flaky or just fail.

    You don't need a radio to test an "A" kit. Just hook up the power and put the 'scope probe on the white output wire. If you don't see a couple of Volts peak-to-peak of RF, it's not running. A frequency counter will tell you the frequency where it's oscillating. Got in the habit of setting the trimmer caps on an Expo kit before it gets installed. If it has to be tweaked to get it on frequency, much better to do it before the radio is apart to install the thing.

    73
     
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