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Cobra 29LTD channel mod questions/help

Discussion in 'CB Radio Modifications' started by dss56, Aug 30, 2018.

  1. dss56

    dss56 Member

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    I want to channel mod a cobra 29ltd and a few other am cb radios that need to be modded with a crystal injector board.
    I know the xtal freqs just need to make an injector oscillator board that will do 2-3 xtals.

    Would anyone out there have a schematic to make a 2 or 3 xtal oscillator injector board and the schematic and parts list.



    I have searched the net with no good luck

    Much appreciated for the help
     

  2. Dmans

    Dmans Well-Known Member

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    Dss56,
    Look at the 29ltd schematic and duplicate the oscillator section with crystal switching.

    Cbtricks.com???

    73’s
    David
     
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  3. nomadradio

    nomadradio Analog Retentive

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    The Expo 100 kit type "A" or "A+" works by injecting a crystal frequency into a mixer chip in the radio's PLL. The mixer chip's output feeds into the PLL input. Has a crystal for each new band, and a one-transistor oscillator circuit.

    Pin 4 of the mixer chip has 15.36 Mhz coming from a single slug-tuned coil that has a 5.12 MHz 5-Volt square wave feeding into it. This gets you a half-Volt or a Volt of the third harmonic, 15.360 MHz.

    The Expo kit has a crystal oscillator with an output level several times the level of the radio's internal 15.36 Mhz source.

    The Expo kit's output overpowers the PLL input, moving all 40 channels up or down. The radio's stock 15.36 MHz signal is still there at pin 4 of the TA7310 mixer chip, but the stronger level from the Expo kit drowns it out. The PLL locks to the stronger one.

    To go 45 channels up, the crystal is 15.810 MHz. Down 45 is 14.91 MHz.

    The Expo kits were sold and designed before the first "export" radios showed up. Those radios move 45 channels per band, so that the lower band gives you 26.955 on channel 40, and the upper band starts at 27.415 on channel 1.

    But the original Expo kits only moved you up or down 44 channels. This caused channel 1 to show up twice. Once where it should be, and again with the selector on 40 with the lower band selected.

    Likewise, channel 1 on the uppers wasn't channel 41, it was channel 40.

    Not sure why they were built with overlap. When they hit the market, nobody cared.

    Later "A" and "A+" kits used a dirty trick to get that '45 channel' jump from the same, old crystals. Must have figured it would sell better if the channel display on your Cobra 29 would read the same lower channels as your buddy's Galaxy.

    They put a disc capacitor in parallel with the lower-band crystal to pull it down from 14.92 to 14.91 MHz

    And they put a capacitor in series with the upper-channel crystal to pull it up from 15.80 to 15.81 MHz.

    Just one problem. This screwed up the feedback in the crystal oscillator circuit. It would reduce the output so much that a radio might "kick in" and get the lower channels when selected, then randomly "kick back" to the normal 40 channels as the radio warmed up. If the output level from the Expo kit isn't big enough, the PLL may have trouble making up its mind which one it will lock onto.

    Using the old crystals was probably a LOT cheaper than getting new ones made.

    One of these days, someone needs to find a copy of the original early-80s instruction sheet. Shows the correct hookup for the "A" kits. Any newer instructions that show the white wire connected to one pin of a tuneable coil are just wrong.

    73
     
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  4. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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  5. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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  6. nomadradio

    nomadradio Analog Retentive

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    That switch is for the crystal-only 23-channel radios, or for a 40-channel radio that can be modified by swapping a crystal only.

    The Cobra 29LTD type radios require both the crystal and an oscillator circuit like the Expo 100 type A.

    73
     
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  7. Handy Andy

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

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    1S2KitMedley.jpg This was a kit installed in a radio that used the TC9109 PLL
     
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  8. LeapFrog

    LeapFrog Sr. Member

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    Thanks Andy, I believe the PLL found in the TRC-427 is a TC9106, BTW I am currently looking for a "UK variant", the TC9119 with the UK specific rom code.

    Dss56,
    I found this page out of Lou Franklin's CB PLL Data book
    I found a free preview on google books.
    The other photo contains an example of install for an expo 100.

    Here is a sample direct from Lou's website.
    http://www.cbcintl.com/docs/plldata-sample.zip

    http://www.cbtricks.com/miscellaneous/vco/expo/cobra.htm

    ~Cheers~
    -Leap
     

    Attached Files:

    #8 LeapFrog, Sep 8, 2018
    Last edited: Sep 8, 2018
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  9. Handy Andy

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

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    You're right - didn't have time to dig out the radio to check but you knew the chassis - so glad it could help.

    I'm sure the way they did that was using a single pole DT switch - all it did was use the main board then floated it and used the onboard.

    So "multipole" expansion should not be a problem.

    I've seen kits that have you reinstall the main Xtal on their card, and you install the card to replace the holes left behind and you trim the Xtals to obtain the best "center slot" you can due to the PLL divider error that as you expand away from the expected (PLL's viewpoint) frequencies it generates using the newer math (aka Oscillation Frequency of the Xtal and your trimpot) so Shadetrees' offer of expansion Xtals seems to be a pretty cool idea.

    Only problem though is to get the main PCB's 1st IF Out (there's a word for this too but it's too stuffed into the back of my brain to fish it out at the moment:)) section to work with the newer frequencies...
     
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  10. kaos513

    kaos513 Well-Known Member

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    Just buy one of lesters board i have a 29 ltd for 2 years with the channel board works good.
     
  11. LeapFrog

    LeapFrog Sr. Member

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    That's an a-okay suggestion for one radio.
    But,
    dss56 stated he wants to do a few radios.
    You can hand a man a single fish if he is hungry, or you can show him how to fish.
     
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  12. Handy Andy

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

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    That's AWESOME...

    Glad you remembered that - sometimes it gets a little isolated in here...
     
  13. kaos513

    kaos513 Well-Known Member

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    Oh ok i see now sorry for the confusion.
     
  14. nomadradio

    nomadradio Analog Retentive

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    The "S" type kits get around the "channel proof" design of this PLL by changing the frequency of the receiver's first IF stage.

    Requires that you remove the first-IF tuned circuit and/or 10.695 MHz filter in the receive section.

    Doesn't do any favors for the radio's rejection.

    Never did sell more than one or two of these kits. They work for only one band. Up or down. A separate kit for each.

    And no, you can't just install both in one radio. Gotta choose, either uppers or lowers.

    73
     
  15. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    I think the deal is that the tuning cans can only bring so many channels into alignment.
     
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