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CB radio conversion to 75 Meters - Anyone ever done it?

Discussion in 'CB Radio Modifications' started by CollinsMan, Jul 24, 2008.

  1. CollinsMan

    CollinsMan Supporting Member

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    Has anyone here ever successfully modified a CB to operate on 75 meters? It seems that this would be a good project for those who want to run decent AM on that band. In particular, I'm interested in converting a Cobra 29. I've got both versions - PLL and crystal-plexer...

    When I was a kid, I remember reading an article where a HAM converted a couple of 3-channel Realistic mobiles to operate on 160 meters AM. However, I've never heard of it being done since then.

    ***I've got several Johnson Viking II transmitters that work, so I'm interested in doing this mainly out of curiousity and in the spirit of experimentation.***

    Thanks in advance for all comments!

    73



    Steve
     
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  2. dudmuck

    dudmuck Active Member

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    Cant say i've heard of such a thing.

    But the only way it would be possible:
    For the transmitter: Change the PLL to generate 13.8 to 14.24MHz instead of the regular 16.725 to 17.165MHz, and then have IC3 down-convert instead of up-convert. The PLL IC1 is mixed with a 10.24MHz reference. Impedance matching TR16, TR15, TR14 would need to be drastically changed to operate on 3.5 to 4MHz.

    For receive, the PLL offsets by 455KHz to generate the first I.F. of 10.965MHz. IC2 would need to down-convert just like IC3. Also the impedance matching needs to be changed for TR7 and FET1 first mixer. The rest of the receiver can be left the same.

    The TA7310 is characterized for a 27MHz output, so i'm not sure it would function as a down converter, or if another mixer is needed. They show the mixer output down to .5MHz, but the amplifier in there only running at 27MHz.
     
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  3. Justme

    Justme Active Member

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    An good transciever will produce good Am, like an FT 100 here with 6 kc am filter, 25 watts carrier, over 100 watts modulated.

    For an cb radio, an downconverter might be the best optiion, leaving the radio as it is, but use the downconverter to go to lower frequencies.

    Did that homemade for 2 meter to 70 cm's and 23 cm's 20+ years back.
    There were even transverters on the market to go from 2 meter all mode to all H.F. ham bands then.
     
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  4. W5LZ

    W5LZ Crotchety Old Bastard

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    Is it possible? Yes. But! Lots of redesigning and work envolved. Worth it? Only so you can say you did it. Actually much easier to just start from scratch.
    - 'Doc
     
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  5. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    Definately not worth the time,money, or effort to do it.It would be better to either build from scratch or build a transverter.I have seen articles how to convert to 10m and 12m and even 6m but never anything else. To go to 75 or 160m would require a complete redesign of the radio.
     
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  6. CollinsMan

    CollinsMan Supporting Member

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    Thanks for the responses! I'll do so some research on the possibilities of making a transverter for the radio. I'm mainly interested in putting a Cobra 29 on the air on 75 meters just for the sake of doing it as I like pursuing unique and strange projects.

    I did convert an old 23 channel Cobra Cam 89 to operate up on 29 MHz, and get lots of compliments on the audio from the operators there. But as y'all say, converting to 75 meters is another thing, entirely.

    Thanks again and 73!
     
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  7. W5LZ

    W5LZ Crotchety Old Bastard

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    Changing frequencies is hard/easy just depending on how far the 'stretch' is. From 27 to 29 Mhz isn't really all that much of a 'stretch', and since the frequency is going 'up' the component 'size' is going down (in most cases).
    A 'stretch' from 27 Mhz to 3.8 Mhz is a very long 'stretch', and since the frequency is going 'down', component 'size' will really go 'up'. Still not impossible, but certainly NOT easy/common/$$$.
    Wanna do it? Great, have at it! (I think when you get sort of part way through it, you'll wonder why you ever thought you wanted to. :))
    - 'Doc
     
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  8. bit_pit

    bit_pit Member

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    There have been a couple of kits/projects down here in Australia, for 11m to 80m transverters. These would not be a stretch to get working on 75m.
    One I built in the 80's. Cannot remember where that came from.
    There was another by the magazine 'Silicon Chip" in 1995. I would guess a crystal change and a few coil / capacitor changes would shift it. I think they would have copies of the project.
    I see http://www.rcsradio.com.au/ has the cct boards listed.
     
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  9. CollinsMan

    CollinsMan Supporting Member

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    Thanks Bit Pit!

    I really appreciate your letting me know where to get the 80-11 meter transverter boards. I figured that I wasn't the only person to have thought of such a project, and I was correct!

    You know, I might even rig one up to work with with my Uniden Grant LT. That would certainly be a QSO starter! Mount it all in an Igloo cooler, and it would make a great portable setup.

    Thanks again, sir!

    R sc 06111951 $ 20.00 2582A 8Om<>11m Transverter. PA1 1995Dec
    R sc 06111952 $ 31.50 2582B 8Om<>11m Transverter. Mix 1995Dec
    R sc 06111953 $ 26.50 2582C 8Om<>11m Transverter. PLL 1995Dec
     
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  10. bit_pit

    bit_pit Member

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    OK, remember these boards are just blank PCB ! You need all the components and schematics etc. to complete.
    If you decide to go this path let me know if I can be of any assistance, sometimes easier to source stuff in the originating country.
     
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  11. CollinsMan

    CollinsMan Supporting Member

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    Hi Bit Pit,

    No problem - I'm happy to just find some pre-made PC boards! I'll definitely keep you in mind when I get the boards in.

    I built a super efficient refrigerator out of a chest freezer based on some
    plans and a thermostat kit I bought from a fellow who lives in your country. He ended up supplying me with the "extra" parts needed to complete the kit that were readily available in Australia but not readily available here. So, I definitely will take you up on your offer for assistance in the future! Also, if I can help you with finding anything here in the states, please let me know.

    Thanks,

    Steve
     
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  12. bit_pit

    bit_pit Member

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    Thanks Steve! if I were to start a list of goodies that you can easily get in the states,that I'd love to get my hands on...I'd be typing for weeks !
     
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  13. CollinsMan

    CollinsMan Supporting Member

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    Hahaha!

    I know what you mean!
     
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  14. loverboy

    loverboy Member

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    CB to HF modifications in the Philippines


    Yes, I would like to share some experiences with CB modifications. The Philippines has over 7000 scattered islands and the communication infrastructure was not good in the late 70's. So HF radio's was kinda expensive that time. I was a frustrated HAM since I cannot afford to buy so modification using the poor mans (CB) radio was my last option.

    I did lots of experimenting with CB radios on both AM and SSB. Tried as low as 80 up to 6 meters and authored 2 books that was never published.

    Modified over a hundred CB SSB radios for 40 meters in the mid 80's using direct conversion and did not used any HF transverter since signal to noise is not good.

    I can make a 7.8 Mhz intermediate frequency SSB CB radio operate on 40 meters and have a good noise level at night using crystal in the broad band front end that serves as a sharp 30 db rejection filter.

    MABUHAY from the Philippines
     
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