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Cobra 29LTD Chrome VFO windup...

Discussion in 'CB Radio Modifications' started by CorruptHawkeyez, Apr 2, 2019.

  1. CorruptHawkeyez

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    I've been trying to figure out where to install the electrolytic cap and resistor on my Cobra 29. I'd like to install a switch also to turn it on and off. Anybody here know where on the board this goes?


     
    Shadetree Mechanic likes this.

  2. loosecannon

    loosecannon Sr. Member

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    not sure what you are referring to here.

    are you talking about adding a swing kit to the radio, or a VFO?
    LC
     
  3. kopcicle

    kopcicle Sr. Member

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    key up off frequency and "wind" onto frequency.
    I'll keep my opinion to myself
     
  4. sonoma

    sonoma Sr. Member

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    I am thinking he is talking about the cap resister mod that goed in the JP36 position. just my guess.
     
  5. kopcicle

    kopcicle Sr. Member

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    any questions?
     
  6. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    So haw many channels does this clobber? I think Motor Mouth Maul used to have something that sounded similar.
     
    Redbeard U812 likes this.
  7. CorruptHawkeyez

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    @sonoma not the J36 mod, that's already done. Talking about what's in the video above. I know all it is is an electrolyric capacitor and a resistor I just don't know exactly where they go. I have an idea that it goes somewhere near the oscillator but not sure.

    @Shadetree Mechanic it doesn't "clobber" any adjacent frequencies. It only slides from about 3-5khz off to on frequency.
     
    Shadetree Mechanic likes this.
  8. sonoma

    sonoma Sr. Member

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    now knowing what you are wanting, maybe some one that has done it will chime in. never did the mod and actually never had wanted it. would like to see just what it is.
     
  9. Low_Boy

    Low_Boy Well-Known Member

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    Mikes radio repair does a vid. on one in a galaxy.
     
  10. nomadradio

    nomadradio Analog Retentive

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    Motor Mouth Maul did this by putting an electrolytic cap on the transmit side of the Cobra 29 clarifier circuit. The front-panel control is powered in receive mode only. when the mike is keyed, the varactor diode's control voltage comes from a resistor and diode connected to the transmit-only voltage.

    Putting a 220uf capacitor at the junction of R99 and D18 will cause the tuning voltage to start out at zero when you key the mike and rise slowly (second or two) and settle onto the channel frequency.

    I have also heard this trick called MMM's "windup".

    The result is that when you key the mike your carrier starts out below the channel frequency and slides upwards to the channel center.

    The capacitor takes a little while to discharge when you unkey. It won't work again until you wait long enough for it to bleed off.

    Kinda cute. Easiest to hear when you're keying on other signals on the channel.

    73
     
  11. kopcicle

    kopcicle Sr. Member

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    .
     
  12. Dmans

    Dmans Sr. Member

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    Begs the question-
    Why?
    Maybe because someone else has one?

    73's
    David
     
    Highway Man and sunbulls like this.
  13. CorruptHawkeyez

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    Pretty sure a resistor is added in series with the electrolytic also. In the video posted by Mikesradiorepair on youtube; he's able to key and unkey the radio fairly quick. I believe the resistor is what decides how long it takes for the channel to slide to center frequency. Also, it takes just as long as it takes to slide to center frequency as it does too receive. This mod also quickly changes receive for a short time after you unkey; which should be the capacitor bleeding off.
     
    Shadetree Mechanic likes this.
  14. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    I wonder if this would help break through pileups? It would seem to put more power into the side band frequency? Would it be better to start high and come down to frequency or start low and come up to frequency? I heard that sometimes shoot out radios are tuned slightly on the high side. Not sure if this is folklore or if there is a real reason to do so.
     
  15. 543_Dallas

    543_Dallas Sr. Member

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    The way nomad mentioned is how I was doing it on the 29. For the 148/2000 chassis put the positive leg of the cap on the wiper of vr5 and the neg lead to board ground.

    No need for a resistor. The cap will bleed off within a second or two in rx mode and once charged in tx it will have no effect
    on the rest of the circuit.

    I've heard about someone modifying the software for a Flex radio to do this. I haven't figured that one out yet.
     
    Shadetree Mechanic likes this.

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