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Midland 77-882 mobile

Discussion in 'CB Radio Modifications' started by Jim Holt, May 5, 2018.

  1. Jim Holt

    Jim Holt New Member

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    Looking for a variable dead key mod for this radio. I have had it a long long time. I bought it brand new back in 1977, the only mods been done to it are the channel mods and a peak job. The radio still works just as good as when I bought it 41 years ago. Any help would be greatly appreciated.


     

  2. kopcicle

    kopcicle Well-Known Member

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    LM336T voltage regulator ...
    If you have the schematic...
    Pin 1 of the 7205P is B+
    Downstream of pin one is connection to C91, R64, then the R66/R73 voltage divider for the ALC (I've played with that but leave it alone)
    Past that connection you can insert the regulator.
    I remember it working but not that well. I don't recall what the issue was but it was along the lines of needing to adjust the Mod (ALC) along with the voltage. I don't remember getting much of a range either.
    There was a mod for PTBM049AOX somewhere in Secret CB but first off I was too lazy to dig and second I was too lazy to dig twice or more to find the correction, the oops and the apology. In short the mods in Secret CB need to be taken with a grain of , well , something .
    That's all I ever came up with other than reproducing the later Uniden AM power circuit . Look at how the 142GTL does it .
    Schema attached ... Midland-77-882-schematic.png cobra_142gtl_main_sch.jpg Midland-77-882-schematic.png cobra_142gtl_main_sch.jpg
     
    ShadeTreeMechanic likes this.
  3. Handy Andy

    Handy Andy Active Member

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    Not sure, but I do believe this one uses a "Single wind" modulator transformer as a reactance modulator - GE did some of these way back when.

    Had one wind to the Driver and Final and a separate tap to the Speaker - when the Speaker opened up (as you keyed) the audio power went to the Driver / Final - with the Driver having a dropping resistor of about 10 ohms or so.

    The RF came from the "exciter" and PLL side - with the TX light showing power to turn on the MPS 1923 or "941" predriver and everything "cascaded" from there...

    If it's similar to any of the above, usually the "Driver" Audio and Bias R21 10 Ohm resistor is your "jp 36" mod entry point...they are AM only units.

    A pic or two of the "innards" would be a big help in identifying the board. We have little info out here about it.

    Regards!
    :+> Andy <+:
     
    #3 Handy Andy, May 6, 2018
    Last edited: May 6, 2018
  4. nomadradio

    nomadradio Analog Retentive

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    Should only need to remove R21, and wire the typical TIP120 and potentiometer into the two holes where R21 was removed.

    Circuit board for this one is PTBM049A0X

    73
     
  5. kopcicle

    kopcicle Well-Known Member

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    #5 kopcicle, May 8, 2018
    Last edited: May 8, 2018
  6. nomadradio

    nomadradio Analog Retentive

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    Too bad I can't make any money selling fewer than five at a time of the "Peel and Stick Variable" module. It's not worth enough to suport our transaction cost. And who wants to buy five modules without first trying out just one of them?

    It's built to line up with the holes in the Cobra 29LTD circuit board when the jumper wire is removed. Double-stick foam tape holds it to the heat sink surface.

    I feel like posting just a schematic of the thing is less useful than a pictorial layout might be.

    But here it is anyway.

    [​IMG]

    Does the job.

    73
     

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