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President Dwight D dead key

Discussion in 'CB Radio Modifications' started by dozerman, Nov 5, 2017.

  1. dozerman

    dozerman
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    hello, its me again

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    I've got a Dwight D with the 858 chip and would like to drop the dead key on this unit. When I first got it, it was around 3 but had be de-tuned so to speak. After replacing all the electrolytic caps in the radio and repeaking, it has a 5.5 watt key swinging about 12. The peak isn't a concern as much as the dead key is. Would a cap and resistor work for this board? It looks a lot like the 29 xlr, so I'm assuming it would work. Also, the clock doesn't work. Not a huge deal, but it would be nice to see it work again.


     

  2. nomadradio

    nomadradio
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    Analog Retentive

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    If memory serves, adding a resistor with a capacitor in parallel with it to the driver of this radio requires cutting and jumpering a foil trace.

    No convenient jumper wire to pull out like you find in the Cobra 29 type radios.

    Pretty sure we have installed a carrier control this radio. I'll see if I can turn up some pics.

    Do you simply want to reduce the carrier to a desired fixed level, or make it variable?

    The resistor+capacitor method is simpler, but there will be some cut-and-try to match the resistor value needed to obtain the desired carrier power. No hard-and-fast formula to follow for that purpose.

    73
     
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  3. dozerman

    dozerman
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    I would like to reduce it and allow full swing like you would in cobra type radios. I run a palomar 300a or a Maco 200 with it, and neither seem to like a 5 watt key. Granted it's not a big swinger, 12 or so pep. And the audio "pinches" now that the key is that high. I've been searching the Internet for this modification but have only come up with the 29xlr mod.
     
  4. wavrider

    wavrider
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    W9WDX Amateur Radio Club Member

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  5. dozerman

    dozerman
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    In that thread, I think it was recommended to use 4001 diodes. I used 3 in series and got a 2.5 watt dk. That what I wanted. But the pep was slow. I swapped for zenier diodes and the action was much peppier. I swapped the driver and final for a 1957 and my last new 1969 and have a good 17 watt pep and by all reports great audio. I've still yet to change the am regulator and some caps in the power supply but, it's doing much better. Now if I could get the clock to work all would be great. Thanks guys!
     
  6. Robb

    Robb
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    Yup

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    Wouldn't just playing with the values of the coupling cap between the pre-driver and the driver be enough to get the desired effect?
     
  7. dozerman

    dozerman
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    It might be enough, yes. But when tested, the original transistors showed less than half the gain of the new ones. Why not start there? I may play around some more in there.
     
  8. wavrider

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    glad you got it going with the dead key you wanted.
    the diode method worked great for my applications and was clean on the o scope.
     
  9. dozerman

    dozerman
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    I'd like to work on the audio section more. Although the diodes worked, it seems like it could use more. I had another hand record my audio using several different mics and while they all sounded clean, it still sounded "thin". Not saying I want splatter loud, just a more robust audio.
     
  10. nomadradio

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    Hmm. Replacing 40 year-old electrolytic capacitors would be a good place to start.

    Our policy here is to replace the audio chip's "blocking" capacitor in radios this old, even if it isn't causing trouble. It serves to separate the 6 Volts DC on the audio chip's output pin from the audio that feeds the modulation transformer. Can't find my copy of Sams CB-121 at the moment, but I do remember the audio chip is the long-discontinued uPC1156. If that blocking capacitor shorts, it blows out that chip.

    As a rule, the capacitance value is either 330 or 470uf. The original will tend to have a low DC-voltage rating, usually 16 Volts or less. We never use one with a rating under 25 Volts.

    As for the other couple-dozen electrolytic caps in that radio, the ones elsewhere in the audio circuits can affect the audio quality when they get old , and not in a good way.

    73
     
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  11. dozerman

    dozerman
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    I
    I just finished all the caps Monday and have it back in line. Modulation increased slightly, not an appreciable difference but improved. Cost more to replace the electrolytics and time invested than the radio is probably worth lol. But I sure like the look of it and the speaker together. Wish I could find another one in this good of a shape. Thanks for all of your help!
     

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