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Sweet & easy variable dead key for cobra 29's

Tim Chambers

Active Member
Apr 9, 2017
36
84
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Works on all cobra 29 radios, ltd and up. After a complete tune up/upgraded final/mosfet/etc,
See the attached PDF. this mod will give you a variable dead key using VR4, the modulation adjustment VR. From 0-max dead key. Clean and easily changed up to use the RF gain on the face is desired.
One resistor, a piece of wire, and a 47uf 25V electrolytic cap.
 

Attachments

  • TinysCBShop29VariableMod.pdf
    244.1 KB · Views: 420

Hi Tim,
here is another version with a transistor, that way you wont have to worry about the VR burning up.

http://www.cbtricks.com/radios/cobra/mods_&_info/graphics/Cobra_Uniden_Var_Pwr.pdf

Also the statement "very similar to what the NPC-RC mod does on ssb & 10 meter radios" not too accurate... it will allow you to lower the key and get "swang", but the radios use different techniques to produce AM, and so the two modifications can only produce similar results to an extent, you won't see the sinusoidal waveform out of a 29 much past 105%, it will start to turn into a triangle waveform, and eventually the envelope looks like a PWM signal or "box car", "loud" but nasty as all get out.. (bye bye adjacent channels)

I'm not dogging on the mod, I've used it; like your paper recommends, a 47 uF capacitor will help to keep it cleaner, a larger cap value would let it over-modulate more. I like to keep the AMC enabled and once you use too large of a cap and lower the key too far, the waveform loses its integrity even if the diode isn't snipped and the VR was turned all the way down.

Sometimes, I just pull the jumper between driver and final, to add an attenuating resistor, reducing the signal from the driver to final, lowering dead key (and max P.E.P.) without mutilating the AMC/Waveform when you modulate the carrier. When I add the resistor I usually have to adjust the AMC VR to compensate. (So it doesn't swang into distortion)

73
 
Last edited:
Hi Tim,
here is another version with a transistor, that way you wont have to worry about the VR burning up.

http://www.cbtricks.com/radios/cobra/mods_&_info/graphics/Cobra_Uniden_Var_Pwr.pdf

Also the statement "very similar to what the NPC-RC mod does on ssb & 10 meter radios" not too accurate... it will allow you to lower the key and get "swang", but the radios use different techniques to produce AM, and so the two modifications can only produce similar results to an extent, you won't see the sinusoidal waveform out of a 29 much past 105%, it will start to turn into a triangle waveform, and eventually the envelope looks like a PWM signal or "box car", "loud" but nasty as all get out.. (bye bye adjacent channels)

I'm not dogging on the mod, I've used it; like your paper recommends, a 47 uF capacitor will help to keep it cleaner, a larger cap value would let it over-modulate more. I like to keep the AMC enabled and once you use too large of a cap and lower the key too far, the waveform loses its integrity even if the diode isn't snipped and the VR was turned all the way down.

Sometimes, I just pull the jumper between driver and final, to add an attenuating resistor, reducing the signal from the driver to final, lowering dead key (and max P.E.P.) without mutilating the AMC/Waveform when you modulate the carrier. When I add the resistor I usually have to adjust the AMC VR to compensate. (So it doesn't swang into distortion)

73
Hi Tim,
here is another version with a transistor, that way you wont have to worry about the VR burning up.

http://www.cbtricks.com/radios/cobra/mods_&_info/graphics/Cobra_Uniden_Var_Pwr.pdf

Also the statement "very similar to what the NPC-RC mod does on ssb & 10 meter radios" not too accurate... it will allow you to lower the key and get "swang", but the radios use different techniques to produce AM, and so the two modifications can only produce similar results to an extent, you won't see the sinusoidal waveform out of a 29 much past 105%, it will start to turn into a triangle waveform, and eventually the envelope looks like a PWM signal or "box car", "loud" but nasty as all get out.. (bye bye adjacent channels)

I'm not dogging on the mod, I've used it; like your paper recommends, a 47 uF capacitor will help to keep it cleaner, a larger cap value would let it over-modulate more. I like to keep the AMC enabled and once you use too large of a cap and lower the key too far, the waveform loses its integrity even if the diode isn't snipped and the VR was turned all the way down.

Sometimes, I just pull the jumper between driver and final, to add an attenuating resistor, reducing the signal from the driver to final, lowering dead key (and max P.E.P.) without mutilating the AMC/Waveform when you modulate the carrier. When I add the resistor I usually have to adjust the AMC VR to compensate. (So it doesn't swang into distortion)

73
Thanks for your input! True,it only is similar to the NPC-RC mod done to radios with the AM regulator instead of a transformer. Using the 100 Ohm resistor keeps VR4 from cooking off, I've not had one come back that burned up.
 
the one that leapfrog posted works a lot better for a 29 variable. does not burn up a pot .
As stated previously, adding the 100 ohm resistor prevents the VR from burning. I find no difference on the scope or spectrum analyzer doing this mod or the "mod kit" cap+resistor mod. I find it to be a cleaner modification and it also allows for easily setting the desired deadkey without having to spend the time wiring up a variable using the transistor method. Less cost forwarded onto the customer as well. I do this mod as part of my standard tune up for the 29, which as no two are the same, you can always drop the carrier to whatever you like, I set the one's I do to a 3 watt dead key. Never have had one come back with any problems. :)
 
The mod leap from posted works extremely well, especially if the radio is tuned properly afterwards.
If you did not notice, there is a 100 ohm to Watt resistor in series with the wire jumpring from VR4 to the solder point on the other side of the board. Of the thousands of 29 and 25 is I've done this mod to not once has the VR burned up. Please read the full modification.
 
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