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Cobra 29 LTD Variable Power Mod

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  #1  
Old 03-24-2009, 06:02 PM
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Default Cobra 29 LTD Variable Power Mod


When doing the swing mod for the Cobra 29, the CBRadioMagazine page suggests adding a variable power with the added 33 ohm resistor and disconnecting the RF Gain wires from the pot and soldering them together. Then, I am to link wires from both sides of the 33 ohm resistor and run them back to the RF Gain pot. But - wouldn't this increase the resistance and thereby lower the dead key to the point where there is no dead key watts???

Can someone explain if this mod is correct - and more importantly why?
I'm trying to learn what is happening.
I know they are acting as two resistors in parallel.
But how does two different resistors behave - when one is variable?
Add the resistance any given variable resistance point in the pot with the fixed resistor - and divide by 2?

In addition, what is the best single-final mod for this radio?
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Last edited by Robb; 03-25-2009 at 02:15 AM.


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Old 03-25-2009, 12:40 AM
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My understanding is yes when adding the variable pot (RF gain) to the circuit, one could draw the Dk down to zilch. However at the other end of the pot range you would have no resistance and effectively be bypassing the 33 ohm resistor to produce a range of zero Dk up to full power.

Here is another means to accomplish it that makes more sense to me.

Super-Talk

Last edited by theshortcircuit; 03-25-2009 at 12:53 AM.

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Old 03-25-2009, 10:39 AM
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Doing it with just the rf gain pot makes it pretty touchy. I'm thinking adding a little more resistance to it will make it more controllable. Personally, i don't like going a variable control in a CB (I never really needed to vary it). I'd rather just make the carrier control on a switch - set it to where I want it for low on one side of the switch, set it normal on the other side of the switch. This is all most people really need, anyway and it's so much easier to do.

Best single final mod for this radio? IMO, the 2SC1969 mod I posted in the mods section, if you can find the transistors. I've done this mod to several 29's and it is just very nice and reliable. For some reason, I like the way these radios sound better with the 1969's over the FETs. There was some discussion about this a a few years ago here. I'll have to see if I can dig up the threads.

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Old 03-26-2009, 12:13 AM
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Jeez I bout fell asleep reading all that.......too many beers I guess.

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