Uniden PC68XL (turn DK down)??

[Mudfoot]

I Picked up a nice looking 68XL today. I found the limiter was cut, so I stitched it back together. It keys around 8 watts and peaks around 20 watts.

I want to turn it down to 4 watts. I've been looking, but can't find the DK reversal instructions. All I see is how to lower it with a resister, or add variable.

How can I turn this back down using common tools a 20 y/o woman would own?

 

[Shockwave]

This radio does not have a carrier control. Whatever was done to make it key 8 watts was not an adjustment. There are so many mods around it's hard to say for sure what was done. Could be anything from changing the final to a MOSFET to jumping the inverse spike protection diode between the modulation transformer and the final.

If you only want to lower the carrier but not the PEP swing, it's a little more complicated because you have to separate the DC power from the audio power feeding the final. The simplest way to do this is to install a resistor inline before the RF choke feeding the modulated DC into the collector of the final. Then add a 1000 mfd cap in across the resistor so that the modulation passes through the cap and not the resistor.

To make it variable you replace the resistor with a transistor and connect a potentiometer to the base to vary the voltage. If you can give up some swing, you may be able to detune one RF stage to reduce the gain. Don't do this on the final stage because it will run hot detuned. Try it on the driver or pre driver if you're looking for a short cut.

 

[Mudfoot]

Sounds easier and more practical going the variable route. I need to scan the board and see what all was done first.

Don't really want to invest much time in it.

Thanks for your help.

 

[Z-MAN 369]

I think it's the same as a Cobra 25, just do the cap & ras mod with a 68 ohm res, should be around a 2w carrier. Skip the variable because it might be unstable, no big difference from a 2-8w besides running a linear, so set it at 2 and put the covers back on, good luck.

 

[Mudfoot]

Thanks, I'll look into trying that.

 

[Grim Reaper]

with 8 watts dead key it might take closer to a 80 or 90 ohm resistor to drop the dead key to where you want it,but the cap will still allow it to swing right bak up.and using slightly diff cap values is not only fun to exp with but also depending on your voice tones will make swing faster or slightly slower,

 

[Z-MAN 369]

The slug in the back might be turned up too high also, my 29 classic was keying at 8w swinging fake 33 ghost watts. L14 was almost falling out, must of been a truck stop tweak & peak, lol. Now it's 2 swinging 17w, perfect for an amp now. If you use the RF gain or SWR cal for a var. it becomes very unstable, creeps up whenever it wants, not cool or safe for a precious box. Personally i found the 2 w carrier was good enough for barefoot talking. I first tryied a pack of 100 ohm res, way too low. Just trying to save ya $2, lol.

 

[Robb]

Why not add 3-7 diodes in series after the modulation xfmr?

I would not detune ANY of the TX coils to lower the output - either; that would be counter-intuitive to what you want to accomplish.

Find the modulation transformer. You'll see a diode in series with it going to the final and driver. Remove one end of the diode, and strap 3-7 more in series, and go from there.

Will lower the deadkey, but keep the pep ratio the same. Makes for a REALLY loud radio.

--Toll_Free

http://www.worldwidedx.com/cb-band-cb-radios/88445-help-need-lower-deadkey-2-watts.html

 

[Robb]

IIRC, you can also put a 2k/ohm VR between the collector and the resistor that feeds the collector of the pre-driver transistor. Not to be confused with the driver transistor - which would require using a voltage regulator. Easier to just put it on the pre-driver than the driver.

 

[AA1QD]

Can some one give me the readers digest instructions for this 3 diode mod on a cobra 29? I tried to read the other post, but my A.D.D. kicked in so hard I almost passed out. What do I use for the diodes? like 1N4001? or 1n4003? And do I do a standard peak first? sorry I just got lost in the discussion.

 

[74IN]

I prefer the variable mod, using the RF Gain control, adding a TIP31 and cap.

Not much more work.

 

[AA1QD]

I prefer the variable mod, using the RF Gain control, adding a TIP31 and cap.

Not much more work.

which mod produces a louder radio?

 

[AA1QD]

Well I tried the 3 diode deal. I have a old cobra 29ltd that gets every thing new I try done to it. I first did the standard peak, stock final. Ended up with about 6 watt dead key. Then I strung together 4 1n4004 at d8, brought the dead key down to 3 watts, swings to 6 average power, and swings 35 pep, actually hits 38 and drops back to 35. I don't have a scope so I have no idea how the waveform may look. Now If I do the rc mod using the same radio, jumping d8, and using a 1000uf at jp36 and the tip31 variable power set up, I see more forward average power (about seven watts from where ever the dead key is set) but less pep. Don't know. Without a spectrum analyzer I guess it's kind of a toss up. Doing the diode string is a lot less work.