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Uniden pc68xl low power on one end of band

twa77

twa77

Well-Known Member
May 2, 2008
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South Dakota
i have a uniden pc68xl when I tune the tx cans for max power on 20, I'll have 4w key with 16w modulated. Then when I go to channel 40 or 1 I'll get like 1.5w with very little forward power. The rf meter on the radio also swings backwards on 1 and 40, and when on 20 operates normal. Is there something else that needs adjusting to even power on both ends of the band?
 

I got this thing on Ebay and the core was gone from L10 , and the limiter was cut. I put a core in from a junk radio and replaced the limiter.
 
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twa77 said:
I tried again this morning but I get it good on one part of the band , but it will be bad on the other end.
Click to expand...


This Link Is the factory service manual for a 25 GTL, should be real close to your 25.
A chart is in the file, I didn't see any mention of balancing the output between channels 1 to 40; but the chart has channel and frequency listed to verify the VCO/PLL is correct.


http://downloads.cobra.com/CB/Models/25-SERIES
 
L18 should be the balance for the upper and lower ends of the freq band. it is a 2 slug can. it has been years since I have done a 25LTD but from memory if you peak it with a scope as said in the service manual it can throw the power off on each end of the band.
I am assuming you have a scope to adjust the radio as the service manual states. one slug does the lower channels and the other does the upper channels. so if you have it peaked on the middle freq could be your problem.
 
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to adjust these properly you need a 8 digit freq counter and a 50mhz minimum scope. if you look at the procedure for adjustments it shows what you need to get the radio aligned properly.
 
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sonoma said:
to adjust these properly you need a 8 digit freq counter and a 50mhz minimum scope. if you look at the procedure for adjustments it shows what you need to get the radio aligned properly.
Click to expand...
Did not know that about that dual coil. But I've never had a channel/power distribution problem after peaking the coils either. I'll amend my notes. A shame that the SM didn't make that distinction - too.
You da man, sonoma!

twa77:
I agree that is important to make sure the radio is on freq, even if it is an AM/only radio, and a scope is only way to be sure that the peaked power is clean and not distorted.

L10 is another failure in the SM notes, as it is used to limit the 54mhz harmonic. One does not want/need to count watts on the meter that don't even work on CB freqs! So, since it was jimmied with, you have another problem to deal with - that a scope cannot even see. It needs to be adjusted with a Spectrum Analyzer and someone that knows how to use it.
 
cobra 29ltd has the same type coil in it and people will turn it and throw the power way off. on the 29 the dual coil the left side effects power output on the higher channels and right side the lower power. you can move the slug about 1/32 of a turn and it will throw the radio way off. some radios you can just bump the slug and it will change the out put a lot. back years ago some of those dual cans even had a piece of tape over them and said do not touch.
 
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Looks like I'm in over my head lol. I'll have to have someone with the right tools do it. Thanks for all the help all of you.
 

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