Lester's Custom Truck Shop
United States
Webmaste
This is an inexpensive kit for the Cobra 25GTL/LTD etc. and similar radio's such as the 21GTL.
The first thing we need to do is isolate the radio receive and transmit mixers from the 10.240Mhz signal coming from the radio's reference oscillator. But we still want the reference signal to reach the PLL. The PLL uses this 10.240Mhz signal to keep our channel spacing at 10khz.
Look at the pictures below. You can see where I have cut the printed circuit run on this 25WX to isolate the mixer circuits. *
* You could also lift the end of R69 and C13 that connect to this point then connect them together. In this case you would connect the green wire to this junction vice JP15.
Now we need to inject the new frequency at this point to mix with the VCO output. JP15 is the perfect place to do this. Connect the green wire to JP15 as shown.
The next thing we need to do is get the newly generated 1st I.F. signal past the 10.7Mhz I.F. filter CF1. We will do this one of 3 ways. These methods depend solely on the radio you're converting;
1st method is to connect a 47 to 100pf disc cap across CF1's outer two pins. This in effect broadens CF1 bandwidth. But still allows it to work somewhat.
2nd method is to bypass the filtering slightly using a 47 to 100pf cap placed from the input winding of L3 using the open terminal pin to the output pad of L4. See the pictures below.
The 3rd method is to use both of the above methods as seen below.
There is a 4th method but I do not recommend it. You may need to use it though in extreme cases. That is to jumper CF1 out completely. That will leave L3 and L4 to do the 1st I.F. filtering. Which is to say hardly no filtering at all. You are in effect changing a dual conversion radio to a single conversion almost. The performance will be about the same. So, not recommended.
What all of this is doing is broadening the passthru of the 10.7Mhz I.F. filtering stage. We need this broader passthru because we're moving the 1st I.F. either up about 450khz or down about 450khz. And this will affect adjacent channel rejection. If your radio doesn't have a 455H CF2 filter, you may want to swap it out for one.
Here is where a CB City 455 Channel Guard would really shine. Just replace CF2 with it and this would really help ACR. But I don't think Lou sells these anymore. You may be able to find an old one somewhere.
I am looking at a selectable I.F. filter but would the cost justify building it? That's the problem.
Of course you will have to realign the receive stages for best coverage from top to bottom.
Ok, next we need to connect the yellow wire to pin 1 of the PLL for the 8 volt power connection.
Connect the Black wire to a good ground.
And you should have already hooked the Green(output) to JP15.
Mount the board and the band switch in convenient locations then power up the radio.
Put the Band switch on the Lo position and align CT1 for 26.945 on channel 40.
Switch to the center position and adjust CT2 for 27.405.
Switch to the Hi position, channel 1, and adjust CT3 for 27.435.
Frequency ranges**;
Lo = 26.505 to 26.945
Mid = CB 26.965 to 27.405
Hi = 27.435 to 27.875
**Subject to change without notice.
With the radio in receive, and connected to an outside antenna, adjust L3 and L4 for the best overall receive coverage. Then proceed doing the same with the remaining coils in the receive path. Adjust L1,2,5,6 & 7 for maximum audio output & minimum distortion, from top to bottom.
You're done! Enjoy your work.
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Lester's Custom Truck Shop
United States
Webmaste