Cobra 29LTD channel mod questions/help

[LeapFrog]

Time for my slice of humble pie.
Honestly I thought the S kit and the A Kit worked along the same principle, a new crystal and an oscillator.

I thought the only difference was the crystal cut, being determined by what model radio it goes in and what direction you want the new 40.

Are all Expo 100 boards the same, and the letter designates what model they work in?
I'm going to buy Lou's book, the one I mentioned.
I don't have a copy, and I'd like to read more about CB frequency synthesizing + mixing schemes.

 

[nomadradio]

The "S" plus or minus kit is unique to that particular PLL chip.

Early Expo 100 kits included versions that had only resistors, wires and diodes for unlocked chips like the uPD858 and PLL02A.

Those disappeared in the early 80s, and only the models with either two crystals or two crystals and an oscillator circuit remained in the product line until they ceased production altogether.

73

 

[Handy Andy]

Not all hope is lost.

CBPhreaker kinda touched on this before - but only in passing...

Dealing with a 148GTL Receive change...

I went back and located the board that pic montage came from ...

Yeah, 9106 but the other issue of the "IF Filter" may seem odd but these things are nothing like a Xtal you'd find these days for narrow filter stuff.

These units the one pic'd below - we used to sell at Radio Shack anyways, they were simpler resonate filters - similar to Xtal but without a lot of "poles" to trim, pare-down or otherwise narrow the response curve. They are quite broadbanded. They were called Ceramic Filters - not Crystal.

They are three legged yes, but they were not very selective - to even use them in a radio - you could only get by with about 20kHz spacing in FM use for the radio platforms at the time.

Radio Electronics did up articles in their hey-days using off the shelf parts - this was one of them. Always used them to help some guy lost or had broken equipment - kinda like MacGyver - only with old spare parts from old radios and they needed to build something fast...one of the articles was to convert an older FM radio to use WX and some TV band - so they elaborated on the issues of Ceramic versus Crystal and how one can be used to obtain a signal while the other you took out and swapped in this ....

 

[Handy Andy]

Why the above, because the radio that kit came out of is using one of those.

So to have that in there, the performance is not a requirement - you need to make a triad of caps to finish this job. The coil and a dropping resistor(s) then can be used to obtain the image you seek - it'll be crappy and not as clean as with a resonator - but you can use a set of caps, and some input and output resistors to make a filter to obtain the bandwidth...