The Cobra 29 series has a different PLL than the 25's do.
The PLL in a 29 uses an offset mixer of 15.360 MHz, or 3/2 of 10.240 MHz (the reference oscillator of most PLLs used in CB), that when injected with a higher or lower offset frequency, like 15.810 Mhz for "Uppers" or 14.910 MHz for "Lowers", the PLL will track the new frequency band. Hence, the Expo 100 "A" kit or any clone of, will work great for that radio.
The VCO frequency for Channel 1 Receive is 16.270.
16.270 - 15.360 = .910 MHz, which is what the PLL is programmed to lock to on CH 1 RX.
To get "Uppers, we change the offset frequency to 15.810. The VCO now goes to 16.720 on Channel 1.
. VCO . - . offset
16.720 - 15.810 = .910 again!
To get lowers, the new offset frequency is 14.910.
The Cobra 25 series, uses a PLL that can directly "read" the VCO frequency, so there is no offset mixer to fool into changing bands.
But, I figured out a way to do it for a 25 series and other radios that have "impossible to crack" PLLs. I haven't finalised the design yet.
Just today while I was at work, I had another idea on how to fix the Receive IF problem with the Expo 100 "S" Kits that I mentioned above, which I have to do some research on before I release my findings.
