This mod is certainly possible with PLL02A based CB
radios. I have done it with two radios: A HyGain 1 (model 2701) that I also had modified to 10 meters, and a Midland Export radio model 7001 that I left otherwise original. Both work great with this mod.
Firstly, it needs to be a radio where the MSB bit of the PLL chip (pin 7 on the chip) is made permanently LOW from the factory (pin is grounded), meaning the synthesizer's programmable divider range in stock condition is only used for half its range. All PLL02 based CB
radios that I have seen in my life (and it was a LOT!) indeed have this condition, so you should be OK here.
What you need to do is two things: (1) Make pin 4 of the PLL chip LOW, meaning grounded (most radios do not have anything connected to this pin because the PLL02A has internal pull-up resistors, so nothing connected means the pin is HIGH)
Grounding pin 4 throws in an extra divide-by-two in the internal reference divider. The result is that the reference frequency on the internal phase detector will now be 5 kHz instead of 10 kHz. remember; the reference frequency on a PLL phase detector determines the ultimate step sizes the sysntesizer can make (which is now 5 kHz the way we want it)
However, you're not there yet. Now comes (2) that I promised you. The programmable divider coding also needs to be changed. Good news is that this is very simple. You need to shift the connections that go from the channel swith to the PLL chip all one pin lower, all of them. Remember from your schooldays (if you had them): Dividing (multiplying) by two in the digital domain is a logical shift right (left) for all bits.
So in practice this means that the switch wire that went to pin 15 (the LSB bit) now goes to pin 14; the one that used to go to pin 14 now goes to pin 13; the one that used to go to pin 13 now goes to pin 12 and so on.
Some of the MSB bits probably did not have switch wires going to them but were either tied permanently to logical 1 (5Volt) or logical 0 (0V) instead. You need to shift the logical state of those as well. For instance, if pin 9 was permanently ONE, pin 8 was ONE and pin 7 was ZERO, pin 9 now needs to become whatever pin 10 was (probably a switch wire); pin 8 will remain ONE (because pin 9 originally was), and pin 7 will now have to be made permanently ONE because pin 8 was.
Sounds complicated but it really isn't; it is basically a logical shift left (one position shift in the direction of decending pin numbers)
Now that you have done this, you will have pin 15 free (the LSB bit of the divider). You need to connect that one to a newly to install toggle switch (use the TONE HIGH-LOW switch if it was there) such that it either is open (then the internal pull-up ressitor will make the pin a ONE) or that it is grounded (making the pin ZERO)
The ZERO condition is the normal frequency for the channels (i.e. channel 19 will be 27.185 as before, and so on for every other channel)
Throwing the switch to the OPEN position will increase the division number for the synthesizer's programmable divider by one, and that will most likely result in the frequency going up 5 kHz (so 17.185 will become 27.190 MHz) . for some radios, the frequency might actually go down 5 kHz when flipping the switch, this depends on what mixing scheme is used in the synthesizer mixer (this is not the same for all radios)
Quite simple, don't you think?
Now comes the BAD news.
Not all PLL02A chips are the same. The PLL02A was originally based on the Motorola MC145109, which chip has the ability to throw in the extra divide by two in the reference divider when you ground pin 4. For some PLL02A chips, pin 4 is internally not connected. So your efforts will not pay off in that case.
The way to solve this is as follows: Solder out the chip, clean the pins, and put an IC socket on the board. In this way you can test different PLL02A chips until you find one that works.
Your solder skills are sub-par to do this? I will give you some solder tips.
First, get yourself a soldering iron with a fine tip (not too fine though, no SMT type) on which you can set the temperature. Most soldering goes wrong because people burn the PCB board with their solder irons, the types intended to repair the barn door. Don't use those big-uns.
Set the temperature for 300C (570F), not higher.
Then, get yourself a good solder sucker and solder wick, to remove the solder on the IC pins. Also, get yourself additional solder paste flux, rosin based (I use Kestler rosin paste flux formula SP-44)
Before you start desoldering, put rosin on EVERY IC pin, don't be skimpy, you can clean it up later. then, warm up the pin until the solder around it is FULLY melted and then suck the solder away with the piston based solder sucker in a single motion. Don't worry about overhearing things; with the tip set for only 300C AND the temperature distributing effects of the rosin you won't destroy the traces, nor the chip.
One other important thing: NEVER put mechanical stress on a PCB trace that is still hot. You WILL pull off the trace.
So if some IC pins are still not free after your sucking action, use the solder wick to remove the last solder.do NOT put mechanical stress on the pins (by pushing or pulling) while the solder around it is not melted properly yet, you WILL pull of the trace.
If you work like this, you can solder on phenolic PCB boards (like most CB
radios have) many times without any problem.
The excessive rosin cleans up nice using lacquer thinner and Q-tips. If , after clean-up, a nice shiny solder joint becomes visible, it is proof you did things right.