The Phillips FM320 UHF CB has the facility to be locked to a particular channel. This is controlled by the existence or absence of a number of diodes on the circuit board. I’ve been looking into this for a telecommand project I’m working on. I wasn’t able to find the information of how to do this anywhere else, but it was pretty straight forward to figure out.
This post describes how modify the FM320 radio to set the nominated “NOM” channel.
Here is the relevant part of the circuit diagram:
The diodes are IN4148. These are commonly available.
Remove the radio cover. With the controls facing you, remove the bottom left and top right screws (Phillips head). Turn the radio over and repeat. Both covers will now just pull apart to expose the circuit board with the controls and backplate still attached. The location of the diode links is marked below:
The table below shows the effect of the diode link being in place. The diodes are listed from left to right as in the above photo and on the circuit diagram. Note that diodes have a specific polarity. Use the other diodes in this photo as a guide for the correct orientation if you are not familiar with them.
Note that these are additive. If you have links in place for D34 & D35 you’ll get channel 30. If you need channel 22, then solder in the links for D35 and D41.
Assembly (as they always said in the Gregory’s car manuals of my youth) is the reverse of the disassembly procedure.