I will have to try carefully modifying the Vex RC using the information that you provided and will post the results. There are still a lot of low cost Vex RC remotes for sale on E-Bay that could be used for experimentation.
I was able to connect an external 5K POT to the Vex RC remote but found that the values read by the Vex Controller ranged from 0 to 50 instead of 0 to 255 as I would expect. I need to experiment some more and will publish my results along with a schematic when I get promising results. The modifications to the Vex RC remote are not very difficult other than carefully cutting the center wire to each internal POT and connecting it to a 2X10 Pin header used as a switch between internal and external resistive devices.
I found out that a 100K linear potentiometers work best for this application. I was able to read digital values between 0 and 255 from the Vex Controller using an external 100K POT instead of the internal joystick channel 1 X-Axis POT.
Er…I noted this is another topic covering range and antennas, but make sure you don’t modify the actual transmitter itself. It’s against the law CFR 47 part 95.201-95.255. I’m not sure if the FCC considers the entire box a “transmitter,” or if they just consider the actualy transmitter PCB the “transmitter.”
I am now able to control the speed of two motors independently using two external POTS. I did not touch the RF Board for this modification. All I did was to switch the internal joystick POTS with two Radio Shack external 100K linear taper POTS using wire-wrap wire and pin headers… The second joystick is left as is, but can also be switched with two additional POTS if needed. These changes are completely reversible by jumpering between the internal and external POTS.