As of January 2010 mods and parts are no longer available for the STUART.

If you want to inexpensively interface a standard RC radio system to your Stuart or Abrams, then this PIC based mixer may be your answer...

The 49mhz receiver board and decoder pins
The 27mhz receiver board and decoder pins
If you're looking to install a different 4+ channel radio in the 1/6th Stuart RC tank, or the 1/9th Abrams, I have a skid-steer mixer based on the 16F628 18 pin microcontroller that allows direct interface. No servos or other electronic switches are required. The only external components required along with the microcontroller is the ceramic resonator, a couple of caps, a couple of resistors, and eight 2N2222 transistors or equivalent. Here's the 27mhz Stuart schematic (right click, download and reopen to see the picture full size), and here's the 49mhz Stuart schematic. Finally, this is the Abrams schematic.

The microcontroller takes 4 channel input (two sticks typically) from a standard Futaba or Hitec (negative shift PPM only) radio (AM or FM). The right stick controls driving (fwd, rev, left, right). The left stick provides switching for 4 functions (controlled by stick movement up, down, right, left). In the Stuart and Abrams, the turret left and right can be controlled by left stick left/right. Left stick up and down can be used to actuate the cannon and MG firing.

The only mod to the tank's receiver board is to remove the decoder IC (the PT8A991A in the 27mhz version and the RS2599 in the 49mhz Stuart and the Abrams) and connect the buffered outputs (transistors) from the mixer to corresponding pads in the receiver. Price for the programmed PIC ONLY is $20 (US shipping included).

I do offer complete mixers ready to plug into the receiver and solder to the tank's decoder board for $65 plus $3 shipping in the US.
Here are installation instructions.

I use Paypal. Simply Paypal $20 (which includes US postage) to and indicate you'd like a programmed skid-steer PIC. Note that assembling the mixer from the PIC requires skill with laying out, assembling, soldering, and testing an electronic circuit using CMOS devices. If you're not familiar with electronics, don't try this conversion.