I've done various hacks and mods on my RC gear over the years. Starting with this one, I'll be posting about them as a series.
The Sanwa SD-5G is one of my oldest 'reputed brand' radios. First, I modded it for accepting a LiPo battery. But then it fell out of use due to lack of firmware features. To overcome that, I decided to replace the main board with an arduino.
Most of these old branded radios use 'uncommon' transmitter chips but they all accept ppm input. So, once you find the ppm input pin, all you need to do is analyze the signal and generate a similar one using a microcontroller.
There were about 15 wires between the main board and the transmitter board. It took about 2 hours to find out what each of those wires were for. Back when I did this hack I didn't have a digital oscilloscope to study each of those wires. But, there's an easy method to find a ppm signal. Check each wire with a voltmeter (multimeter) and one of the wires with a fluctuating voltage between 1 to 3 Volts is likely to be carrying the ppm signal.
After shortlisting the wires all you need is a ppm reading sketch on an arduino to find the ppm wire. For the SD-5G, the 4th solder-pad from the right (bottom of the transmitter board) is where the ppm signal goes. It's shown with a red arrow in the attached picture.
I found out that the 3rd pin from right (at the top) needed a voltage of 0.01 volts to bind with the Rx. I used a voltage divider for this. The left resistor is 10k and the one on the right is 22E. (Shown in the attached pic)