Now for a description of the hardware. Here is a Merit Megatouch XL 'Telco' brand motherboard. It has 3 pieces, the motherboard, the I/O board and the riser board that connects the two.
I've outlined the CR2032 motherboard battery in this picture. It's a good idea to replace it, because if it's the original one it will either be dead or close to it.
The motherboard ports from left to right are: keyboard, mouse, com1, com2, parallel and video. The touchscreen goes into com1, the original Megatouch also has a network jack that plugs into com2. I don't know what the parallel port is for.
This next picture has 4 things outlined.
The 2 sets of jumpers at the top of the board should be shorted if you're using the original CD drive, and open if using a hard drive.
The lower right red connector is where the speaker is connected. Connect one lead to the top pin, and one lead to the bottom pin.
The blue object next to the speaker connection is the volume potentiometer. There's a volume control in the software menu, as well as this hardware volume control.
The bottom center white connector is where the pins are that connect to the buttons for bringing up the software setup menu, touchscreen calibration menu and coin input.
Here's a closeup showing where to wire up buttons to be able to access the software setup menu, and the touchscreen calibration.
Just connect one lead of a momentary pushbutton to a ground, and the other to the specified pin. I'm sure one of the other pins will add credits to the game, but I've got mine set for free play so I didn't bother trying to find which one(s).
If you don't have a button handy, you can strip both ends of a piece of wire and carefully touch one end to a ground pin, and the other end to the setup or touchscreen pin.