Breaking out the Mame Cab from storage to turn into an X-Box Cab, so I thought I'd share the information I've gleaned by doing this. I've torn apart a few pads, but the one that works the best, in my opinion, is the Intec Cyber Pad 2. It's about $19.99 anywhere you go, so it's not very expensive.
When you tear it open, it's not very complicated inside. The usual control PCB setup. All of the pads are very, very easy to solder to, as well. Here's a simple little helper datasheet that I managed to get together.
First thing's first, make sure, if you're going to use it, to unsolder the rumble motor leads and the analog triggers. The analog sticks can be undone as well, but it's not neccessary. You can just remove the tops to the sticks and leave it at that.
Note: All connections do not share a common ground. Grounds are shared between D-Pad button groups, ABXY/WB Button groups, and the start/back buttons.
DPAD:
Very easy, small nodules here and there off the main traces, but otherwise uses MASSIVE half-moon shaped pads of copper.
For Up/Down, the Left pads are ground, right is signal.
For Left, the Upper pad is signal, lower is ground
For Right, lower pad is signal, upper is ground.
Buttons:
A little different. They all have carbon over the upper left and right pads. Just scrape it off with a hobby knife. If you look at the connections, it's roughly an inverted "U" shape.
Lined up like so:
Wh/Bl
Y/B
X/A
They all have four pads as well in a square shape. The upper left and right are square, the lower left and right are rounded and look like droplets almost. There's only one exception, which is the upper most connection, white, which is turned upside down. The rounded points are on top, while the lower points are squared.
We'll only be working with the Square points that need to have the carbon scraped from them, so IGNORE the round droplet shaped points.
Now that you're familiar, all inside points are a shared, common ground. All outside points are signal.
Ie. Wh/Y/X, left is signal, right is ground
For Bl/B/A, right is signal, left is ground
LTRIG/RTRIG
Simple. There's three points moving in a vertical line.
Upper being one, center being two, bottom being three, on both Trigger points. Connect the bottom two points on either side and it registers a full depression of the trigger.
Start/Back
This is on a little daughtercard attached to the main PCB, lifted up. Ignore ALL the pads, as it's totally unneccessary to solder to any of them. If you look at the bottom, moving from left to right, there are eight pins, all of which are crudely labeled. What you want is pin 1, 2, and 4.
Pinout:
1: Start
2: Back
3: VCC (+3.3v)
4: Gnd
5: PL (Program function, no idea)
6: Turbo Function LED (Connect to ground, it lights up?)
7: Turbo Function
8: PRO (Possibly Program)
By grounding pin 1 or 2, you make the START and BACK signals, so you won't have to solder to the strange, easy to mess-up pads of Start and Back.
So, there you go. A nice little write up on how to hack a Cyber Pad 2. Hope it helps some folks.