The controllers are drop dead simple.
If you have a wiring diagram, thats all you need. **Note, most diagrams out there have the diodes labeled backwards**
Most multimeters have a diode tester, I've yet to see a diode go bad.
Most of the time, it's the "leaf" spring contacts or fire button that are having issues. I find cleaning them, etc usually works.
I tear a very small strip of 600 sand paper, and pull it through the connection, once up and once down.
The biggest issue is the cords. You say you tested it, I find most are broken right beside the rectangular block.
Most often it's an intermittent connection. It comes from them being yanked out of the socket by the cord.
The other big issue is the push on connection to the PCB, they usually oxidize, I usually add some solder and attach them directly to the board.
If you have a multimeter and a soldering iron, you can fix the controller.
The keypad?... it's another story. (then your into silver conductive pens, etc... )