This 12-keys keypad failed after years of daily use. No schematic or replacement parts from the original manufacturer. The only option was to replace it with an off-the-shelf telephone-style keyboard. However, I had to discover (by reverse-engineering) the original connector pinout to adapt the new keyboard. Following the maze of each and every PCB track in a circuit is boring and error-prone, so I looked for an easier way to get the job done.
Reverse Engineering with Photoshop - [Link]