Few days back I was planning something quite different but ended up fixing a broken Sega Saturn 3D Control Pad – the analog variant of Saturn controller that came bundled with Nights into Dreams.
This one was only slightly broken. It was otherwise ok, but the other analog trigger did not response at all. I remember opening it up in the past, but now that I took second look inside I realized what was wrong.
The magnet. It had fallen off from its socket and attached itself to the other trigger’s magnet.
Unlike just about any other controller with analog functions, this controller is making use of magnetic sensors instead of the usual potentiometers. There are sensors that measure changes in the magnetic field as magnets move closer and away. Both the analog triggers and the “3D” pad work this way. The advantage of this is probably that they don’t wear over time as there is no mechanical strain to the sensors itself. The much similar Dreamcast controller uses this same principle.
So fixing this pad was as simple as putting the loose magnet back to its socket.
Update (22nd July 2019): These magnets need to be facing the right direction – check details posted in comments by retron8.