Yesterday was really, really annoying.
I built a prototype of the Neo Geo adapter board. Took about an hour. Plugged it in, realised all the button mappings were wrong. Looked at the 74hc157 datasheet again and noticed that I had connected all the multiplexing backwards. That means the schematic and PCB are both wrong… Built a NOT gate out of an NPN transistor to invert the data select line on the prototype.
All looks good except that the down and left d-pad directions don’t work.
Checked all relevant connections. Re-flowed a lot of solder just in case of dry joints. Gamepad clicks nicely so sounds like the switches are okay, but a multimeter test proves otherwise. Google the problem, see that it’s pretty common, in fact 75%+ of these pads suffer from the same issue. Turns out SNK used switches that re-define the meaning of “cheap and crappy”.
Apparently you can open the switches up and clean them. They ever never designed for that though, so it’s not easy. Tried it anyway, after all what do I have to loose? Ended up with mostly-working-if-you-push-hard-enough d-pad which doesn’t click in either of the repaired directions, making it feel rather weird.
I wish I had bought a Neo Geo joystick instead now, but it was too large to bring back from Japan with me. Should have posted it.