I wired mine up just before and did some testing. I was hoping I could leave the gamecube c-stick on the board it comes on and just wire to the 4 wires coming off it.. ..but that didn't work, for some, now, obvious reasons
after I removed the analog stick from the board and wired directly up to the pots pins it worked but the values were all wrong. the N64 controller went: left: 0 - - middle: 128 -- right: 255 in values (values from a joypad controller mapping program called 'Controller Mate' on OSX) but with the c-stick hooked up the values were something like: Left: 78 - - middle: 128 - - right: 138 ..so as you can tell didn't work too well, tried it out on Super Mario 64 and you can only just walk. However, I didn't run into that Diagonal problem Diminuendo had. Although diagonally down to the left might not have worked, couldn't quite tell, but I think it was fine.
So my solution! I just popped the pots off of the C-stick and took the Pots out of the analog unit that came with my N64 controller. I'm just going to attach the N64 Pots to the C-stick. Will take some hot-gluing, but shouldn't be too hard, although for some reason I don't have a hot glue gun! So I'm gonna go out and buy one tomorrow and get this thing working!
