Il Mio Problema
My old GameCube controllers had never been cleaned in the 14 or so years I've owned them. Decided to finally do something about it after seeing a video.
La Mia Riparazione
It went surprisingly well, heh. I was afraid of frying the circuit boards both times.
Il Mio Consiglio
Definitely come to this site for specifics on how to fix whatever you need.