Does anyone have experience fixing these keyboards? Some buttons work fine, others work only when pressed really hard, and others don't work at all. I've taken everything apart and I can see nothing obviously wrong. I'm usually pretty good with fixing my own stuff, but this has me confused. It has been in storage for the last 20 years or so. Everything else about the system seems to work fine. I know that Best Electronics sells them, but I'd prefer not to go that route if at all possible. Any help would be greatly appreciated!