My DX7 mk1 behaves strangely. The lowest octave of keys has a velocity problem. Sometimes velocity works, sometimes works for a second, then not, then again and again not, sometimes it doesn't work at all. When I give it a love slap it starts working again. I opened it up whe I decided to fix some key that had different response than others, but I couldn't really locate the problem. I thought the problem is in the long ribbon calbe going from keyboard to the main board, but I'm not sure.
The other problem is that when I set the pitch wheel range at more than 3 it never returns to zero (when I swing the wheel forward and releas it the pitch ends up one or two semitones higher). It's not a big problem, because I usually have it set for 2 semitones, but it's always good to have more options.