Korg Poly800/EX800 Users

Hello to all people in the group,

i've been watching for some time, but it is my first post here.

I have a poly800 mk1 with reverted keys.
Bought it used a year ago.
It was working great, and suddenly a few weeks ago I noticed that the tune slider is not working. The joystick is working, pitch and all. 
I used some cleaning spray and also did a visual check, but I didn't notice any obvious problems.

Any ideas what might be the problem? Should I replace the slider?

thanks in advance 
ilias