Rhodes Chroma Polaris

Hi folks,

i've got a problem with the master volume slider
it doesn't work when i try to regulate the volume.
but it looks like the physical fader is ok, 
the "tune one" works. i did this LF D4 check
and the LED lits when i turn the slider up.
(even though i don't know exactly whta this checks? that there is an amount of 256 volume steps available?)
any ideas for that issue, cause changing the fader seems to
be a bit complicate..?

cheers Harald