Yamaha CS80

i bought a near mint CS60 yesterday, looks nice with my CS80.

But the CS60 has a few (minor problems) the aftertouch for instance is 
always on. Any touch response settings are directly audible, when i 
press the aftertouch nothing happens. I know that the aftertouch works 
with a light bulb. I opened it up the light bulb still works, if i put 
a piece of paper between the light and i think the sensor then the 
touch reponse stops. So somewhere there is a fault perhaps the sensor? 

Does somebody have any experience with this problem, can i fix it 
myself or do i have to bring to a (experienced) technician.

Thanks in advance,

Alessandro