Vintage Synth Repair

Hi. I'm writing on behalf of a guy who bought a Roland JP-8000 from me not long ago. The synth was in mostly great condition, but he claims that the ribbon controller is giving him trouble. The JP-8000 has a "hold" feature on the ribbon which will hold the current position when you finger is removed. He showed me a video that demonstrates that this hold feature isn't working when the button is pushed on. When the button is off, the ribbon seems to engage a little sometimes by itelf making the sound a little quirky. Does anyone have any idea if this is a serious problem or indeed a problem at all? Could it just be some parameter somewhere which interferes? If you think the ribbon is indeed faulty, how would one approach repairing it? Easy or difficult fix? Please help! Thanks.