I had a PSR-510 a long time ago , and the spring on the pitch bend somehow came off,
So I just turned it upside down and removed a bunch of screws , took the back off and put the spring back on he wheel.
It work great after that and never came off again !

But having both the pitch bend and the modulation wheels not working at all , I don`t know , that might be more electronic than mechanical.
Could be that the wheels work mechanically , but no difference in sound is produced.

I was going to ask that question , but , if an S900 is in my future , I want to buy where I can return it if need be.

Better paying a few dollars more for piece of mind !!