I don't think there's going to be a whole lot you can do without opening it up. You probably have an internal component failure. You could get a schematic and hit the test points with your meter. Not a lot of players have those diagnostic and repair skills.
My suggestion is to contact the authorized repair center Yamaha can refer you to.
If you really think you're up to cracking it open on your own, get a schematic and forward me a copy - I'll help. If you've never soldered on a circuit board before I wouldn't consider it. It's possible to find something as simple as a volume pot needs to be sprayed out with electronics cleaner, but I doubt it.
One last thing - I wouldn't take it anywhere except who Yamaha refers you to.
Good luck, keep us posted - Jim