I'm afraid that I don't know where you can find a service manual for this instrument either. I suppose you could try to contact Yamaha and ask if they could sell you one or perhaps they'll give you one for free if you ask nicely. Anyway, if you can tell me what problems you're having I might be able to offer some advice, I have worked on a number of synthesizers over the years. I can't say that I'm an expert, but I usually know how to get things working again.