I would have a look at the ribbon connector from the keys to the circuit board. Check it is in place and not loose. Also, dies the demo work? or the drums... Is there any sound at all out of the speakers? (even a low hiss?) If demo or drums sound then that's good news... means that the d to a converter is working and the digital side is ok. Worst case is the keyboard interface chip (not common).. If no sound at all then it could be the power amp. more info needed.
The problem isn't usually the board.. its the rubber contact that either need replacing or cleaning. Clean with pure alcohol.. I use Isopropyl alcohol and a lint free pad like chamois. Don't use cotton bids. If the fault is on the first octave the part number for the contact is V3413800 Top Octave? V3413700 All the other octaves: V4313600 If it is the board its in two halves... lower half. Pt number is V3414200 and the top half is V3414300 Contact Yamaha in your country and ask for spares dept, In UK the number is 01908366700 You can pay by card. cheers Iain (the engineer) Keyboard Repair Centre, Birkenhead. UK