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