Korg Poly800/EX800 Users

Hi,

I have a poly 800 MK1 and have a problem. The 4 key doesnt make any 
sound or send midi data, this is the same every 8 keys (4 12 20 28 36 
44). From the circuit diagram, I see they are all on the same track, 
but am unsure as what the cause could be...any ideas?

Lander =).