Vintage Synth Repair

Hi, the top 5 keys (intermittently) fail to sound on my D5. I can 
always see a time multiplexed signal for each key on the main board and 
can just about see a difference in pulse width when the second velocity 
contacts come in (but not as pronounced as for the working keys). I 
have a paper copy of the service manual but it does not describe the 
keyboard. Does anyone have a description or schematic of the keyboard 
operation please? Thanks in advance, Andrew.