Yamaha CS80

Hi,

It is either the velosity key contacts(the ones on the right of each 
pair) or the TSB input curcuit for that key. There will be three wires 
to the TSB board for that key all the same colour. One of these wires 
is the velosity amount.
To make sure the key contact is not the fault press down the right 
contact while pressing the none working key down repeatedly to see if 
it does increse the volume, if so ajust the contact until you get the 
volume to match the other keys when hit at the same amount.

Hope this helps