thanks for all the replies unfortunately I don't think its the vca gain control because the cs 10 doesn't have one.
Here is a pic: http://www.proun.net/img/yamaha/yamaha_cs10.jpg
The sound is still affected by the keyboard in that I can play melodies and the sound reacts just as if i was normally playing the synth with the vca hold switch on however, in the mixer section everything is turned down and the volume is all the way down. Could it be something to do with with the trigger as the sound only comes on after a key is hit?
any ideas are appreciated although I'm slowly coming to the realization that I might need to take a trip to the synth technician.