Kawai Synthesizers

Hello
I bought Kawai k1 II synth (second hand)severall days ago.I cleaned 
everything including the contactacts.The problem that I have is 
this: 
 Every 9-th key from the keyboard (going downwards from the top 
key "C" to the first key "C")sounds louder than the other keys.They 
don't have sensitivity,they just sound louder than the rest of the 
keys even when you apply the same pressure.
 I don't think that the dirt and dust are the problem,because every 
9-th note (including the last key"C")makes the problem.
 Can anyone help me with this problem and tell me more specific what 
to repare?
Thank's everyone in advance
Igor