Yamaha CS80

Hello all.
After  switching the machine on, here is the problem: 
MIX I : everything works fine.  I move the "channel" selector down to Mix II 
MIX II: panel and memory do work as well.  Only the Preset buttons show the following fault:
when I play the first note it does not work (sometimes it seems to fire a note, but at a very low level... ), the following 6 in a row do work. then the following does not work, and so on. 1 yes 6 no. 
Moreover, when I keep the first (faulty) key down, and play another key in a repeated sequential way, all of the notes are normally heard. 
I tried to move and check the boards' cables, no clue.  The fact that all of the voices work when panel or memory 2-4 are switched on means that the voice board of the first note (21)
is not faulty. Do I have to check at T54 or BA?
Please help me!  
Ciao
Fernando from Italy


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