Yamaha CS80

Hello! I'm one of the new members mentioned a while ago. :)

I just bought a Yamaha CS60, which according to the seller was in
great working condition, but upon arrival it is exhibiting a problem.
Perhaps somebody has come across this before or could guide me on what
to check?

Basically some of the keys are playing incorrect notes. On each
octave, when I press c, c#, d, f#, g, g# they all play fine.

However when I play the other notes they do the following:

I press d#, I hear a c#
I press e, I hear a c#
press f, I hear a d
press a, I hear a g
press a#, I hear a g
press b I hear a g#

All the notes are perfectly "in tune", but exactly half of them are
playing the wrong note. This happens for all octaves, each wrong note
is in the same octave as the other notes (ie when I press f3 I hear a
d3, when I press f4 I hear a d4).

The seller assured me it was working fine when it left him, and he
tuned it before it left. It travelled via a courier company from Paris
to Berlin so perhaps something has moved around a bit inside (the
seller seems genuine so I believe that it left his place in working
order).

This is my first Yamaha CS synth so don't know much about them (for
example, how do you open up the front panel? :)) I do own a multimeter
and would be willing to measure voltages to find the source of the
problem. Has anybody come across a fault like this before, or knows
what I could check?

One other thing which may be worth nothing, according to the seller, a
French technician called Jean-Loup Dierstein has installed a simple
little kit inside the CS60 which tells you via 8 leds which voice you
are playing when you press a key. (this might come in handy!) I can
hold 8 keys down (ones which play different sounding notes anyway) and
hear 8 notes at the same time .

So, what to check next? (thanks in advance)

-Stephen
-- 
http://www.skkatter.net