Well, Roel,
if so, you may be able to help me with another keyboard I got !

It's an old Farfisa electric organ ("Lido") ...

There are 3 main sections ("conductors") for the upper manual - flutes, piano/guitars, and strings/brass.

The piano/guitars section plays 100%, but on the flutes section, the highest C# key just doesn't play at all while the F# key on the same octave play only 2 of the 4 flutes (it plays only the '16 and the '8 flutes while missing the '4 and '2). When using the strings/brass section, the same C# doesn't play at all while the same F# play but you can hear that the string sound is not "full".

I do have the schematics for the machine -

Can you give me some directions please ?

Thanks !!!