Ok, done the testing.
Not a chance to fix it doing it the reversed way. Even when I get 6V at the top, there's the same voltage jump past the highest E, and then all the other lowest scales got flat. Maybe I should have explained it better on the first message, but there is a drastic sudden voltage drop when passing from the Eb to the E. Not something linear happening as you go up the keyboard. Only on the highest notes (E to C) there. And they are quite in tune with each other, just about 30 cents flat. If I am making myself clear... if not, I'll try to make an audio recording or a video.
About those resistors you mention, as far as I see:
R19 is 14.3K. Are you sure the values written there in the schematics are not 28.9K and 14.3K?
R21 is 14K. I think you misread 196 on what is actually 1% and the new value is written below, hardly legible, but it could be 14.0K.
I don't think that there's a calibration problem. I'm thinking there may be some faulty component. Maybe I'll try replacing IC10-11/4069 or IC8/TL072. What do you think?
Thank you very much again. Your help is really appreciated.