First, thanks again for your explanation, but I must say that my electronics knowledge is so limited. So, although I've read the message several times, I do not fully understand how a D/A works. But I think I got the basics.

I made the test with the Tune Point opened. If closed, the voltages of the keyboard do not show on the CV Out. But anyway, no matter what I do with the key assigner tuning VRs, there is always the same voltage drop when I reach the highest E.

Then I removed the board and replaced R52 with 1K. Making the VR adjustment turned out to be more difficult. Seems that each trimpot affected more to the others, but that may be my perception. Anyway the voltage drop actually increased. I can tune all the octaves the same as before, but when I reach the highest E, now it comes as Eb +28cents, 5.27V. Highest C is 5.94V.  Maybe I should increase it's value instead

If that fails...

I just have one spare 4069 here at home. Which IC do you think I have better chances replacing IC11 or IC10 I could remove another one, just for test purposes, from somewhere on my DIY modular, say for example on the WASP VCF clone.

Thanks.