OBERHEIM SYNTH

Hi all,
I have a xpander since some month and all was nearly fine. It began few month now with a dead voice, changed the VCO chip and since all good.
But now since two week its impossible to tune the voices. 
When I use the autotune feature the result are random, sometimes all voice fail sometimes only 2 or 3 or more but never all pass. Even those who pass doesn't sound same.
The PW test pass, RES pass, VCF fail, VCA pass.
DAC monotonicity test fail I hear plenty of blips.

I have equipment to repair it, I have learned electronic in school but the lack of description on how the cpu's works is a wall to me.

If someone knows how the DAC monotonicity test is done perhaps that would help to locate the problem.
I have done all calibration procedure and nothing wrong except the problems above.