Thanks a lot you've helped me.
By comparing the DAC section of the clone boards with the original boards (
by luck i have a few old boards and a few clone boards).
I found an error on the clone board:
IC33 pin 1 et pin 2 are linked on the clone where there is no link on the original boards.
That is why the D/A adjustment was impossible.
Now D/A adjustment has been done OK.
Now I've got a new problem, I'm going to open a new topic for this new problem in order not to confuse readers.