Nothing resolved, but some news.
The pin 3 is not really the 12v feedback, the pin 3 is a sort of safe switch to turn off the HV board! Without pin 3 the psu is free to startup, skipping the overvoltage or undervoltage issue. The key is the chip UC3903, it have a reference voltage of 2,5v, it take four rails (with reduced voltage) and if all four is 2,5v it put the pin 14 to high and the psu stay up, if one of this four input is not 2,5v the IC3903 put pin 14 low, the signal go to pin 3 of brown connector and HV board turn off! This is the explanation because some Indigo2 psu try to start and then off... removing pin 3 make the psu to force on for diagnostic (can be dangerous but is the only way).
Understood this I sure have an electrical and not feedback/logical issue! To be sure I have removed the pin 3 to my working 8001, and 12v is perfect and stable.
On the IC3903 have three 2,5v and one 1v.... and it slow rising to 1,7v because it follow the my 12v rail, which start at 3,5v and slow rising (in many minutes) to the max of 7v.
Now for those with good knowledge on switching can be more easily. HV board is excluded (this board work with 8001 LV board), the rectifier on 12v line of LV board is excluded (I have replaced it without success).... what can it be? What can make the 12v rail start at 3,5v and after 2 minutes is at 5v, and after 4 minutes at 7v? One component sensitive to temperature?
IC3903 Datasheets