Xbox Series X - Southboard missing 12v - diagnostic help

I have purchased a faulty Xbox Series X with no power.
I have done some basic checks and the southboard is not getting 12v in standby.
I have checked the power supply and seem ok (12V will be delivered).

In standy mode I cannot measure and voltages on the 12v point from the southbridge board. I have checked for shorts on the testpoints and cannot find anything

I have currently no idea where the voltage will be dropped, as I cannot measure anything one power supply is connected to the board.

Any ideas, advice are appreciated.