Thank for answering.
My mobo has Intel CPU and Nvidia 512MB integrated. The schematic I use is "Qanta QT6 Feb2008, Rev.1" (if you need I can send it by p.m.). I've found many other differences among my mobo and QT6 schematic but could be due to many version/revision they've made.
Based on my checking the fault should be chip PU2. Schematic, on pag 39, shows a ISL6237IRZ-T while my board has a RT8206B installed (Richtek Technology Corporation : Products : RT8206A/RT8206B
). Exactly same circuitry is on your schematics for AMD, on page 33, with PU8 chip, a similar ISL6237 (I wonder what is installed in the mobo: a RT8206 ?).
Anyway, PU2-RT8206B chip seems to be the main power supply, since it has an internal fixed regulator that immediately produces +5V LDO used by power switch and kb/peripheral chip. After pwr +5V is generated, an internal switch bypass this LDO to it.
These my findings:
pin6 VIN = is 20V as expected to supply the chip;
pin7 LDO = no voltage, while it should start immediately with +5V, 70mA capable.
Some say that RT8206B and ISL6237 are interchangeable but I cannot find none of them on the market. So, I should either buy a new mobo or a faulty one to retrieve the chip but is a mess since many mobo have either no Nvidia or weaker version or, worse, AMD cpu. About getting a faulty board, the risk is to get one with the same fault.