HP Z820 not powering up at all

[hhajj]

When i press the power button on the workstation it lights green on the LED next to the button as long as i'm pressing it but it doesn't power on.

I changed the PSU and it's still the same, i removed all pci cards and RAM but it just doesn't power at all.

Any troubleshooting i can do more? what could be the problem? 

[DGroves]

read the HP user manual and the service manual that hp provides online for this model

and follow the troubleshooting steps, or take to a qualified repair shop

[hhajj]

I already did that but there's nothing there about such issue 

[DGroves]

did you perform the "BIST" test?

[hhajj]

Yes when i unplugged power cables from the board the LED in the PSU from the back turned green, when i plug them to the board it doesn't turn on .

it says that it's a motherboard issue but i was able once to power it on through discharging power by pressing a holding the power button. but that worked one time . how can i further troubleshoot and diagnose this issue and are there any detailed pcb diagram for the board to check if there's a short somewhere and fix it? 

[hhajj]

Do you have the motherboard pcb circuit diagram? 

[DGroves]

the board appears to have failed and needs replacement however it's actually cheaper to either buy a complete bare bones z820 (case pwr supply and cpu) than just a ver 2 motherboard

also unless you require the dual cpu feature a newer model like the z440 is a better value

check ebay for current pricing

last, there are no free schematics of the z820's motherboard available