PowerEdge R715 server won't POST - no lights on the hard drives except for a quick flash when i turn on the power

The server powers on and i see the hard drives light up for a brief second....but no POST, no video no nothing....

I've re-seated all of the drives, the memory, and the cables attaching the drives to the motherboard and backplane (waiting the 2 minutes to power on after I've put the power cables back in)

I've re-seated the power supplies

I've tried in order the steps here including removing the drives and the optical drive before powering on and still no luck:

https://dell.com/support/kbdoc/en-us/000152161/dell-poweredge-troubleshooting-a-server-that-does-not-start-no-power-no-post-no-boot-and-no-video#4

any suggestions to resolve are appreciated - thank you