I was recently given a dual xeon Mac Pro 3,1 which had been working up until very recently. Before it was passed to me the OS drive (on-SSD) was removed, but it's otherwise as-is. Turning the machine on with either of my displayport screens using a known-good DP-mDP cable I'm greeted with a black screen. The monitor does turn on as if a signal is detected, but there's nothing on-screen that indicates the mac is booting. The system does make the usual system-on bong sound, looking at the diagnostic LEDs (and pressing the diagnostic button) nothing appears to be up. The white LED above the power button doesn't flash either. After 30/60 seconds the system shuts off. I've re-seated the RAM boards and individual modules, re-seated the GPU (fan spins when it turns on) and tried both mDP ports on the radeon card. Any suggestions as to what I can try next? From what I can tell the machine had been running 10.13 from the SSD fine, does part of that process re-flash anything on the board itself, is it related to the lack of the old boot drive? If not is it something more fundamental?