Seonding Peter, listing all the things that can cause a complete blank computer screen would fill this forum to the brim...
emunstar wrote:The Mackie tech seemed to be pretty positive that the blank computer monitor at boot was a good indication of a bad motherboard. Monitor mouse and keyboard all check out as good.
Does your keyboard's NumLock/ScrollLock indicators flash when you power up the beast? You might want to try another VGA card, Mackie OS will probably not boot with it, but it should be good enogh to get into the BIOS. You can also connect a small speaker to the two outer terminals of the 'speaker' row (4 pins), if it beeps, then your motherboard does work and the processor runs - the number of beeps might tell you something about the nature of the problem preventing proper booting. You might also want to check out another PSU (AT type).
Just a handful of ideas to start with...