Have you tried to use the moldex to PCIe adapter an run it off another rail?
Take out the Video card and you sould get beep code of Video failure
If you have a floppy drive unplug the floppy or check the data cable see if it is pluged in backwards.
If that does not work. Reset the CMOS ( unplug the power supply, remove the battery and change the jumper). Let it sit for about 20 mins, then put back together and try to boot up.
If still nothing, take out the RAM, you should get beep codes. If you do not get beep codes it could be a power problem.