I have an early 2009 Mac Pro 4,1. The system will not power on at all. When I connect the power cable, I can see a red light through the front vent flash for a second, but when I press the power button, nothing. No sound, fans etc. I have tried removing GPU, Ram, HDD etc but no change. Is this likely to be a power supply? As the system must be getting some power for the red light to blink. Thanks in advance.