At this point it could be another bum board or other part. Have the board supplier show you the board functioning. Be sure to leave ANY OPTIONAL part out. When a machine is this gone I won't install the hard drive, CD/DVD and may use one or none memory.
I have a Gateway M320 laptop which I recently replaced the motherboard on due to bad USB ports.
The laptop is reassembled, but now all I get is a flashing power light. There seems to be no control of the power button. The power comes on as soon as it is plugged in, (no need to push power button).
The fan is working normally, I have reseated the memory and hard drive and have gone as far as replacing the processor, all with same results.