macbook wont boot

im sure this has been asked before but i have a macbook and when you power it on it immediately seems like it goes to sleep. the led stays on. ive read its a ram seating issue but i reseated the ram multiple times and i know its in all the way. ive tried different sticks. i got a brand new ac adapter. ive taken it apart and disconnected everywire and reconnected it and it still does it. i tried the firmware restore disks everything. the screen doesnt even come on. sometimes it flickers very fast but nothing more. also the cd drive and hard drive spin up when you press power. if you just press power it shuts off after about 5 seconds but if you hold it the sleep light starts to blink and then stays on steady and the computer will just stay like that and get hot until you shut it off. if it needs a new logic board i just want to be sure before i go and spend 300 dollars on one. ive looked through forums for hours and have not found my exact issue. any suggestions or advice would be greatly appreciated thanks