You may try to remove completely the top case, disconnect the hard drive, the optical drive, airport card, bluethoot cable, well everything you can disconnect from the logic board but let the dc in board connected then find the two power pad contacts on the logic board (you should see a white power icon, or PWR written somewhere). Connect the AC adapter and try to briefly bridge the two power pads with a small jeweler flat screwdriver. If the MBP powers up and you see the question mark onscreen then you know the logic board is OK and you can connect the disconnected components once at the time powering the machine between each connection until you find the culprit.