well the 14" uses a 14 volt battery and the 12 uses a 10.4v so I suspect that your problem is there, at the battery and not with the power supply or the dc board. Usually what happens is the 14v battery fries the DC bus and the battery will no longer charge but the laptop runs ok otherwise. Basically the logic boards are not really interchangeable for this reason. You may be able to figure out a way around it though not sure how. But my 2 cents is its the battery not the power board.