No response from phone even with different components

phone was left to me by a customer was dropped on the floor said the lcd was sending lines through... . OK so first tried to plug it in and hard reset it no response. tried new screen new battery and charge port (and plugging it in to power on). NO response. using a test phone i tried the customers screen to see the damage, it came on briefly through my test phone( with no damage) and shut its self off, not sure why. But Im only left thinking the mother board is the problem. If anyone else has a suggestion for this loop i seem to be stuck in that would be AMAZING! Thanks >v<