ACMT here, former Apple Genius. Everything is SMD/SMT on the boards, this translates to: too small to repair or measure by hand, except you're hardcore into microelectronics and have the equipment.
For this exact reason parts like MLB (main logic board) get replaced in Apple Stores instead of 'repaired', as a skilled worker with equipment would be more expensive than replacing modules altogether.
Your best bet would be a smc-reset first, if that doesn't help try to replace the magsafe connector (under the board). Please give attention to ESD - might do more damages only by touch.
Schematics aren't available except for a few older machines, even within the company. Usually everything is Intel reference design_except_for_apple_own_stuff which you're handling here.
Your question implies you might be firm with powered devices, but not with microelectronics as the magsafe connector (who has a high chance of being faulty) has only a few caps and resistors, so...maybe you are not the right person to try this on your own, no offense.
Good luck to you