Logic Board won't power up; where to start?
I buy broken MacBooks and try to refurbish them. I don't do it to make money, really, but more of a hobby. If I can repair one to working condition and make a profit, great, but I certainly don't buy a machine with the assumption it will be an "easy fix".
That being said, I have a number of these 2009/2010 MacBook Unibody (model A1342) machines that have had various issues (liquid spills, no power, etc). I've gotten to a point with a few of them where I simply cannot find any problem with them. I've tested the top case/power buttons to ensure their functionality. I've checked over the logic board with a fine-tooth comb and have found no signs of liquid damage or burning. I cannot get the board(s) to power on even with a manual jump (using a known-good magsafe board and known-good charger). All boards receive power fine (and all will even charge batteries when attached). They simply won't power on.
My question (after all of that backstory) is: Where should I go from here? I could of course take a known-working logic board and compare/test each component with my volt meter, but with 10+ boards that would take a while. Is there a particular part of the circuitry I could start with--perhaps a "power on" circuit? Is there some main power fuse or gateway that I should check after confirming the top case and DC-in board are working fine? Is there anything obvious that I could be overlooking (something similar to not having the top case ribbon cable unplugged, etc)? I realize that sometimes the logic board can just be "dead", but certainly there's always some particular cause for failure (even if I can't identify is visually).
If I failed to provide any pertinent information, feel free to let me know! Also, if there's someone who specializes in troubleshooting this particular model, I'd love to pick your brain about a few things (in general) regarding the A1342 via email. Let me know! Thanks in advance for any replies.