Quoted from Chisox:

Never work on the game with the power on, not even chainging bulbs.
Always check the most simple things first. Re-seat those ribbon cables. I once re-soldered a bunch of connections, changed two transistors on the power board, and wasted a good four hours when it ended up being a ribbon cable re-seat was all I needed. Doh!

Quoted from dasvis:

Of course, after putting the backbox on you realize you forgot to route the power cord...