What else can I do?

Hi there, I have been trying to fix this Gameboy Pocket for a while now. I have cleaned everything on the board with rubbing alcohol, reflowed all the pins next to the chips, cleaned off some battery corrosion on the pins of these really tiny chips near the battery terminals, and replaced the battery contacts. But for some reason, the Gameboy will get power and a black bar will appear, 20% of the time, but no further. Won't play a game. I have a multimeter but need to learn how to use it. Any suggestions? This is my first project, I really want to make this work.