Fixing old consoles is roughly...

a) inspect for obviously damaged components. Capacitors that leaked, chips that released the magic smoke, etc.

b) confirm voltages are good

c) inspect the inputs and outputs of the ics to see if they're doing what you expect

d) depending on the boards involved, a lot of checking if pin A is electrically connected to pin B when it should be. Sometimes traces get broken and need to be fixed up.