Is there anything happening on the boards at power up? Iirc Defender has some lights on the rom board that indicate where problems are. Did the cab work previously? Have you tested the voltages from the power supply are at the correct levels?
I thought that to start with, "why is defender displaying space invaders"
Isn't there a colour bar test in book keeping mode ?. Check the batteries, make sure they haven't leaked. Download a copy of the defender manual and go through the book keeping options if it will let you.