I would firstly remove the barrel battery. It looks like it has corrosion damage. I finished restoring a A2500HD this summer that had battery damage to it. Try to cleanup around the that area as good as you can. Next remove everything extra in it such as expansion boards, drives, extra memory. Reseat all the chips and power it on. Take note of the power light flashing if it does and the sequence. If no flashing start adding back peripherals one at a time. Also look for corrosion around caps, resisters and such and if any are in question use a voltage meter to see if they are still good or need replacing. One other thing is to look at the underside of the motherboard. This is often overlooked.
