My second 840av has the same issues, turns on with no bong or video. Caps have been replaced but they had leaked over a long time and there is some crud around chips even after cleaning.

The ROM on an 840av is on the motherboard, the only 68 pin stuff should be VRAM installed 4 at a time.

I would try recapping that machine and see if it works, if you recycle it any chance I can have the back plate?