Happened to notice there is a GARY chip where DENISE should be! WTF!?!?!?
If you have the knowledge, I would compare the pinouts of the Gary and Denise chips. Are the same pins tied to ground, Vcc, etc on each one? Then make an EWAG (Educated Wild A**ed Guess) about what would likely suffer if a previous owner got cute with swapping chips around. Would signals from the support chips feeding inputs into that socket be shorted to gnd (ground) if Gary were put into there?
With a schematic, meter, and maybe a scope, you should be able to go pretty deep into it. Confirm power rails, gnd, and clocks to all chips.
You could probably save it.
