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.
Yes, totally valid point. Those chips could be OK, but they might not be.
In my mind the board probably had a RAM or datapath fault, possibly due to battery leakage, and some idiot probably tried swapping the chips thinking they were interchangeable, or by accident. Personally I would test those chips in another 500 first - work out if any of the socketed chips have failed, then start looking at the PCB for acid damage etc. My guess is there will be 2 faults here at least. If you live in the UK and want someone to fix this, I can. Or you could stick it on EBAY as is.
It's a shame, many of these 500 and 500+ boards are being destroyed because when there's a fault with datapath, demux or RAM people are either making more faults or stripping them for parts and destroying the PCB.
I bought 4 'RAM sections' a few weeks back - part PCB cut from 500 motherboards =\
