This is part of the "hopeless" NEO GEO boards bought as scrap - and yeah, i have few dozen . Corrosion in battery area and usually other problems as well.
The lower board was a pain to fix, all components in battery area been removed-replaced and traces re-due under microscope (i do not like kynar patches).
It remains only one problem: Slot 3 had corrupted graphics.
Upper board looks like new, i suspected one of the custom IC`s but no.
Nothing was visible with bench magnifier so i switched to microscope for inspection. Noticed some slight corrosion - not visible with naked eye- in the area above the battery. But no obvious damage.
Corrosion on the upper board from battery below? Ugh, that board was stored for ages to get in that condition.
Time to pull out my notes with every signal on the custom chips. This really speeds up troubleshooting this kinda issues.
Focused on C5 NEO-253, linked to Slot#3 and found pin 35 dead (as per my notes that should toggle). Found one trace cut - at least from measurement.
Cleaned a little the through-holes and found a microscopic cut on the trace connection. All area been cleaned-soldered to prevent further damages, corrosion expansion.
Board saved - going into proper storage...