Yep, that's corrosion. Usually best to completely remove the battery holder from the board if one or more of the contacts is corroded & there's also some corrosion on surrounding traces.. just to check underneath. Then clean up the affected areas. I usually spot-sand after removal of affected components, then use an acid brush to spot-clean with distilled white vinegar, then water, then isopropyl. Following that, replace components and either tin with solder or brush on conformal coating I bought in a bottle years back. You can use clear nail polish too. Just apparently not a great idea to leave sanded traces exposed as it's said the air will allow corrosion to continue or come back (that said, I've not experienced that *yet*).

I don't know those boards well, but I can't imagine the 18v lamp circuits being in that area of the battery holder or memory. That said, if the memory is wonky it could do weird things on some boards. Not saying that's it, but it's easy enough to swap the NVRAM with the old SRAM chip or memory from your other game to see if it makes any difference at all in how the board is functioning.

Yep, I saw

It's rare to hear of any issues with the nvram that goes out -- once in a while someone installs it backwards in the socket (notch facing opposite direction) and that immediately shorts the chip. That's usually the extent of any issues I hear about on them and luckily is pretty minimal at that.

I just figured the RAM is near the corrosion area and since it's easy-enough to swap out it might be worth trying. Game EPROMs are another possibility. EPROMs can go bad after a bit and data gets mangled, which would definitely prevent a board from booting and likely cause other issues as bad data is read in. The RAM & eproms are easy enough to swap out. Just a matter of having an original memory chip & either buying new eproms or burning your own (the GQ-4X is relatively inexpensive at around $100-115 and if you have a large collection of games or plan to, it's worth its weight in gold).

Anyway, in terms of troubleshooting.. those are the few things I'd try swapping out if it was me. Have been bitten by dying eproms too often to not suspect those. The suggestion of swapping out the RAM was it's already in a socket, so easy enough to rule that out as well. Not too familiar with Whitestar boards to offer much more for suggestions unfortunately.

Had you swapped eproms when you were swapping the Monopoly board into SP or did you just swap the boards and leave the Monopoly eproms in there? If you hadn't swapped eproms, that might be something to try (ie. swapping SP eproms into the Monopoly board that was also having issues in SP). May need to verify eprom sizes are the same for both games via the manual and/or check jumper settings for the game eproms.