If you only check one thing in the backbox, check the batteries. The repair guides for System 11, Data East and White Star all call leaky alkaline batteries the number one killer of pinball boards. The damage is easy to miss in a listing because it sits behind the backglass or translite, and plenty of sellers never think to photograph it. Most of the games in our catalog carry batteries on the CPU board, and how bad a leak gets depends on which board system the game uses.
Why the batteries are there
Solid state games keep their settings and bookkeeping in RAM, and that RAM forgets everything when the power goes off. Makers added a battery to the CPU board to hold that data while the game is switched off. That covers things like high scores, audits and adjustments such as free play. On Williams WPC games you can't simply pull the batteries and forget about it, because the game boots with a "Factory Settings Restored" message every time.
What a leak does
Old batteries always leak eventually. On a Williams WPC board the electrolyte drips onto the switch matrix circuits and eats the traces under the green solder mask, leaving them lumpy. It has to be caught early, and it can leave a board unrepairable. The damage also creeps. Corrosion has been seen travelling up the wires of a remote battery holder and back onto the CPU board. On White Star boards, alkaline damage is the most common reason the CPU won't boot.
The symptoms can look like something else. On WPC games, whole rows or columns of switches stop working because the corroded traces are the switch matrix. The connector right below the battery holder often gets alkaline drips on it, too.
What to look for in the photos
Ask for a sharp, well-lit photo of the whole CPU board, plus a close-up of the battery holder and the parts below it. Blue-green or white crust on the holder, the board or nearby connector pins means a leak. Dull, lumpy traces around the holder mean the corrosion has already been at work. Check for new parts as well. Boards repaired after alkaline cleanup often have chips put in sockets, so sockets or fresh solder below the holder usually mean an earlier repair. Ask who did the work. If there are no batteries on the board at all, ask where they went. A remote holder or an NVRAM chip is good news. Bare contacts with nothing connected mean the game will lose its settings.
If you don't do electronics, the safest advice is to pass on machines with battery damage unless you're ready to buy replacement boards, since damaged boards fail in the end.
System by system
Gottlieb System 1. Genie and other System 1 games carry a rechargeable 3.6 volt NiCad soldered to the CPU board, and its fumes corrode the edge connectors and chips nearby. Take it off as soon as you find it. Aftermarket System 1 boards exist if the original is past saving.
Williams System 11. Games like High Speed use three AA batteries on the CPU board. When they die the game can show "Adjust Failure", and the battery contacts sometimes look fine while the rivets underneath have been eaten away.
Gottlieb System 80. Black Hole and Haunted House run System 80, which was plagued by battery corrosion, bad connectors and bad grounding. The original battery is a rechargeable 3.6 volt NiCad soldered to the CPU board, and the black Data Sentry pack has the worst record for board damage. System 80 is also one of the systems best suited to a memory capacitor in place of the battery.
Data East. The Simpsons, Star Wars and Jurassic Park use three AA batteries on the CPU board. Move them off the board, or use a memory capacitor instead.
Williams WPC. This covers most games in the DMD golden era, and the three board generations differ. The WPC-089 board drips straight onto the switch matrix. WPC-S moved the batteries to a remote holder on a daughter board, which mostly ended the damage, though used boards are often missing that card. WPC-95 put them back on the board in a plastic shell, in a spot above the ASIC socket that's even worse than before.
Sega and Stern White Star. White Star began with Sega's Apollo 13 and runs through Stern games like The Lord of the Rings and The Sopranos. It also uses three AA batteries on the board. It's not a matter of if they'll leak but when, and the very fine traces make repair difficult or impossible.
Stern S.A.M. Games like Iron Man and AC/DC use a single 3 volt CR2430 lithium coin cell. These last a long time and aren't prone to leaking, so relocation isn't needed.
Stern SPIKE. The coin cell sits upright on the board in the backbox.
The usual fixes
There are four choices. A remote battery pack moves the cells off the board on a pair of wires. Mount it below all the boards, since leaking batteries have harmed boards up to 6 inches away, and some people put the pack in a zip-lock bag. Write the date on the batteries and change them every year, and use lithium AA cells, which are much less likely to leak. NVRAM replaces the RAM chip with one that keeps its data with no power at all. A memory capacitor holds a charge in place of a battery but needs the game switched on regularly.
There's one catch with NVRAM. Games with a real-time clock need a battery to keep time. Twilight Zone is one, since its playfield clock shows the real time, and Medieval Madness is another, once its midnight madness feature has a special setting turned on. With NVRAM only, the clock won't advance while the game is off.
On delivery day
Pull or check the batteries before you do anything else. Don't just flip the power switch on a game in unknown condition, either. Go through the line cord, fuses and connectors first. If you see leak damage, the PinWiki guide for that board system walks through the cleanup, and the listing red flags and playfield guide cover the rest of the inspection.