After not being powered for a long time the game lights up but does nothing, and leaking alkaline batteries are found in the board's battery holder.

Lights up but dead after storage: leaked batteries on the board

Cause: Battery electrolyte leakage around the on-board battery holder.

Checks

• Remove the batteries and look under and around the holder, and along the traces (darkening under the green mask), for corrosion.

Fix

• Remove the holder, neutralise the area with a vinegar and water mix, rinse with high-strength isopropyl alcohol and let it dry fully.
• Fit a remote battery holder in the cabinet (or non-volatile memory) instead of batteries on the board.

Battery electrolyte is corrosive: wear gloves and eye protection and neutralise it before it spreads.

Systems

• Williams System 6 / 6A

Games

• Firepower

Related tips

• Battery holder looks fine but makes no contact
• Batteries drain in days or get hot
• Move the CPU batteries off the board
• CPU will not boot after battery corrosion
• Clean battery corrosion off a CPU board
• Check the battery blocking diode D17
• Replace batteries of unknown age right away
• Number-of-players backglass lamps dead after playfield work: reseat the MPU plugs

Source: Pinball Repair help group!! (Facebook group)