Every time the game is switched on it comes up in a maintenance/service mode instead of the normal attract mode, as if its settings had been lost.

Starts in the service menu at every power-up: broken battery connection

Cause: The memory backup battery was not connected: one of the battery holder's connections on the CPU board was broken, so settings were lost at every power-off.

Checks

• Measure the battery voltage at the CPU board's memory chip supply with the game off.
• Inspect the battery holder and its solder joints and traces for breaks or corrosion.
• Look for any battery leakage damage nearby while the board is out.

Fix

• Repair the broken battery connection (a jumper wire fixed it here), and ideally move the batteries off the board to a remote holder so future leaks can't damage it.

Systems

• Williams System 9

Games

• Comet

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