The upper left flipper fires when either the left or the right flipper button is pressed.

Upper left flipper fires with either flipper button: terminal block diode reversed

Cause: One of the diodes on the terminal block behind the flipper boards had been soldered in backwards by a previous owner, so the right flipper switch signal fed back into the left side.

Checks

• Find the terminal block with diodes behind the flipper boards and check the band (stripe) direction on each diode against the manual's wiring.
• The brown/green wire runs to the CPU; gray/black goes to the left flipper and gray/violet to the right flipper.

Fix

• Resolder the reversed diode so its banded end faces the CPU (brown/green) side, matching the other diode.

Systems

• Data East/Sega CPU Version 3b

Games

• Baywatch

Related tips

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• Flipper board LEDs show the cabinet buttons are seen
• Solid state flipper goes up but will not stay up
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• Flipper EOS switch role from Jurassic Park on
• Measuring voltage at a solid state flipper coil
• Solid state flipper flutters when held

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