The game resets in the middle of one particular loud speech call-out (here during the Dracula catapult shot when he is unlit), and now and then at other loud moments. Coil, burn-in and sound tests all pass, and with the speech ROMs removed there are no resets.

Game resets halfway through a loud speech call-out: volume set too high

Cause: The volume was turned up so high that sound peaks pulled the supply voltage down far enough to reset the CPU.

Checks

• Turn the volume down and see whether the resets stop at the same shot.
• Check the power supply's small filter capacitors near the bridge rectifier and look for leakage, since they are a common cause of resets too.

Fix

• Lower the volume. Recapping the sound board and power supply should let it run louder without resetting.

Systems

• Williams System 11B

Games

• Taxi

Related tips

• MPU won't boot even though a test ROM passes every PIA and RAM: bad game ROMs
• The D-11581 sound board: chips and ROMs
• Loud constant hum that the volume control does not change
• How System 11 makes sound
• Which System 11 sound board each game uses
• Using a D-11581 sound board in other System 11 games
• Power-on sound tones and what they mean
• D-11297, D-11298 and D-12338 sound boards

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