As soon as the game is switched on one of the tones (chimes) sounds continuously, and the fault stays the same after fitting a second, different driver board.

One tone sounds constantly at power-up, even with another driver board: MPU fault

Cause: When two driver boards give the same result, the fault is upstream on the MPU: the open-collector driver chip (a 7417) that sends that tone's signal to the driver board is the likely culprit. A shorted driver board transistor or a bad connector at either board can give the same symptom.

Checks

• Unplug the driver board connector that feeds the tone board; if the tone stops, the signal is coming from the driver board or the MPU.
• Try a known-good MPU board if one is available.
• With the board out, diode-test the pins of the TTL driver chips to ground and compare similar pins: one reading unusually low points to a failed chip.

Fix

• Replace the faulty MPU driver chip (or the MPU board).
• Fit the protection (isolation) diodes to an older driver board that lacks them, to stop the MPU driver chips being damaged again.

Systems

• Gottlieb System 1

Games

• Count-Down

Related tips

• Chime coils used with System 1 must be 12 ohms or more
• Kickout hole fires nonstop once a game starts: blown 7405 in the switch circuit
• Many lamp faults are driver board connectors
• System 1 driver board connectors
• The CPU to driver board harness
• Testing a driver board with a Ni-Wumpf CPU
• Testing the 2N6043 coil drivers
• How many coils the driver board can fire

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