The MPU (even a replacement) will not complete its flashes at power-up. Unplugging the solenoid-voltage feed to the sound board lets it boot, and the sound board then works when reconnected.

MPU stops before the 7th flash until sound board power is unplugged: replace CR3

Cause: The diode CR3 on the sound board that isolates its supply from the solenoid voltage line had failed (leaky). That let the sound board's filter capacitor smooth the line, which upsets the MPU's zero-cross detection, so it never reaches the 7th flash.

Checks

• Disconnect the sound board's power plug and see whether the MPU then boots.
• Test CR3 on the sound board out of circuit; it may read good on a simple diode test and still be faulty.

Fix

• Replace diode CR3 on the sound board with a 1N400x-series rectifier diode.

Systems

• Bally MPU AS-2518-35

Games

• Star Trek

Related tips

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• Faulty sound board blanks the displays and confuses the lamps
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• Sound module -32 and -50 test points
• Identifying AS-2518-51 and AS-2518-56 sound boards
• AS-2518-51/-56 sound board jumpers
• Bench-test a -51/-56 sound board with a test EPROM
• Sound Plus -56 and vocalizer test points

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