How a Bally display board drives its glass

The only chip, U1 (listed as E-620-38), is an MC14543 BCD-to-seven-segment decoder. Each of its seven outputs drives an MPS-A42 segment driver. Each digit has an MPS-A42 level shifter and a 2N5401 driver that connect high voltage to that digit when the MPU enables it (page 50); the six digit-enable lines are daisy-chained to every display.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• Replace a score display glass
• Test MPS-A42 and MPS-A92 display transistors
• Which transistor drives which WPC solenoid and lamp circuits
• First checks before repairing Bally score displays
• Making Bally and Stern 7-digit displays swap between makes
• Flickering score displays
• Rebuild the high-voltage section of a 520-5047-00/-01/-02 supply
• Rebuilding the +100 V section of the power supply

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)