What the MPU VR1 zener does

VR1 in the MPU reset circuit keeps the RESET line low briefly at power-on while the +5 V supply and its filter cap come up to voltage. Its replacement is a 1N4738A (8.2 V) or 1N4739A (9.1 V).

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35

Related tips

• MPU VR1 is an 8.2 V zener; the manual's "1N9598" is a misprint
• MPU boots only some of the time
• MPU LED locked on with no flicker: likely causes
• Testing the MPU reset circuit at U9 pin 40
• Garbage in the bookkeeping totals
• Reading the MPU flash code
• Replacing the MPU reset section with a DS1811
• What each MPU LED flash proves

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