The +5 V on an 80B game wanders, swinging from about 4 V up to 7 V or more, causing crashes and risking the boards.

System 80B +5 V drifts or jumps

Cause: The 500 ohm trim pot on the 80B power supply is poor quality and develops dead spots; adjusting a worn one has put 12 V on a CPU board.

Checks

• Read the CPU board +5 V across the input electrolytic next to its +5 V plug; look for 5.0-5.2 V DC.

Fix

• Replace the trim pot with a good one, or with a fixed 1/2 W resistor of about 270 ohms (adjust the value by trial until +5 V is right).
• Don't try to adjust the original pot.

Systems

• Gottlieb System 80B

Related tips

• Ground mod for the System 80B power supply
• Set the 80B +5 V after replacing the trim pot
• While the 80B power supply is out
• Which System 80 power supply boards are interchangeable
• First power-up, step 3: run the power supply on its own
• Ground mod step 1: CPU board ground wire
• Swapping CPU boards between System 80, 80A and 80B
• Making a System 80B CPU board run an earlier game (or the reverse)

Source: Repairing & Upgrading Gottlieb System 80 Pinballs from 1980 to 1989, Part One, Clay Harrell (pinrepair.com)