Pressing one or both flipper buttons during a game drops the game back to attract mode; with the backglass off you can see the CPU LEDs flash (System 7 shows "0" on its LED display) as it reboots.

Game reboots when the flipper buttons are pressed

Cause: A cracked, unsoldered or missing diode on a flipper coil lets the coil's collapse spike (about twice the coil voltage) back into the supply. Other causes are AC ripple from a weak filter capacitor or, on System 7, a weak 5/12 V bridge rectifier.

Checks

• Tug gently on each flipper coil diode to find cracked bodies or loose leads.
• With the game on, measure AC volts across the large power supply filter capacitor: under 0.2 V AC is good on System 7, under 0.3 V AC on System 3-6.

Fix

• Replace any bad flipper coil diode (1N4004, band toward the power lug).
• If ripple is too high, replace the filter capacitor: 15,000 MFD on System 7 and on System 3-6 games with more than two flippers, 10,000 MFD is enough on two-flipper System 3-6 games.
• On System 7, also fit a new 35 A 200 V bridge rectifier for the 5/12 V supply.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• Reset or lock-up only when one device fires
• Addams Family: both right flippers stick up until the left button is pressed
• Fliptronics: flipper will not hold up (flutters)
• Fit a remote AA battery pack to a System 80 CPU board
• Random resets or lock-ups during play: broken flipper coil diodes
• Two lamps flash together in the single lamp test
• System 80B alphanumeric displays dark: check the display board supply
• Pressing both flippers resets the game: replace big filter cap C15

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)