During the final I/O test one of U6's port pins does not toggle between 0 and 5 V.

RIOT I/O test: U6 output not pulsing

Cause: Both U6 ports are outputs. Either the RIOT output is dead or the chip it drives has a failed input that holds the line.

Checks

• Probe every PA and PB pin on U6; all should pulse.
• Remove the chip driven by a stuck line; if the line then pulses, that chip is bad.

Fix

• Replace the loading chip, or the RIOT if the line stays dead.

Systems

• Gottlieb System 80
• Gottlieb System 80A
• Gottlieb System 80B

Related tips

• Swapping CPU boards between System 80, 80A and 80B
• Swap the three 6532 RIOT chips when a CPU board dies
• Test EPROM stalls at the switch matrix RIOT (U4)
• Test EPROM stalls at the display or lamp/solenoid RIOT
• Which System 80 CPU board goes with which games
• Later single-PROM CPU board won't boot in an early System 80 game
• System 80 and 80A CPU boards swap only with the matching U2/U3
• Making a System 80B CPU board run an earlier game (or the reverse)

Source: Gottlieb System 80 Test EPROM V3.2 (February 2015), Pinitech