80B displays stay blank although the display board has power.

System 80B displays stuck in reset

Cause: The reset line from the CPU board is not going high, or transistor Q1 on the display board has failed.

Checks

• Power off: test Q1 (2N3906) on diode test.
• Power on: the reset line starts low and should go high about 50 ms after boot. The same signal appears on CPU RIOTs U4-U6 pin 34.
• Trace it: it leaves CPU U5 pin 17, enters Z17 (7404) pin 1, leaves inverted on Z17 pin 2 and goes out on A1J2 pin 24 to the display and sound boards.
• Display chips U1-U3 can also be checked for their supply voltages.

Fix

• Replace Q1, or repair the broken point in the reset path.

The 80B display board turns 32 V AC into -45 V and -15 V DC; switch off before touching it and probe carefully with the power on.

Systems

• Gottlieb System 80B

Related tips

• System 80B display board connector A4J1
• How the Sega opto receiver works
• The A15 flipper sensor board
• What the System 80 reset (watchdog) board does
• Replace the System 80 reset circuit with a DS1811 chip
• Dead CPU board: check the reset line first
• 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 Two, Clay Harrell (pinrepair.com)