Of the two LEDs on the power supply, the 12 V one is lit but the +5 V one is dark (if the 12 V LED is out, +5 V will be too).

80/80A power supply has 12 V but no +5 V

Cause: Most often the UA723CN (NTE923D) regulator, then pass transistor Q3 (PMD10K40); also the crowbar SCR1 (S107Y1), which shorts +5 V to ground if it rises past about 6 V.

Checks

• Check Q3 is tight on its bolts, well soldered to its eyelets, and not shorted to the plate.
• With the supply off and unplugged, put the meter on diode test, red lead on ground and black on TP4: about 0.3 to 0.5 is normal for SCR1.

Fix

• Replace the 723 regulator (socket it), then Q3 or SCR1 as the checks show.
• A TIP31C is a good substitute for the SW4F013 at Q1 (NPN).

Systems

• Gottlieb System 80
• Gottlieb System 80A

Related tips

• The 5 V output has a crowbar for overvoltage
• Reassemble and set up the 80/80A power supply
• Bench-test an 80/80A power supply in the game, part 1
• Bench-test an 80/80A power supply in the game, part 2
• Parts for the 80/80A power supply rebuild
• Mandatory 80/80A power supply rebuild
• Recommended extras for the 80/80A power supply
• Test and remove the 80/80A power supply's big transistor

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