The +5 V output is missing or reads about 2.4 V.

No +5 V or only 2.4 V from the power supply

Cause: Q1 pass transistor, IC1 regulator or the crowbar SCR101.

Checks

• 2.4 V on the +5 V line means Q1 is bad.
• With no +5 V, remove Q1 and check IC1 (UA723CL) pin 7 for 14-15 V; if not, IC1 is bad.
• If Q1 runs hot with no +5 V, SCR101 (S107Y1) is shorted.
• Make sure Q1 is insulated from the back plate by its mica washer.

Fix

• Replace the failed part; Q1 can be a 2N6057 or 2N6059 (60 V 8 A NPN darlington).

Systems

• Gottlieb System 1

Related tips

• System 1 power supply outputs
• Power supply output connector pins
• Power supply voltage drifts or overvoltage trips
• Replacing power supply filter capacitor C1
• System 1 power supply shows about 110 V where 4 V and 8 V belong: wrong ground
• Getting to the power supply solder side
• Power supply bring-up, step 1: inputs
• Quick checks on the similar System 1 power supply

Source: Gottlieb System 1 Pinball Repair from 1977 to 1980, Clay Harrell (pinrepair.com)