The 5/12 V supply is only half-wave rectified

There is no bottom-panel bridge for the logic supply. Two 3 amp diodes (CR1/CR2) on the power supply board rectify it, and the stock C1 is only 2900 mfd, so ripple is high. That is why C1 must be replaced with a 6800-10,000 mfd part.

Systems

• Gottlieb System 1

Related tips

• System 1 power supply outputs
• Replacing power supply filter capacitor C1
• Acceptable System 1 supply readings
• Getting to the power supply solder side
• Power supply bring-up, step 1: inputs
• Power supply output connector pins
• Quick checks on the similar System 1 power supply
• Display high-voltage section of the power supply

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