Power supply J1 plug fits upside down

The bottom plug (A2-P1 on J1) has no key and goes back on whenever the backbox is refitted, so it can be fitted inverted. That ruins the power supply, can overheat the small transformer until it fails, and may harm the CPU. Look for the "This Side Up" label; facing the board, the green ground wire goes to the left. Pin 1 (white/blue) is the leftmost pin below the two big diodes.

The bottom panel carries 120 volt line power; unplug the game before touching the transformers, line filter or fuse block.

Systems

• Gottlieb System 1

Related tips

• System 1 power supply outputs
• Getting to the power supply solder side
• Power supply bring-up, step 1: inputs
• Power supply output connector pins
• Power supply bring-up, step 3: displays
• Quick checks on the similar System 1 power supply
• Display high-voltage section of the power supply
• Logic and display reference section of the power supply

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