System 7 CPU LEDs are swapped versus System 6

The two diagnostic LEDs are wired the other way round on System 7: LED1 on System 6 is LED2 on System 7. So on System 7 the top LED lit after pressing the test button means a ROM fault, not a RAM fault (the same as the bottom LED on System 6).

Systems

• Williams System 7

Related tips

• Add two test LEDs to a System 7 CPU board
• Identifying a System 3 to 7 CPU board
• Late System 7 CPU boards lack LEDs and DIP switches
• System 7 CPU board test points
• A System 7 CPU board can run any System 3 to 7 game
• Is it the CPU board or the driver board? Boot the CPU alone
• Run a System 6/7 CPU board on +5 V only
• CPU LEDs flash once, then come back on and stay on

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)