Testing U25 RAM with Leon's EPROM

Steps

• With the test EPROM running, press the CPU diagnostic button.
• Good RAM: the indicator LED on U15 pin 15 pauses briefly then carries on. Bad RAM: the LED stops.
• Reference readings at U25: data pins 9 to 11 and 13 to 17 about 1.5 to 2.5 V; address pins 1-8, 22 and 23 about 2 V; A10 (pin 19) about 0.5 V; pins 12 and 20 0 V; pin 21 about 3 V; pin 24 about 5 V; pin 18 about 3.5 V.
• If those readings are right but the test fails, replace U25.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• CPU RAM at U25
• U15 CPU and U25 RAM substitutes
• Setting up Leon's System 11 CPU test EPROM
• System 11 NVRAM upgrade not working: fit it at U25, not U23 (sound RAM)
• Leon test EPROM does not run: check the 6808 and its pins
• Using the Williams System 11 test fixture EPROM
• Game EPROMs with missing window labels may be erased
• System 11 CPU board won't boot after cleaning: worn ROM socket contacts

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Two, Clay Harrell (pinrepair.com)