Free-running the CPU to check address lines

Steps

• Remove the EPROM from 5C and power the board; with no program the CPU steps through every address.
• Measure CPU address pins 9 to 25 (pin 21 is ground): each should switch and read about 2 V.
• Check the address buffers at 6C and 6D the same way.
• If a line is missing, bend the suspect chip's pin out of its socket and measure again; a signal returning means a short in that chip, none means the CPU has failed.

Systems

• Data East CPU Version 1/2
• Data East CPU Version 3
• Data East/Sega CPU Version 3b

Related tips

• Other CPU jumpers and what they set
• Bench-testing a CPU board on +5 V
• CPU boards are only downward compatible
• CPU EPROM jumpers J4 and J5
• CPU board versions 520-5003-01, -02 and -03
• Where the TIP122 drivers are on the CPU board
• CPU board PIA positions and jobs
• CPU jumpers J4 and J5 set the EPROM size at 5C

Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Two, Clay Harrell (pinrepair.com)