Test the switch row inputs with a diode jumper

Steps

• In the switch levels ("TST") test, with CN8 and CN10 unplugged from the CPU board.
• Clip a lead to CN8 pin 1 (right-hand pin) with a 1N4004 on the other end, band toward the clip.
• Touch the plain end of the diode to CN10 pin 1 (right-hand pin): switch 8 should show closed.
• Step along CN10, skipping key pin 4: switches 7, 6, 5, 4, 3, 2 and 1 should follow.
• A row that never shows, or shows with nothing connected, means a CPU board fault.

Systems

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

Related tips

• Switch row (return) circuit on the CPU board
• Always check the coil diode when a driver transistor has failed
• Fit a replacement switch diode the right way round
• Test each lamp column driver with a test lamp
• New batteries but still a memory error: check D25
• False switch readings only when several switches are closed
• Testing CPU switch rows and columns with a diode and clip lead
• Testing a switch diode out of circuit

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