Test the switch column drivers with a diode jumper

Steps

• Remove the backglass, power up and open the switch levels ("TST") test.
• Unplug CN8 and CN10 from the bottom of the CPU board.
• Clip a lead to CN10 pin 9 (the left-hand pin as you face the board) and fit a 1N4004 to its other end, band facing away from the clip.
• Touch the band end to CN8 pin 1 (right-hand pin): switch 1 should show closed.
• Step along CN8, skipping key pin 6: switches 9, 17, 25, 33, 41, 49 and 57 should show in turn.
• A column that never shows, or shows with nothing connected, is a CPU board fault, usually its 2N3904 at Q48 to Q55.

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
• Test each lamp column driver with a test lamp
• Always check the coil diode when a driver transistor has failed
• Fit a replacement switch diode the right way round
• New batteries but still a memory error: check D25
• False switch readings only when several switches are closed
• Testing a coil diode on the driver board
• Testing lamp-column drivers with a test lamp

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