A diode-mode transistor test is right about 98% of the time

Meter tests check a transistor with no load, so a part that passes can still fail under load. Failed transistors usually short, reading zero or near zero instead of 0.4-0.6. When the board is out for another repair, test every transistor while it is accessible.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Testing loose transistors on the bench
• Testing a coil diode on the driver board
• 20-second test for shorted coil driver transistors
• Testing a 74LS374 driver latch with the diode setting
• Replace the pre-driver along with a failed driver transistor
• Which transistor drives which WPC solenoid and lamp circuits
• Left popper fires at random with the right pop bumper: shorted J127 header pins
• Coil locks on as soon as the coin door closes: transistor shorted by the old coil

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Two, Clay Harrell (pinrepair.com)