Test a playfield switch diode

Steps

• Every matrix switch needs a 1N4148 (or 1N914) diode, fitted with the band the same way as on the other switches.
• Game off: unsolder one diode lead and test on diode range; 0.4 to 0.6 V one way and open the other.
• Game on in switch test: bridge the two switch wires on the far side of the diode. If the switch now registers, the diode is bad.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• Lower playfield center target fires the kickout and ends play: switch wired backwards
• Spare diodes for coils and switches
• Check the battery blocking diode D17
• Bench-testing a -133 MPU: swap the R113 diode for a resistor
• Firepower 2732 change when IC15 is a 74139 (diode and pull-up variant)
• Battery power not reaching the RAM: diode D2
• Settings lost although the batteries are good (blocking diode D2)
• Settings are lost at power-off even with fresh batteries: bad battery diode D2

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)