Four lamps stuck on or off together, or one lamp stuck bright or dead.

Groups of four lamps or single lamps misbehaving

Cause: A failed 74175 affects its group of four; a failed driver transistor, the more common fault, affects one lamp.

Checks

• Use the ground-the-leg test; if the lamp lights that way but not in play, test the transistor.

Fix

• Replace the MPS-A13 or MPS-U45, or the 74175.

Systems

• Gottlieb System 1

Related tips

• How CPU-controlled lamps are driven
• Finding which transistor drives a lamp
• No CPU controlled lamps light
• Testing a CPU lamp circuit by grounding its driver
• Lamp test can hold coils on for five seconds
• Game over relay won't hold in: poor 5 V pin from the CPU to the driver board
• One tone sounds constantly at power-up, even with another driver board: MPU fault
• Spare transistors for System 1 boards

Source: Gottlieb System 1 Pinball Repair from 1977 to 1980, Clay Harrell (pinrepair.com)