Testing a CPU lamp circuit by grounding its driver

Steps

• Each CPU lamp has one wire back to its driver transistor, which switches ground through its 74175.
• Clip a lead to the copper ground strip or backbox metal.
• With the game on, touch the right-hand leg (facing the installed board) of the MPS-A13 or MPS-U45 for that lamp.
• Lamp lights: wiring and bulb are good, so test the transistor.
• Lamp stays dark: check the bulb, socket, wire and driver board connector; light sanding of the fingers or new pins often fixes it.

This check is done with the game on; keep the clip lead away from other parts and touch only the named point.

Systems

• Gottlieb System 1

Related tips

• How CPU-controlled lamps are driven
• No CPU controlled lamps light
• Shorted lamp sockets kill MPS-A13 drivers
• Finding which transistor drives a lamp
• Groups of four lamps or single lamps misbehaving
• The CPU to driver board harness
• Testing a driver board with a Ni-Wumpf CPU
• Early driver boards lack isolation diodes

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