Build a tester LED for PIA and driver checks

Steps

• Solder a 150 ohm resistor to the flat-side (cathode) lead of an LED.
• Solder a wire or clip lead to the other LED lead; this connects to +5 V.
• Use the free resistor lead as the probe. The LED lights when the probed point is low.
• Buy LEDs from a reliable source; some surplus LEDs have the flat side on the wrong lead, which reverses the readings.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• Add a blanking LED to a driver board
• Making the test LED for the test EPROM

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Two, Clay Harrell (pinrepair.com)