Checking a UDN driver with a logic probe and scope

Steps

• Put the game in display test and step through the digit patterns (all 0s, all 1s and so on).
• Probe the input pins 1-8 with a logic probe; they should pulse and change as you step.
• Check the output pins 11-18 with an oscilloscope on a 100 V range, never a logic probe; they should pulse too.
• Inputs active but an output stuck means the chip is bad. Example: segment b stuck on for players 1 and 2 traced to output pin 18 of the 7180 at U14.

The display supply runs at about 100 V DC (plus and minus) with large filter capacitors; work with the game off and unplugged, and measure live only with insulated probes.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• UDN display driver chips
• Telling a shorted UDN chip from a shorted display glass
• One segment lit on many characters of the top display: unsoldered leg on a driver chip
• High-voltage fuse blows once the display connector is plugged back in
• System 9 versus System 11 display data
• Display board chips shown as 4050 are really 4049
• Same segment missing across one 16-digit display
• Characters missing from a display

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Three, Clay Harrell