Telling a shorted UDN chip from a shorted display glass

Steps

• Carefully remove the suspect UDN chip without damaging it and fit an 18-pin socket.
• Check the socket for shorts and that every trace reaches it.
• Repeat the diode test on the empty socket, pins 11 to 17. No reading means the chip was bad; a reading means the display glass is probably shorted.
• A shorted glass cannot be repaired; replace it, then plug the UDN chip into its socket and test it again.

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

• Display board chips shown as 4050 are really 4049
• System 9 versus System 11 display data
• Characters missing from a display
• Checking a UDN driver with a logic probe and scope
• One segment lit on many characters of the top display: unsoldered leg on a driver chip
• Displays dead after the CPU board was removed
• Same segment missing across one 16-digit display
• High-voltage fuse blows once the display connector is plugged back in

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