Diode-test UDN chips and score glasses for shorts

Steps

• Game off. Unplug the high-voltage connector 3J5.
• Meter on diode test, red lead to ground.
• Black lead on each UDN pin 2-8: expect 0.5 to 0.7. Black lead on pins 11-17: expect no reading. (Skip corner pins 1, 9, 10, 18.)
• A shorted glass can also show up on pins 11-17. To tell which, carefully desolder the chip without damaging it, fit an 18-pin socket and test pins 11-17 of the empty socket.
• Empty socket reads open: the chip was bad. Socket still reads short: the display glass is shorted and must be replaced; then retest the chip in the socket.

The display supply carries about 100 V DC (plus and minus); keep fingers and probes off the master display board and power supply high-voltage section with the game on.

Systems

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

Related tips

• All score displays dead
• One score display dead
• A segment missing on a score display
• Four digits missing on the player 2 and 3 displays: bad driver IC12 on the master display
• Score displays dead but digits glow: bad wire in the CPU blanking plug (1J3)
• Bottom segments missing and master display resistors burning: shorted display glass
• Master display board versions
• A half-working UDN7180 can still be used

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