Displays stay weak and the 100 V section reads below 90 V even with the displays unplugged.

Display supply low (under 90 V) after the resistors are changed

Cause: The 3.9 V zeners ZR1 and ZR3 (1N5990, 1/2 W) leak.

Checks

• Change the 39k resistors first, then re-measure with the display connector unplugged.

Fix

• Replace ZR1 and ZR3 with 1N4730A (3.9 V, 1 W).
• If the output is still under 90 V, rebuild the whole high-voltage section.

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 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7
• Williams System 9
• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• Rebuilding the +100 V section of the power supply
• Rebuilding the -100 V section of the power supply
• Game will not boot consistently: power-on reset (POR) circuit
• Low +12 V (below 10.5 V)
• Test the power supply with its outputs unplugged
• Low +5 V (below 4.9 V) from the power supply
• Fix the display short before trusting a rebuilt supply
• Measure a rebuilt supply before reconnecting the displays

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