At power-up the game beeps four times, then gives its usual start-up tones, but only the GI comes on and the display stays blank. Pressing the CPU reset button sometimes gets it to attract mode.

Four beeps at power-up, GI only and no display: game has no batteries

Cause: There were no memory batteries in the game, so the CPU could not keep its settings and failed its start-up check.

Checks

• Check the battery holder has fresh batteries and that the holder and its traces are not corroded.
• Look up the beep code in the manual; four tones points at the memory chip area.

Fix

• Fit fresh batteries (or a remote battery holder), then do a factory reset if the settings are scrambled.

Systems

• Williams System 11B

Games

• Black Knight 2000

Related tips

• Game will not boot consistently: power-on reset (POR) circuit
• "Adjust Failure" at power-on
• "IRQ Failure" at power-on
• Resets continue after diode repair: power and connector checks
• CPU keeps resetting: reset line at U15 pin 40
• Displays flash bright and speakers noisy, game does nothing
• MPU won't boot even though a test ROM passes every PIA and RAM: bad game ROMs
• Displays lose digits and coils buzz once the game warms up

Source: Pinball Repair help group!! (Facebook group)