At power-on the 5 V LED lights but there are no displays; after about a minute the game boots. The 12 V supply starts at about 2.4 V and only reaches 12 V after 30 seconds or so, and the flippers work erratically.

No displays for a minute after power-on: hidden delay timer in the power wiring

Cause: A previous owner had wired an aftermarket 90-second power delay device into the supply, hidden under tape.

Checks

• Watch the CPU's blanking LED: no displays appear until blanking comes on.
• Measure the 12 V supply from switch-on; a slow rise points to the supply or something in its path.
• Look along the power wiring for added devices, splices and tape.

Fix

• Remove the added delay device and restore the original wiring, then recheck the supply voltages.

The device was in the power wiring; unplug the game before working on it.

Systems

• Williams System 11A

Games

• Pinbot

Related tips

• Display voltages at 3J5, power supply D-8345 (High Speed to Swords of Fury)
• Low +5 V (below 4.9 V) from the power supply
• What the System 11 power supply board feeds
• Fix the display short before trusting a rebuilt supply
• Rebuilding the +100 V section of the power supply
• Rebuilding the -100 V section of the power supply
• Measure a rebuilt supply before reconnecting the displays
• GI connectors left the power supply after Cyclone

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