After replacing the large filter capacitor on the power supply board, the game will not boot and both CPU LEDs stay on. The 12 V line measures only about 8 V DC.

CPU LEDs stay on after recapping, only 8 V on the 12 V line: cap in the wrong hole

Cause: The new 12,000 uF capacitor's negative lead had been put in a pad that is not connected to ground (there are several holes on that side).

Checks

• Measure the 12 V test point.
• Check the new capacitor's leads go to the pads the old one used, and that the negative pad has continuity to ground.

Fix

• Refit the capacitor with its negative lead in the grounded pad.

Large filter capacitors can hold a charge; discharge before handling.

Systems

• Williams System 4

Games

• Phoenix

Related tips

• What the line filter does
• Replace the +5/+12 V logic filter capacitor
• What the power supply board takes in and puts out
• Test the power supply with its outputs unplugged
• Swapping power supply boards between generations
• The backbox bridges and can capacitor are not logic power
• Judging the logic filter cap by AC ripple
• No +5 V on a System 3-6 power supply

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