The two lower playfield pop bumpers lock on as soon as their power comes up, or blow the 2 A slow-blow fuse.

Lower playfield pop bumpers lock on or blow their fuse: shorted driver transistors

Cause: Both drive transistors on the lower pop bumper driver boards were shorted.

Checks

• Note that the lower pops' 24 V only switches on while the lower playfield is in play, so there is no voltage at fuses F21 and F22 in attract mode; start a game and trip a lower playfield switch before measuring.
• Test the driver transistors on the pop bumper boards.
• Check the ground wiring from the pop bumper boards back to the power supply, a common cause of these boards failing.

Fix

• Replace the shorted transistors (TIP102 was used here).

Systems

• Gottlieb System 80

Games

• Black Hole

Related tips

• Replacements for the 2N5875 under-playfield power transistors
• Adding the 4.7k base pull-up to playfield power transistors
• Coil stuck on that is driven through a playfield-mounted power transistor
• Which wire goes where on a playfield 2N5879
• Testing a playfield-mounted 2N5875/2N5879 with a meter
• Fuse F21 blows at game start: find the shorted pop bumper driver board
• Coils pulse briefly at switch-on on Volcano, Black Hole and Haunted House
• Add power and fire LEDs to a pop bumper driver board

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