Several pop bumpers don't fire, and fuse F21 blows as soon as a game starts, but only while one particular pop bumper driver board is plugged in.

Fuse F21 blows at game start: find the shorted pop bumper driver board

Cause: A shorted part on that pop bumper driver board. Other pop boards in the game had also failed.

Checks

• Unplug the pop bumper boards one at a time to find which one blows the fuse.
• Test the board's transistors and diode with a meter on diode test.
• The pop boards get 5 V and ground through the MPU switch matrix connector, so check that connector too.

Fix

• Rebuild or replace the faulty pop bumper boards (updated versions are available), then fit a new fuse of the correct rating.

Systems

• Gottlieb System 80

Games

• Black Hole

Related tips

• System 80 driver board transistor types
• Lower playfield pop bumpers lock on or blow their fuse: shorted driver transistors
• Both pop bumpers fire together when one is hit: spoon switch contacts too close
• Up-kicker fires by itself when the flippers are pressed
• Black Hole up-kicker on a pop bumper driver board
• Pop bumper switch error only with the playfield down: weak pop spring
• Sound supply fuse F8 blows after the game runs a while

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