The three pop bumpers, both slingshots and the up-post are very weak while the flippers are normal. The backbox boards work perfectly in another game, the coils measure correctly, and bypassing the fuse or feeding the coils directly from the supply doesn't help, but jumpering a coil to the cabinet ground braid makes it fire at full strength.

Pop bumpers, slings and post weak, flippers fine: bad coil ground return plug

Cause: A poor connection in the solenoid ground return path. The coils' return goes back through a common wire and connector on the driver board, separate from the flippers' return; the old plug at that connector (A3J3 here) was bad.

Checks

• Fire a weak coil with its drive side jumpered to ground: full strength points to the return path, not the coil.
• Inspect and repin the driver board and rectifier board connectors, especially the one carrying the solenoid ground return.
• Check the coil power daisy-chain wire for a strand hanging on by a thread.

Fix

• Replace the housing and pins of the solenoid ground return connector (and any other tired connectors on the driver and rectifier boards).

Systems

• Bally MPU AS-2518-35

Games

• Fireball II

Related tips

• Coils go weak or the wrong coil fires mid-game: reseat the solenoid driver plugs
• MPU gives 3 flashes after reassembly: two 15-pin plugs swapped
• Knocker fires instead of the outhole kicker: bad solenoid driver J4 connector
• Pop bumpers and slings die minutes after a driver board swap: playfield fuse
• No coils work at all though fuses are good and game starts: 43 V not reaching playfield
• Pop bumpers fire nonstop after a solenoid expander swap: board jumper set wrong
• Coil in the drop target assembly is always on: burnt trace on the solenoid driver board
• Backbox GI out and fuse F6 blowing: burnt rectifier J3 plug and a shorted bridge

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