A circuit breaker on the transformer assembly trips every time the game is switched on, even with the transformer disconnected from the power supply board, so the transformer looks faulty.

Circuit breaker trips even with the transformer unplugged from the board: bad breakers

Cause: The circuit breakers themselves had failed and were tripping well below their rating. Even the first replacement breaker was bad.

Checks

• Before condemning the transformer, temporarily substitute a fuse of the same rating for the breaker; if the fuse holds, the breaker is the problem.
• Test each breaker; they are a much more common failure than the transformer.

Fix

• Replace the faulty breaker(s) with good ones of the correct rating and recheck every secondary voltage.

Games

• Fireball (Home Model)

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