If bell doesn't ring or bulb doesn't light at button or bell box, both are okay. By process of elimination, you now know problem must be in transformer or wiring. You'll usually find transformer mounted on electrical junction box, subpanel, or main entrance panel. Bell wires are attached to exposed terminal screws on transformer. Connect test bell directly to exposed low-voltage transformer terminals; don't touch any other screws. If bell doesn't ring, transformer is defective or not getting power. Caution: Transformer is connected directly to power supply, and it carries current that can hurt you. Before working on transformer, deenergize branch circuit that supplies power to transformer. Remove appropriate fuse, trip correct circuit breaker, or throw main switch to shut off all electricity in your home.