If transformer is defective, deenergize circuit, and remove transformer. Buy replacement transformer of the same voltage and wattage. You can find electrical information stamped on transformer, and you should find installation instructions on package. Follow instructions carefully. Use crimp-on connectors or wirenuts to attach new transformer to circuit line wires of your electrical system. Then connect bell wires to low-voltage screw terminals on transformer, turn power back on, and press doorbell button. If you've installed transformer properly, you should hear bell or chime.