Test the Terminals
If you have a chime assembly with three or more wires, tag them with masking tape: "T" for transformer, "2" for front door chime, and "1" for the back door chime. Loosen terminal screws, remove all wires, and connect wires labeled "T" and "2" to screw terminals on test bell or bulb. If test bell rings or bulb lights when you push front door button, old chime set is faulty. To check this conclusion, connect wires labeled "T" and "1" to screw terminals on test bell. If bell rings when you push back door button, then you're certain chimes must be replaced.