If they re tight, hold a screwdriver across the transformer s bell-wire connections. If you don t get a weak spark, replace the transformer. If you do see even the faintest spark, the transformer is fine and your search narrows down to the bell itself or to the wiring. The easiest way to check the bell is to disconnect it, clean it thoroughly, rub the contacts with emery cloth, then hooks it up directly to the bell-wire connections at the transformer. And that includes a couple minutes to go to the service panel and cut the power to the circuit.
Some brands come with grip holes instead of screws.
A switch is wired only into the hot line, with the source feed usually connected to the top terminal. The neutral wires in some circuits are independent of the switch and continue all the way back to the service panel. Sometimes they face the side. If a receptacle goes bad and you re elected to replace it, all you need to do is wire the new outlet exactly as the old one. In newer homes and some older ones, there s often a third ground wire that connects to the receptacle.