When the bell doesn’t ring: Assume that the bell does not operate when the button switch is pushed. The two most common problems are a bad switch or a bad bell. To track down the trouble, measure the voltage across the push button switch. With the switch open, 10 volts should appear across the open switch. There is no current in the circuit and thus no voltage across the bell (M3). When the switch is closed, meter M should read zero. If a voltage appears across the switch even when it is closed, this indicates that the circuit is not operating properly. Check the contacts to see if they are corroded or broken. The switch can possibly be repaired by simply scraping and cleaning the contacts. However, it may have to be replaced. If 10 volts appears across the bell when the button is pushed and the bell does not ring, the bell is probably defective. Disconnect it and check its resistance to see if it has an open coil.
