In most cases, repairing a doorbell is a relatively simple, quick, and inexpensive project. Familyhandyman.com has a good how-to article that will guide you through the process, and there are lots of helpful videos available on the web.
You don’t need to be afraid of working with electricity. Most of the doorbell circuit is “low-voltage” current that won’t give you a shock if you come in contact with the wires. A transformer reduces the 120-volts carried by most electrical circuits to the 10 to 16 volts necessary to run the doorbell.
Start with the doorbell button — in most cases, it will be the cause of the problem. Unscrew the doorbell button and touch together the two wires fastened to the back; if the contact between the wires makes the bell ring, then the button is broken and should be replaced. Connect up the replacement button the same way the original button was wired. Treat the wires gently, especially in an old house, to prevent them from breaking (If one does happen to break, you can buy a spool of 18-gauge doorbell wire and connect a piece to the remnant coming from the wall, using a small wire nut or Scotchlok™ connector.)
If the button isn’t the problem, check the bell unit itself. Yours may be an inexpensive bell or a fancier chime unit. In either case, remove the cover and check that the wires are attached securely to the screw terminals. If so, use a low-voltage tester or voltmeter (multimeter) to determine whether the electrical connections are working. Put one probe on the common terminal and the other on the front door or back door terminal, and then ask someone to press the button at the appropriate entrance. If the tester lights up, power is coming to the chime, but the bell itself isn’t working.
Before replacing the chime, check whether there is a mechanical problem. Dust, smoke, or grease can obstruct the hammer mechanism; clean the armatures with rubbing alcohol and manipulate them back and forth until they move smoothly. Then, if the bell still doesn’t ring, replace it. (As with the button, you’ll be working with low voltage, so you can replace the chime without turning off the power.) Be sure to identify the wires as you remove them from the old bell, so you can attach them the same way to the new one.
If the tester doesn’t light when the button is pressed, then you’ll need to look at the transformer — a small black or silver box, usually attached to a beam or an outlet box in the basement, that supplies power to the system. You can use your low-volt circuit tester or voltmeter to test whether the transformer is working. Touch the probes to the screw terminals; if the tester doesn’t light up, the transformer should be replaced.
Replacing the transformer requires that you work with 120-volt power, so remove the fuse or shut off the breaker for that circuit before you start. (Use a circuit tester to make certain the current to that line is off, or shut off all the power in the house to be sure.) Remove the old transformer and connect the new one the same way the old one was wired. (Note: if your transformer is inside your main service panel, leave this job to a licensed electrician.)
If your bell, chime and transformer all work properly, the only other place to look is in the wires that make up the doorbell circuit. It can take some tracking to find just where one of these very thin wires may have broken. While the wires may be visible in your basement ceiling, breaks usually occur behind one of the door trim pieces. If you can locate the break, you don’t have to replace the whole length of wire – just the part that is broken. Join the new portion together with the unbroken length using a small wire nut.
If you can’t find the break in the wire, you might opt for a “wireless” doorbell. Whichever solution you choose, you’ll no longer need a “Doorbell out of order” sign on your door!