Checking a doorbell transformer is the easy part; sometimes just locating it can be the real challenge.
Sometimes finding a doorbell transformer is a challenge. It may be located inside the chimes or bell enclosure but often is located elsewhere.
If you know where the doorbell wires run, trace them back to find the transformer’s location. Newer homes often have doorbell transformers conveniently located in plain sight, high on a garage wall.
Sometimes it’s mounted on a floor joist in the basement, below the doorbell, in a closet, in the attic (possibly buried under insulation), or in a crawlspace. Or, it may be located inside an electrical box, near the electrical panel or the doorbell.
The proper voltage rating should be stamped on the transformer and/or the doorbell mechanism.
To check the doorbell transformer:
1 Adjust a volt-ohm meter’s scale to 25 VAC. Attach the two probes to the two flat screws that connect the small-gauge doorbell wires to the transformer.
2 Compare the reading to the device’s voltage requirements. If it’s too low or exceeds 16 volts, shut off the power to the transformer before doing any more work on the doorbell system. Replace the transformer with a new one, readily available at home centers.