Transformers are key electrical components in power supplies and audio circuits. Though they are reliable and have no moving parts, sometimes transformers fail, rendering the equipment it powers inoperable. If you suspect a bad transformer in a piece of equipment, you can perform a simple test with an oscillator that checks the transformer's basic functions. The oscillator's AC signal tests the integrity of the primary and secondary windings; broken wires or shorts inside the transformer will produce no output. The method is safe as it avoids the high voltages usually present inside electronic equipment.
Disconnect the equipment containing the transformer from the wall outlet, if the transformer is in a circuit.
Turn the oscillator on. Set its frequency to 60 Hz.
Touch the digital multimeter's probe tips to the oscillator's output. Set the multimeter to read AC volts.
Set the oscillator's signal amplitude to 5 volts by monitoring the multimeter's display as you adjust the oscillator's amplitude control. Disconnect the multimeter from the oscillator.
Connect the oscillator's output to the transformer's two primary wires.
Touch the multimeter's probe tips to the transformer's two secondary wires. A good isolation transformer reads about 5 volts AC on the meter. A good step-down transformer produces from 100 millivolts to 1 volt AC, depending on its rated output voltage. If the output reads only a few millivolts or less on the multimeter, the transformer is bad.