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.
Calculate the voltage output for a step-down transformer by dividing the rated output voltage by the input voltage, then multiply by five. For example, if the transformer's output is rated at 24 volts with a 120-volt input, divide 24 by 120 and multiply by 5 to obtain 1 volt.
If you are testing a transformer inside equipment plugged into the wall outlet, it is important that you disconnect the plug from the wall before testing. Even with the equipment switched off, you might receive a shock or damage the oscillator if the equipment is still plugged in.
Extra high voltages exist in some kinds of vacuum-tube equipment, even with the equipment unplugged. Unless you have experience testing these kinds of circuits, refer any testing to a trained technician.