Safety First

Power supplies can contain dangerous voltages even after the equipment has been switched off. Large capacitors may remain charged.

Never work inside mains-powered equipment unless you understand the hazards and have appropriate test equipment and safety procedures.

Start With a Visual Inspection

Before applying power, inspect the board carefully. Look for burned components, cracked solder joints, damaged tracks, swollen capacitors, broken wires and blown fuses.

Check the Fuse

A blown fuse is often the first indication of a power supply fault. Do not simply replace a repeatedly blown fuse with a larger fuse.

A fuse that blows again usually indicates another problem such as a shorted rectifier, transistor, MOSFET, capacitor or transformer.

Check the Transformer

In transformer-based supplies, check the transformer windings and connections before proceeding to the rectifier stage.

Resistance measurements can help identify an open winding, but a resistance test alone cannot prove that a transformer is operating correctly under load.

Check the Rectifier

A bridge rectifier converts AC into pulsating DC. Individual diodes can be tested with the diode-test function of a multimeter when the circuit is safely disconnected.

Check the Filter Capacitors

Large electrolytic capacitors smooth the rectified voltage. A failed capacitor can cause excessive ripple, low output voltage or unstable operation.

Capacitance, ESR and visual inspection can all provide useful information when diagnosing electrolytic capacitors.

Check the Regulator

Linear and switching regulators control the output voltage. Measure the input and output voltages according to the circuit design.

If the input voltage is correct but the output is missing, the regulator or another component connected to its output may be faulty.

Work Through the Circuit Stage by Stage

Stage	What to Check
Input	Mains, connector, switch and fuse
Transformer	Primary and secondary windings
Rectifier	Diodes and bridge rectifier
Filter	Electrolytic capacitors and ripple
Regulator	Input, output and control signals
Load	Short circuits and excessive current

Conclusion

The safest and most effective troubleshooting method is to divide the power supply into stages and test each stage systematically. Avoid replacing components randomly; measurements can usually narrow down the faulty section first.