If a switching power supply is faulty, 90% of faults can be located by following the four steps of "Inspect - Test - Disconnect - Replace":   1. Inspect After powering off for 5 minutes, visually inspect the housing for bulging, burning, blown fuses, or capacitor leakage, taking photos for record.   2. Test Use a multimeter in the DC range to measure the main capacitor: 300 V ±10% at no load. Below 250 V indicates a defective rectifier bridge or filter. Measure the output: No load should be ±5% of the nominal value; short circuit should be 0 V, the fuse is intact, and the secondary rectifier is broken.   3. Disconnect Unplug the load and use a 30 W/12 V automotive light bulb as a dummy load: If the light bulb lights normally, the power supply is OK, and the problem lies with the load. If it does not light or flickers, proceed to the n...