What is the common fault of switching power supply? I've found that only a small percentage of components fail in switching regulator power supplies. A common failure is the switching transistor itself. The transistor shorted out, causing a lot of current to flow through the transformer and blow the fuse.

Transistor failures are usually caused by bad capacitors. It is common to find bloated or leaky output filter capacitors. Capacitors that look bad should be replaced. To prevent this common failure from recurring, the output filter capacitors should be replaced with capacitors. These capacitors are designed to handle the stringent filtering in switching power supplies. Most power supply manufacturers do not install low ESR capacitors as original equipment because they are a bit more expensive than traditional capacitors. However, it is worthwhile to use them as replacement components, as they will greatly extend the life of the power supply in the field. When I use the power supply, I replace the output filter caps with low ESR caps, no matter how good or bad they look. Since service calls cost much more than capacitors, this is a prudent practice.

Secondary tube failure is another common problem. There are many diodes in the switching power supply, and the failure of one diode will cause the power supply to blow the fuse or shut down. A common diode failure is a short circuit to the +12 volt or -5 volt output rectifier. Failure of these diodes will not blow the fuse. The power supply detects a short circuit and shuts itself down. Some of these failures can be caused by using the +12 or -5 volt output to power the coin gate light. In the power supply, none of the outputs are overcurrent protected. A short circuit in the lamp holder can blow the diode by drawing too much current from the power supply. If you inadvertently use a 6 volt bulb instead of a 12 volt bulb, the +12 volt diode may blow. The high-voltage input diode may also be short-circuited.