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Recently one of my friend who work in the maintenance field sent me a switch mode power supply board for repair. This SMPS board comes with a control board for controlling display. The complain was no power and the fuse blown into a very dark color. There must be some components that had developed a direct short circuit that could have cause the fuse to turn into black and by replacing the fuse only will not solve the problem. My next step was actually to use a series light bulb trick but have decided to just scan from above to see if there is any visual indication of components blown. Scanning from above i could see the varistor have some problem. After taken out the part and check with ohmmeter and found that it was totally shorted.
As usual one have to check for any corresponding parts first before you put back a new component and power On it. I have tested the bridge rectifier, power fet and check the big filter capacitor and components in the EMI filter circuit and found all of the components to be good. Conclusion, this switch mode power supply problem was caused by only a shorted varistor and a blown fuse. The indicators (LED’s) in the controller board lit proof that the power supply was working fine upon power On.