The Power Supply Need Grounding
If you are repairing a switch mode power supply (SMPS) make sure the power supply board have earth ground connected to it. If you look at the photo of a LCD Monitor power supply, you could see that the earth ground is connected to the casing and the power supply board grounding connection did not attach to the casing because you want to perform the voltage test. In this case, if you put your black probe to the power supply board ground and try to measure the output voltages from the SMPS, there will be no reading. So you will think that the power supply have problem. You begin to inspect the power supply board this will waste your precious time. The moral of the story is to let you know that before you want to take any measurement in a SMPS you need to make sure the power supply secondary side is properly grounded.