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A computers power supply is a very complex device. A very quick diagnostic is to simply replace the power supply and then wait to see how the system behaves. But, before we replace the power supply, let’s check a couple of things.
First of all, we are going to check the CPU power line. There is no point to replace the PSU when there is a short circuit somewhere in the CPU's power line. For this, we will use a simple multimeter and check the Resistance of a 4-pin auxiliary connector.
Usually the Resistance in the connector on the motherboard is near to infinity. When the multimeter is connected for the first time, it will show 150 ohm and then quick to infinity. This is normal and this is the way it should be on the motherboard. Of course we need to do this on a motherboard that is not powered.
The second thing to check is the CPU Resistance. Usually, it is near 7 ohms or more. Something less than 4 ohms would indicate a CPU fault. The CPU fault can sometimes look like a PSU fault as the system cuts off after a power on or does not even turn on at all. This happens because the power supply has a short circuit protection and will not start at all if the short circuit is on any power lines.
There are also PSU testers that do not cost a fortune. The advantage of such testers is that they show all the voltages at once and this is convenient for even a non-experienced user to test. But, they do have one disadvantage. The PSU testers can’t see the actual signal form like an oscilloscope can. There is a very high risk that PSU testers will not show any voltage fluctuations or spikes that can be dangerous for hard drives.
The PSU can also be diagnosed using an oscilloscope. This is my recommended way to troubleshoot your computer power supply. As I mentioned above, PSU testers do not show a complete signal, which can be perfectly flat, or if the capacitors in the PSU are gone, the tester will show fluctuations or spikes. Sometimes it is hard to see voltage fluctuations as it can be only mili Volts in amplitude. When I measure the voltage on a PSU, I always set a scale to 2 mV or the lowest Volts per cm possible. It might also be beneficial to adjust on a horizontal scale. If you know what you are looking for, then you will know how to find it.
Observe visually. Visually observing can also tell a lot about the PSU. First of all, check the temperature by placing your hand on the metal surfaces on every side that you can access. Usually, the power supply is just a bit warm. If it is hot then something is wrong with it or a component on the motherboard. The capacitors also can be seen trough the ventilation gaps in the power supply. It is important to look for the top of the capacitor. If the top of the capacitor is flat and shiny silver, then everything is okay. If the top of the capacitor is bumpy, has a darker color, or a liquid coming from it, then these are the signs of a faulty capacitor.
I do not recommend opening the computer power supply unless by a qualified engineer.
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Me holding a laptop hard drive