Computer Repair Lessons: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
I will try to describe here the most common computer power supply problems out there.
First of all, let's check the power supply with an oscilloscope and see how the +5V and +12V look. To see small amplitudes better, then we need to switch the oscilloscope to AC coupling. There can be a little noise or fluctuations in the oscillogram coming from the power supply. A small spike is also okay until they are extremely small. One thing to look out for is a small quire form that can be seen, this will indicate capacitor problem. If this occurs, it is better not to take the risk damaging the hard drive and simply replace the power supply.
There is also one fault that even prevents the PC from powering on. This fault is in the same place on PC desktops and on laptops. This is called burned MOSFET.
MOSFET’s are simple electronic switches used in power circuits. They usually are on a heavy load and are on a much higher temperature compared to other components on the motherboard. When the MOSFET burns, we usually get a short fan spin which may sound like a strange noise and the computer will cut off. This is because when the MOSFET burns it makes a short circuit and the modern smart PSU protects itself by cutting off. If the system would fail to protect itself, it would fail permanently. So, this may look like a PSU fault, but in reality it is motherboard fault. The most common symptom that can be found and measured with a multimeter is the resistance in the socket where the +12V power line is next to the CPU.
Take the plug out and measure the resistance with a multimeter. If it shows less than 100 ohms and stays like this, it means that one of the MOSFET’s is faulty. If it shows a reading for a second and then goes to infinity or a very big value, then everything in this area is okay and the fault is somewhere else. Usually, the short circuit occurs between the Gate and the Drain. And most likely, this also blows something on the motherboard, such as a driver chip. The drain pin is the biggest pin on the MOSFET. Usually, when the MOSFET fails, it fails with smoke. The smoke also indicates serious trouble with the motherboard or the PSU.
There are several PSU testers that available from online retailers. These tester devices will show voltages and will tell you if the power supply is okay or not. The tester is a basic device and cannot show oscillograms.
If the computer does not have a surge suppressor, then buy it today. We have seen computer PSU’s fail because of lightning or because there was a spike throughout the power line from a close factory.
And, as a reference, an office with 20 or more computers are a potential electronics damager because all of their power supplies pollute the main line.
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Me holding a laptop hard drive