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There is not a lot of good information about computer hardware diagnostics out there. A few books mention information about troubleshooting, and about the operating system and diagnosing software issues. But, there is not much information about hardware diagnostics overall.
I already mentioned on this site about hardware diagnostics. Hardware diagnostics is done by using probabilities, and with certain probability, I could tell what is wrong with your computer using this method. With about a probability of 70%, I could I say that something specific is wrong with your computer hard drive or that you have a virus. This is because a very large amount of repairs are related to the hard drive.
There is no consumer hardware out there for hard drive diagnostics, but there is a small software available for free that does a very good job at helping called GSmartControl. You can download this software as it can also prevent a disaster from happening by determining how long the hard drive will live. And, knowing this information is very important.
For the motherboard diagnostic and everything electronics diagnostic, a multimeter and oscilloscope can be used. A multimeter measures the voltage and resistance which is what we need to diagnose short circuits, overheating, etc. With a multimeter bad capacitors can even be diagnosed, but I personally would use a oscilloscope for this.
Not only because the problem is visual, but because an oscilloscope can show you what is actually going on in there. If you touch a probe to the capacitors, as well as the pin, there must be a straight line which does not do anything when you change the trigger level. If there is movement following the trigger then the capacitor is faulty. The weird thing is that it seems to follow the trigger level.
There are motherboard diagnostic boards with digital indicators. But, I never saw any usage to it. First of all, every BIOS has a different error code meaning and second, I never got any useful information from such cards.
For me, it is better to use a multimeter or oscilloscope.
It is good to familiarize yourself with computers by taking a good working computer and then measuring everything possible with a multimeter and an oscilloscope. You will discover there are step down converters, MOSFETs, the PSU, and most importantly, you will learn what voltages are and when they are working properly. This is how I learned, I took a working computer and measured everything I could on it and then investigated it when it was powered down.
Take a cheap desktop computer to inspect so it will only be small loss if you damage it accidentally.
Learn typical faults on laptop and desktop models. After you see typical faults, you confirm this information by measuring or replacing the part, skipping the diagnostic process.
Computer Problem Troubleshooting Guide
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Me holding a laptop hard drive