A while back I had a blog on Google+ linked to my freelance IT business and I started recording some case studies. I think I only did a few in the end but I thought I'd share them anyway.
Let me begin by saying this actually happened some time ago, before I started this blog. I'm adding it because it was an interesting and fairly unique case, but the specifics may be a little sketchy. I went to look at this PC, neither old nor new, and it was turning on but nothing happened after that. Just a black screen, with no signal to the monitor, but the power light was on. At this point you might think there's nothing that can be done and you're almost right.
When a computer starts up you normally see the logo of the manufacturer of your PC. Behind this, the computer is checking itself to make sure all the important bits are working properly (older computers pre-2002-ish just showed the test taking place). If the test fails, it will beep at you a certain number of times to let you know what the problem is. In this instance even that wasn't happening. If the computer didn't turn on at all that would be easy to solve, because it would be one of the following:
1) it's not plugged in,
2) a fuse has blown,
3) the power supply is faulty.
The first two are obviously easy to fix and the last one costs about £15 (or free if, like me, you've got spares lying about). This computer, however, turned on but didn't beep either. A PSU fault could cause this symptom if it's only working partially. Unless it's able to output steady voltages at 3.3V, 5V and 12V the PSU could cause instability at best and damage to other components at worst. The PSU tested good, so there are only two parts of the computer that could be wrong at this point: the motherboard or the CPU. The motherboard is the main circuit board that you can see when you open up a computer. Everything connects to this - think of it as the torso with the blood supply etc. The CPU is the 'brain' of the computer - it processes all the data like we do with what we see, smell and touch, etc. Fortunately the CPU is a removable part in a desktop PC and in some laptops. It is, however, one of the most expensive parts of the computer. New top-of-the-range CPUs cost upwards of £300 but, once they're a few years old, they drop in value to more like £50 so it's cheap to replace an older CPU. Motherboards are more complex but usually cost about £100 for a standard model but you can pay upwards of £500 for something really advanced.
It's relatively easy to work out which one of these has gone wrong. Fortunately I had a similar computer to this one so what I did was take the CPU out of the client's PC and put it in mine instead. The good news was that it worked fine. At this point we can assume the motherboard is at fault but, to check for sure, we take another CPU that we know works and put that in the suspect motherboard. As expected, it still didn't work. This is bad news. CPUs are mass-manufactured and very easy to find second-hand on eBay and the like, but motherboards are often specific to a particular computer or manufacturer in branded computers (proprietary). Custom-built or home-made PCs just use off-the-shelf boards. Finding a proprietary one second-hand is a game of luck - unless someone is selling one you're stuck. While CPUs generally fit in standard socket types, motherboards are different sizes and have different functions built in so it can be very hard to find an equivalent replacement.
Repairing motherboards used to be a lot easier. Today's many-layer PCBs, surface-mount components, high levels of integration and precise engineering make it very difficult for a person with a multimeter and a soldering iron to perform any meaningful repairs beyond replacing capacitors.
This time we got lucky - someone was selling the exact same board on eBay for around £100 and it was relatively local. The client's choice was simple: spend £100 to get the old computer working or put the money towards buying a new one. This is often a tough decision but the client decided to keep their computer going and it was only £30 labour to fit it.
The cost difference between repairing a PC and replacing it is slimmer than it has ever been. For some time now, PC manufacturers have been making their hardware increasingly proprietary. Dell were probably one of the first companies to really do this, with their 'build-to-order' website. Nearly every component that was previously an 'add-on', such as video and sound were built into the board to save cost. This does unfortunately mean you can replace components as they fail. Retailers and manufacturers would much rather you buy something than repair it, hence the rise of the Right To Repair movement. It requires a level of independence and integrity to keep hardware going these days rather than outright bin it.