A typical ATX power supply circuit is shown in the figure. Structurally, it is a classic pulse unit on a TL494 PWM controller, triggered by a PS-ON (Power Switch On) signal from the motherboard. The rest of the time, until the PS-ON pin is pulled to ground, only the Standby Supply with a voltage of +5 V at the output is active.

Let's take a closer look at the structure of the ATX power supply. Its first element is
:

Its task is to convert alternating current from the mains to direct current to power the PWM controller and standby power supply. Structurally, it consists of the following elements:

• Fuse F1 protects the wiring and the power supply itself from overload in the event of a power supply failure, leading to a sharp increase in current consumption and, as a consequence, to a critical increase in temperature that can lead to a fire.
• A protective thermistor is installed in the neutral circuit, which reduces the current surge when the power supply is connected to the network.
• Next, a noise filter is installed, consisting of several chokes ( L1, L2), capacitors ( C1, C2, C3, C4) and counter-wound choke Tr1. The need for such a filter is due to the significant level of interference that the pulse unit transmits to the power supply network - this interference is not only picked up by television and radio receivers, but in some cases can lead to the malfunction of sensitive equipment.
• A diode bridge is installed behind the filter, converting alternating current into pulsating direct current. Ripple is smoothed out by a capacitive-inductive filter.

Standby power supply is a low-power independent pulse converter based on the T11 transistor, which generates pulses through an isolation transformer and a half-wave rectifier on the D24 diode, powering a low-power integrated voltage stabilizer on the 7805 chip. Although this circuit is, as they say, time-tested, its significant drawback is high voltage drop across the 7805 stabilizer, which leads to overheating under heavy load. For this reason, damage in the circuits powered from the standby source can lead to its failure and subsequent inability to turn on the computer.

The basis of the pulse converter is PWM controller. This abbreviation has already been mentioned several times, but has not been deciphered. PWM is pulse width modulation, that is, changing the duration of voltage pulses at their constant amplitude and frequency. The task of the PWM unit, based on a specialized TL494 microcircuit or its functional analogues, is to convert DC voltage into pulses of the appropriate frequency, which, after an isolation transformer, are smoothed by output filters. Voltage stabilization at the output of the pulse converter is carried out by adjusting the duration of the pulses generated by the PWM controller.

When a very nasty buzzing or rattling noise starts coming from the computer's power supply, this is a sure sign that replacing the cooler (fan) in the power supply is just around the corner. But it happens that it is not possible to replace it “this second”. But it cannot be left as is for two reasons. The first is a nasty sound from the system unit (more precisely, the power supply). Secondly, the fan may simply jam at some point in time, and the power supply will burn out from overheating. And this risks replacing half of the “filling” of the system unit, which will cost a pretty penny!

Lubricating the fan (cooler) of the power supply.

For this procedure we need a fairly simple set of tools:

• heat-resistant silicone grease or, at worst, machine oil;
• screwdriver (phillips);
• scalpel or stationery knife;
• wide tape;
• several toothpicks or matches;

This set of tools is quite enough to lubricate the fan in the computer power supply.

Read also: How to make a flash drive from a hard drive?

There is nothing complicated in the procedure of removing the power supply from the computer case. Tighten the bolts that hold the left side cover, slide it back and remove. After this, disconnect all the wires going from the power supply to the boards and disks (be sure to remember what goes where!!!). After this, outside the computer on the back wall, unscrew the 4 bolts that secure the power supply itself.

And here is the power supply on the table. There are two most popular types (based on the location of the coolers) of power supplies. With fans with a diameter of 80mm (it is located on the back wall of the power supply, which faces outward) and a diameter of 120mm (installed on the power supply cover, which faces down and inside the system unit).

In both cases, the first action will be to open the power supply cover. To do this, you need to unscrew the four bolts on the top edge (in the picture). After that, simply pull the cover up.

Read also: The computer began to work loudly (buzzing)

After removing the casing, you can unscrew the fan itself.

If you have a large, 120mm fan, then on the same cover that you just removed, unscrew the four screws that hold the fan.

If you have a cooler with a diameter of 80mm, unscrew the same 4 screws, but on the back wall of the power supply.

After this, carefully use a knife to peel off the sticker from the back of the fan. Then use the same knife to pry and pull out the rubber plug that covers the bearing, and drop some oil there. There should be very little of it - just to wet the surface of the bearing (the bushing in which the fan axis rotates). If you can’t accurately measure the oil, no problem! Take a toothpick and sharpen the tip into a narrow spatula with a knife. Then dip this spatula in oil and gently lubricate the bearing with it.

Problems with unstable voltage in AC networks are the scourge of domestic electrical networks, which leads to the failure of many household appliances. For example, a desktop computer. Both during operation and when switched off, this device is exposed to the negative effects of power surges. The thing is that the negative impact is mainly directed at the power supply, which still works even when the computer is turned off. And that means this is the most vulnerable place. That's why it most often fails. And here many ordinary people have a question: what to do: buy a new one or repair the computer power supply with your own hands?

The question is actually posed very correctly. Everything will depend on how the computer is built. If the power supply is assembled from nameless parts (specialists usually call them non-name parts), then this is a cheap option that makes no sense to repair. It will be easier and cheaper to choose and purchase a new one. Although you can try to repair a computer power supply using it. Even if you don't succeed, it will be a good experience. So it's worth tinkering with it at your leisure.

But if your computer has a branded power supply, then replacing it with a new one will cost a pretty penny, so it makes sense to understand its configuration and circuitry and carry out the repairs yourself.

By the way, there is an easy way to check the functionality of the power supply. To do this, it must be disconnected from the motherboard. Simply disconnect the connectors of the wires leading from the block to the plateau. Connectors can be 20 or 24 pins (4 or 6). To check whether the unit is working, you need to short-circuit contacts 14 or 15 with each other (if the connector is twenty-pin) or 16 and 17 (if it is twenty-four pin). That is, green (sometimes gray) and black wires are connected to each other. Then the unit itself is connected to the network through an outlet. If the cooler fan starts working, then everything is in order, it is not the cause. We need to look for other faults.

Repair process

So, let's start with a disclaimer that will determine the root cause of the question, how to repair a computer power supply? Remember that the power supply itself, unlike the computer, operates at 220 volts. Therefore, high-capacity capacitors are installed in his circuit. They accumulate tension that can be stored for a long time.

Do-it-yourself repair of any electronic device is based on working with a soldering iron. And if you don’t have much practice, then you should abandon this idea. After all, a computer power supply unit is a responsible device, on which it depends whether the computer will work or not.

Plus, you will have to figure out the diagram as events go, because you are unlikely to find an exact diagram, even on the Internet. There are schematic diagrams, but this does not mean that it will be exactly the same in your power supply. Therefore, everything will have to be done as the repair progresses.

Where to begin

First of all, you need to remove the cover and clean all the insides from dust. A thick layer of dust becomes a barrier that prevents temperature transfer from working parts. So this is also the reason for the failure of the unit.

Now pay attention to the fuse. Usually a 5 A part is installed here. This is a glass bulb with a thin metal thread running inside it. If there is no thread, then the fuse is blown and needs to be replaced. But sometimes there seems to be a thread present, so it’s worth checking the fuse. How?

• It will be necessary to solder a copper wire with a diameter of 0.18 mm to the ends of the part.
• Then plug the unit into a power outlet.
• If the cooler fan starts working, then the fault is the fuse.
• Unsolder it from the circuit and install a new one.

Capacitors

Typically, power supplies contain capacitors with large capacitance. It is in them that tension accumulates. Therefore, these are the parts that most often fail (in 80% of cases).

The first thing that should catch your eye is swelling and leakage of electrolyte. If all this is available, then it is one hundred percent that the capacitor is not working.

Attention! Poor fan operation causes capacitors to swell. The thing is that the fan must cool the capacitors, which are heated due to the accumulation of voltage in them. Therefore, experts recommend periodically cleaning the fan bearings and cleaning the entire cooler.

But sometimes there are no visible defects in capacitors, so it is worth checking them with a multimeter to check the resistance. If the resistance is high (compared to the nominal value), then this indicates that there is a gap between the inner lining and the terminal. Experts call this situation a broken capacitor.

There are also electrolytic capacitors in the power supply circuit. They can also swell, but there is no point in replacing them with new ones, because you must first find the cause of their swelling and then replace them. Usually the cause is a failure of the voltage stabilization circuit. So until you figure it out, there is no point in changing electrolytic capacitors. It won’t help, they will still swell. But repairs to computer power supplies of this type can only be carried out by a specialist; you cannot do it yourself. Plus, you will need professional measuring instruments. So the best option is to take the power supply to a workshop. In this case there is no choice.

Transistors

This is another part that can cause the PC power supply to malfunction. Pay attention to the design feature of the transistor. He has three legs:

1. Base.
2. Collector.
3. Emitter.

So, to determine whether a part is working or not, you need to ring it with a multimeter. And here you need to know how to call. Dialing can only be done in two directions:

• The base is a collector.
• Base - emitter.

If you change the polarity of the dial, then nothing will happen. The same applies to the direction between the collector and emitter. To correctly carry out the ringing, you need to connect the probe with the red wire to the base of the transistor, and the black wire to the collector or emitter. If the display shows an indicator within 650-800 mV, then everything is fine, the transistor is intact.

To check, you can ring the collector-emitter. Here the resistance should be infinite, the display will show one. If this transition is broken, the multimeter will emit a characteristic signal. But note that this is not necessary; other transitions also do not work.

As for diodes, these small devices are practically the same as transistors. That is, a transistor is two diodes connected in series, but with cathodes at one point. Therefore, ringing them is practically a test of the base-collector or base-emitter transition. The resistance indicators are exactly the same.

Rework

What is the purpose of converting a computer power supply? That is, is it possible to replace some of its parts to improve the operation of the device? Some masters try to make some changes, and thereby achieve better results. We will not go into details of all the modifications, because we are talking about independent repairs. And some of them cannot be done with your own hands.

The simplest modification is to reinstall the capacitors that are built into the power bus. They are designed for a voltage of 5 V. So, the more voltage these devices can withstand, the better. It would be nice to install capacitors rated for 10 V in their place, but they are large in size, so they may not fit on the plateau. Therefore, it is still worth choosing a capacitor with a high voltage that would fit on the plateau, for example, at 6.5 V.

Attention! Replacing a capacitor depends on its correct installation on the plateau. Therefore, pay attention to the negative output strip. It is wide, vertical and light. So, the new device must be installed in exactly the same position so that the strip ends up in the old installation location.

Conclusion on the topic

So, if you have replaced all the failed parts, then the power supply should work. The easiest way to check this is to plug it into a power outlet. The cooler fan should spin. There is another more reliable option - check the main connectors for voltage with a multimeter. Their value should be 12 and 5 volts.

As you can see, repairing a power supply is actually a difficult process. Although if you figure it out and go through the diagram several times, changing first one device or another, then you can already consider yourself a home craftsman. But the most important thing, as practice shows, is the ability to work with a soldering iron.