DIY power supply repair

In detail: DIY repair of power supplies from a real master for the site my.housecope.com.

In the modern world, the development and obsolescence of personal computer components occurs very quickly. At the same time, one of the main components of a PC - an ATX power supply - is practically has not changed its design for the last 15 years.

Consequently, the power supply unit of both the ultra-modern gaming computer and the old office PC work on the same principle and have common troubleshooting techniques.

A typical ATX power supply circuit is shown in the figure. Structurally, it is a classic pulse unit on the TL494 PWM controller, which is 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
mains rectifier:

Its task is to convert AC from the mains to DC to power the PWM controller and the 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 result, 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 unit is connected to the network.
Next, a noise filter is installed, consisting of several chokes (L1, L2), capacitors (C1, C2, C3, C4) and a counter-winding choke Tr1... The need for such a filter is due to the significant level of interference that the impulse unit transmits to the power supply network - this interference is not only captured by television and radio receivers, but in some cases can also lead to incorrect operation of sensitive equipment.
A diode bridge is installed behind the filter, which converts alternating current into pulsating direct current. The ripple is smoothed out by a capacitive-inductive filter.

Video (click to play).

Further, a constant voltage, present all the time the ATX power supply is connected to the outlet, goes to the control circuits of the PWM controller and the standby power supply.

Standby power supply - this 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, supplying a low-power integrated voltage regulator on the 7805 microcircuit. high voltage drop across the 7805 stabilizer, which under heavy load leads to overheating. For this reason, damage to the circuits powered from the standby source can lead to its failure and the subsequent impossibility of turning 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, the change in the duration of voltage pulses at their constant amplitude and frequency. The task of the PWM unit, based on the specialized TL494 microcircuit or its functional analogs, is to convert the constant voltage into pulses of the appropriate frequency, which, after the isolation transformer, are smoothed by the output filters.The voltage stabilization at the output of the pulse converter is carried out by adjusting the duration of the pulses generated by the PWM controller.

An important advantage of such a voltage conversion scheme is also the ability to work with frequencies significantly higher than 50 Hz of the mains. The higher the current frequency, the smaller the dimensions of the transformer core and the number of winding turns are required. That is why switching power supplies are much more compact and lighter than classic circuits with an input step-down transformer.

A circuit based on the T9 transistor and the following stages is responsible for turning on the ATX power supply. At the moment the power supply is switched on to the network, a voltage of 5V is supplied to the base of the transistor through the current-limiting resistor R58 from the output of the standby power supply, at the moment the PS-ON wire is shorted to ground, the circuit starts the TL494 PWM controller. In this case, the failure of the standby power supply will lead to the uncertainty of the operation of the power supply startup circuit and the probable failure of switching on, which has already been mentioned.

The main load is borne by the output stages of the converter. First of all, this concerns the switching transistors T2 and T4, which are installed on aluminum heatsinks. But at high load, their heating, even with passive cooling, can be critical, so the power supplies are additionally equipped with an exhaust fan. If it fails or is very dusty, the probability of overheating of the output stage increases significantly.

Modern power supplies are increasingly using powerful MOSFET switches instead of bipolar transistors, due to the significantly lower resistance in the open state, providing a higher efficiency of the converter and therefore less demanding on cooling.

Video about the computer power supply device, its diagnostics and repair

Initially, ATX computer power supplies used a 20-pin connector (ATX 20-pin). Now it can only be found on outdated equipment. Subsequently, the increase in the power of personal computers, and therefore their energy consumption, led to the use of additional 4-pin connectors (4-pin). Subsequently, the 20-pin and 4-pin connectors were structurally combined into one 24-pin connector, and for many power supplies, a part of the connector with additional pins could be separated for compatibility with older motherboards.

The pin assignment of the connectors is standardized in the ATX form factor as follows, according to the figure (the term "controlled" refers to those pins on which the voltage appears only when the PC is turned on and is stabilized by the PWM controller):

Most modern consumer electronic equipment has in its design independent or located on a separate board electronic modules that reduce and rectify the mains voltage.

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There are several reasons for this, but the main ones are:

fluctuations in the mains voltage, for which these step-down-rectifier devices are not designed;
non-observance of operating rules;
connecting a load for which the devices are not designed.

Of course, it can be very offensive when you need to do urgent work, and the power module of the computer is faulty, or while watching your favorite TV show, this device fails.

Do not immediately panic and go to a repair shop or rush to an electronics supermarket to purchase a new unit. Often the reasons for inoperability are so trivial that they can be eliminated at home, with minimal expenditure of financial resources and nerves.

Of course, in order to try not only to repair the switching power supply, but also to determine its malfunction, you must have basic knowledge of electronics and have certain electrical skills.

As part of any power supply, whether built-in, like in a TV or installed as a separate device, like in a desktop computer, there are two functional blocks - high-voltage and low-voltage.

In the high-voltage side, the mains voltage is converted by the diode bridge into a constant voltage, and smoothed on the capacitor to the level of 300.0 ... 310.0 volts. Constant, high voltage is converted into a pulse voltage with a frequency of 10.0 ... 100.0 kilohertz, which makes it possible to abandon massive low-frequency step-down transformers, replacing them with small-sized pulse ones.

In the low-voltage unit, the impulse voltage is lowered to the required level, straightened, stabilized and smoothed out. At the output of this unit, there is one or more voltages required to power household appliances. In addition, various control circuits are mounted in the low-voltage unit, which make it possible to increase the reliability of the device and ensure the stability of the output parameters.

Visually, on a real board, it is quite easy to distinguish between the high-voltage and low-voltage parts. The network wires are suitable for the first, and the supply wires go from the second.

Switching regulator in the transistor power supply

A person who is going to try to repair a power supply unit for household electronic equipment must be prepared in advance that not every power supply device can be repaired. Today, some manufacturers produce electronics, the blocks of which are not subject to repair, but complete replacement.

Not a single master will undertake the repair of such a power supply unit, because initially it is intended for the complete dismantling of the old device with replacement with a new one. Often, such electronic devices are simply filled with some kind of compound, which immediately removes the question of its maintainability.

As statistics show, the main malfunctions of the power supply are caused by:

malfunction of the high-voltage part (40.0%), which are expressed by the breakdown (burnout) of the diode bridge and the failure of the filter capacitor;
breakdown of a power field-effect or bipolar transistor (30.0%), which forms high-frequency pulses and is located in the high-voltage part;
breakdown of the diode bridge (15.0%) in the low-voltage part;
breakdown (burnout) of the choke windings of the output filter.

In other cases, the diagnosis is quite difficult and without special devices (oscilloscope, digital voltmeter) it will not be possible to perform it. Therefore, if the malfunction of the power supply is not caused by the four above-mentioned main reasons, you should not engage in home repairs, but immediately call a master for replacement or purchase a new power supply device.

Faults in the high-voltage section are easy enough to detect. They are diagnosed by a blown fuse and a lack of voltage after it. The third and fourth cases can be assumed if the fuse is good, the voltage at the input of the low-voltage unit is present, and the input voltage is absent.

It is advisable to check all the details at the same time. If several electronic elements are burned out, when one of them is replaced with a serviceable one, it may burn out again due to a complex malfunction that has not been eliminated.

After replacing parts, you must install a new fuse and turn on the power supply. As a rule, after this, the power supply begins to work.

If the fuse has not blown, and there is no voltage at the output of the power supply, then the cause of the malfunction is the breakdown of the rectifier diodes of the low-voltage part, the burnout of the inductor or the output of the electrolytic capacitors of the secondary rectifier unit.

Malfunction of capacitors is diagnosed when they are swollen or fluid leaks out of their body. The diodes must be evaporated and checked with a tester in the same way as checking the high-voltage part. The integrity of the choke winding is checked by a tester. All defective parts must be replaced.

If you cannot find the desired choke, then some "craftsmen" rewind the burned-out one, picking up a wire of a suitable diameter and determining the number of turns. Such work is quite painstaking and is usually performed only for unique power supplies, it is difficult to find an analogue for which.

As already mentioned, most power supplies for modern computers and TVs are built according to a typical scheme. They differ in the size of the electronic parts used and in the output power. The diagnostics and troubleshooting procedures for these devices are identical.

However, a high-quality repair requires an appropriate tool, the range of which includes:

soldering iron (preferably with adjustable power);
solder, flux, alcohol or refined gasoline (Galosha);
device for removing molten solder (desoldering pump);
Screwdriver Set;
side cutters (nippers);
household multimeter (tester)
tweezers;
100.0 watt incandescent lamp (used as ballast load).

In principle, simple TVs can be repaired without a circuit, but the main difficulty in repairing some models is that the power supply device generates the entire range of voltages - including the high-voltage used to scan the kinescope. The power supplies for household computers are made according to the same type of scheme. Let us consider separately the methodology for determining the malfunction and repairing the TV and desktop.

A malfunction of the TV power module is first of all evidenced by the absence of a glow of the "sleep" mode diode. The first repair operations are:

check for the integrity (absence of breakage) of the supply voltage cord;

disassembly of the television receiver and release of the electronic board;

inspection of the power supply board for the presence of externally faulty parts (swollen capacitors, burnt spots on the printed circuit board, burst cases, charred surface of resistors);

checking the soldering points, with special attention paid to soldering the contacts of the pulse transformer.

If it was not possible to visually establish the defective part, then it is necessary to sequentially check the performance of the fuse, diodes, electrolytic capacitors and transistors. Unfortunately, if the control microcircuits are out of order, their malfunction can be established only indirectly - when, with completely serviceable discrete elements, the operational state of the power supply does not occur.The most common reasons for the inoperability of television units are:

breakage of ballast resistance;

inoperability (short circuit) of the high voltage filter capacitor;

malfunction of the secondary voltage filter capacitors;

breakdown or burnout of rectifier diodes.

All these parts (except for rectifier diodes) can be checked without removing them from the board. If it was possible to identify the faulty part, then it is replaced and they begin to check the repair performed. To do this, an incandescent lamp is installed in place of the fuse and the device is connected to the network.Several options for the behavior of the repaired device are possible here:

The light flashes and goes out, the sleep mode LED lights up, a raster appears on the screen. In this situation, the line scan voltage is measured first. If its value is too high, it is necessary to check and replace electrolytic capacitors with guaranteed serviceable ones. A similar situation manifests itself in the event of a malfunction of the optocouplers.

If the lamp flashes and goes out, the LED does not light up, the raster is absent, then the pulse generator does not start. In this case, the voltage level on the electrolytic capacitor of the filter of the high-voltage part is checked. If it is below 280.0 ... 300.0 volts, then the following malfunctions are most likely:

one of the rectifier bridge diodes is broken;
large capacitor leakage (the capacitor is "old").

If there is no voltage, it is necessary to re-check the continuity of the supply circuits and all diodes of the high voltage rectifier. If the light is high, you must immediately disconnect the power module from the network and re-check all electronic parts.The above sequence and test scheme allow you to identify the main malfunctions of the power supply device of the television receiver. Image - DIY power supply repair

Today, ATX devices of various capacities are most widely used for powering desktop (desktop) designers. The reason for their repair should be:

the motherboard does not start (the computer is completely inoperative);
the cooling fan of the device itself does not rotate;
the block “tries” to start itself many times.

Before starting the repair of ATX devices, it is necessary to assemble the load circuit (figure). The repair is carried out in the following sequence:

the device is removed from the computer and the cover is removed from it;
a vacuum cleaner and a brush removes dust from electronic boards and surfaces of parts;
external examination of electronic elements and printed circuit boards;
a load device is connected.

If, when turned on, the lamp flashes brightly and continues to burn, then the diode bridge in the high-voltage part or the filter capacitor is out of order. Burnout of the high-voltage transformer is possible.

If the fuse is intact, then the reason for inoperability may be:

failure of the transistors of the pulse generator;
malfunction of the PWM controller.

In these cases, it is easier to purchase a new device, which, depending on the capacity, costs from 600 to 800 rubles.

With repeated self-starting of the device, the reason for inoperability is usually the failure of the reference voltage stabilizer. In this case, the computer system cannot pass the self-test mode, it turns off and turns on the power module.

One of the important components of a modern personal computer is a power supply unit (PSU). The computer will not work if there is no power.

On the other hand, if the power supply generates a voltage that goes beyond the permissible limits, then this can cause the failure of important and expensive components.

In such a unit, with the help of an inverter, the rectified mains voltage is converted into an alternating high frequency, from which low voltage flows necessary for the operation of the computer are formed.

The ATX circuit of the power supply consists of 2 nodes - a mains voltage rectifier and a voltage converter for a computer.

Mains rectifier is a bridge circuit with a capacitive filter. At the output of the device, a constant voltage of 260 to 340 V is generated.

The main elements in the composition voltage converter are:

an inverter that converts direct voltage to alternating voltage;
high frequency transformer operating at 60 kHz;
low-voltage rectifiers with filters;
control device.

In addition, the converter includes a standby voltage power supply, amplifiers of a control signal for key transistors, protection and stabilization circuits, and other elements.

The reasons for faults in the power supply can be:

power surges and fluctuations;
poor quality product manufacturing;
overheating associated with poor fan operation.

Malfunctions usually lead to the fact that the system unit of the computer stops starting or, after a short time, turns off. In other cases, despite the operation of other units, the motherboard will not start.

Before starting the repair, you must finally make sure that it is the power supply that is faulty. In this case, you must first check the functionality of the mains cable and the mains switch... After making sure they are in good working order, you can disconnect the cables and remove the power supply from the case of the system unit.

Before re-enabling the power supply unit autonomously, it is necessary to connect the load to it. To do this, you need resistors that are connected to the corresponding terminals.

First you need to check motherboard effect... To do this, you need to close two contacts on the power supply connector. On a 20-pin connector, this would be pin 14 (the wire that the Power On signal goes through) and pin 15 (the wire that matches the GND pin - Ground). For a 24-pin connector, this would be pins 16 and 17, respectively.

After removing the cover from the power supply, you must immediately use a vacuum cleaner to clean out all the dust from it. It is because of dust that radio parts often fail, since dust, covering the part with a thick layer, causes overheating of such parts.

The next step in identifying faults is a thorough inspection of all elements. Particular attention must be paid to electrolytic capacitors. The reason for their breakdown may be a severe temperature regime. Defective capacitors usually swell and leak electrolyte.

Such parts must be replaced with new ones with the same ratings and operating voltages. Sometimes the appearance of a capacitor does not indicate a malfunction. If, by indirect indications, there is a suspicion of poor performance, then you can check the capacitor with a multimeter. But for this it needs to be removed from the circuit.

A faulty power supply can also be associated with faulty low voltage diodes. To check, you need to measure the resistance of the forward and reverse transitions of elements with a multimeter. To replace faulty diodes, you must use the same Schottky diodes.

The next malfunction that can be determined visually is the formation of ring cracks that break the contacts. To find such defects, you need to look very carefully on the printed circuit board. To eliminate such defects, it is necessary to use careful soldering of the cracks (for this you need to know how to properly solder with a soldering iron).

Resistors, fuses, inductors, transformers are inspected in the same way.

In the event that a fuse is blown, it can be replaced with another one or repaired. The power supply uses a special element with soldering leads. To repair a faulty fuse, it is soldered from the circuit. Then the metal cups are heated and removed from the glass tube. Then a wire of the required diameter is selected.

The wire diameter required for a given current can be found in the tables. For the 5A fuse used in the ATX power supply circuit, the diameter of the copper wire will be 0.175 mm. Then the wire is inserted into the holes of the fuse cups and fixed by soldering. The repaired fuse can be soldered into the circuit.

The above considered the most simple malfunctions of a computer power supply.

One of the most important elements of a PC is the power supply, if it fails, the computer stops working.
The computer power supply is a rather complex device, but in some cases it can be repaired by hand.

Despite its apparent power, the personal computer is fragile. To disable any part, just careless handling of it is enough. For example, do not clean the system unit and its components. As a result, a lot of dust forms on the parts, which negatively affects the operation of the device as a whole.

One of the most important components of a PC is the power supply. It is he who distributes electricity to the system unit and controls the voltage level. Therefore, the breakdown of this device can be attributed to one of the most unpleasant. Nevertheless, everyone can do the repair and fix the problem with their own hands.

The most critical situation is when the computer does not respond to the power button... This means that important points were missed that could indicate an imminent breakdown. For example, an unnatural sound during operation, a long turn-on of the computer, an independent shutdown, etc. Or maybe similar malfunctions were noticed, but it was decided not to resort to repairs.

In addition to the most critical moments, there are several signs that help identify problems in the operation of the computer power supply:

The occurrence of various errors when turning on the PC.
Sudden computer restarts.
Increasing the volume of coolers (small fans).
Various errors when the PC is turned on.
Stopping the hard drive or some coolers.
Loud beeping from the system unit (indicates overheating).
Electric shock when touching the cabinet.

Such signs indicate the need for an early repair, which can be done by hand. However, there are also more serious problemsclearly indicating a serious malfunction. For instance:

"Screen of death" (blue screen when the device is turned on or working).
The appearance of smoke.
No reaction to inclusion.

Most people, when such problems arise, turn to a repairman for a repair. Typically, a computer technician advises purchasing a new power supply and then replacing the old one. Nevertheless, with the help of repairs, you can "reanimate" an inoperative device with your own hands.

To completely solve the problem, you need to understand why it could appear. Most often, the computer's power supply fails for three reasons:

Voltage drops.
Poor quality of the product itself.
Inefficient operation of the ventilation system leading to overheating.

In most cases, such malfunctions lead to the fact that the power supply does not turn on or stops working after a short time. In addition, the above problems can negatively affect the motherboard. If this happened, then do-it-yourself repairs will not be enough here - it will be necessary to change the part to a new one.

Less often, malfunctions in the computer's power supply occur due to the following reasons:

Poor software (poor OS optimization has a bad effect on the operation of all components).
Lack of cleaning of components (a large amount of dust makes the coolers run faster).
A lot of unnecessary files and "garbage" in the system itself.

As mentioned above, the power supply is a rather fragile thing. Nevertheless, it is very important for the computer as a whole, so this component should not be ignored. Otherwise, repairs are inevitable.

The power supply in the computer is responsible for distributing and converting electrical current. The fact is that each element in a PC needs its own voltage level. In addition, AC power is used in electrical circuits, and computer components operate on DC. Therefore, the device of the power supply is quite specific and you need to know it for do-it-yourself repairs.

In each power supply unit there are 9 important components:

Without at least an approximate idea of the structure of the power supply, it is impossible to fully carry out an independent repair.

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Before you start solving a problem in a computer with your own hands, you must think about your own safety... Repairing such a device is a dangerous undertaking. Therefore, first of all, you need to work thoughtfully and without haste.

For greater safety, there are several important rules to keep in mind:

Work only with the power supply off. Despite the banality of the advice, this is a very important point. No one is immune from the "fool's syndrome", so it is better to check once again that everything is turned off, and only then start repairs.
To preserve the components and also avoid "fireworks", it is recommended to install a 100 watt light bulb instead of a fuse.If the light remains on when the power supply is turned on, then the network has shorted somewhere. If it lights up and immediately goes out, then everything is in order.
Power capacitors are energized especially for a long time. Therefore, even after disconnecting the power supply from the network, you should not immediately get down to work.
It is better to check the operation of the device away from flammable substances, since there is a risk of short circuit and "fireworks" sparks.

To make repairing a power supply unit simple but effective, each home craftsman will need certain tools to work. All these products can be easily found at home, asked from neighbors / friends, or purchased in a store. Fortunately, they are inexpensive.

So, for renovation you will need the following tools:

Soldering station with built-in power regulation or several soldering irons, each of which is designed for a certain power.
Solder and flux for soldering components.
To remove solder - braid or suction.
Several screwdrivers with different tips.
Multimeter.
Side cutters (devices for cutting plastic "clamps" that hold the wires together).
100 watt light bulb.
Tweezers (for removing small components).
Alcohol or refined gasoline.
An oscilloscope may be required (if the cause of the problem is not determined).

First you need disassemble the power supply... To do this, you only need a screwdriver and accuracy. When unscrewing the bolts, you do not need to shake the PSU in order to quickly fix the problem. Careless handling of it can lead to the fact that do-it-yourself repairs will be simply useless.

For the correct formulation of the "diagnosis", it is necessary to conduct an initial diagnosis, as well as a visual inspection of the device. Therefore, first of all, you need to pay attention to the fan of the power supply. If the cooler cannot spin freely and gets stuck in a certain place, then this is clearly the problem.

In addition to the fan of the product, you should also inspect the entire device. After a long service life, a lot of dust accumulates in it, which has a negative effect and hinders the normal operation of the power supply unit. Therefore, it is imperative to clean the product from dust accumulation.

Also, some products fail. due to voltage surges... Therefore, it is necessary to carry out a visual inspection for burnt parts. This sign is easy to identify by swollen capacitors, darkening of the textolite, carbonized insulation or broken wires.

Finally, it is worth moving on to the most important point - do-it-yourself BP repair. For convenience, the entire process will be presented in the form of a list. Therefore, it is recommended not to "jump" from one point to another, but act in a specific order:

It so happens that outwardly everything is in order: the components are not melted, there are no cracks or contact violations. What is the problem then? It is best to carefully examine all the details again. It is possible that a malfunction was missed due to carelessness. If no problems were detected during the secondary inspection, then in 90% of cases the malfunction lies in standby power supply or in the PWM controllerusing wide pulse modulation.

To fix the standby voltage problem, you need to know the basics of how the power supply works. This PC component works almost always. Even when the computer itself is turned off (not disconnected from the network), the unit operates in standby mode. This means that the power supply unit sends 5 volt “standby signals” to the motherboard so that when the PC is turned on, it can start the unit itself and other components.

At system startup, the motherboard checks the voltage for all elements. If everything is in order, it is formed feedback signal "Power good" and the system starts up. If there is a lack or excess of voltage, the system start is canceled.

This means that first of all, on the board, you need to check the presence of 5 V on the PS_ON and + 5VSB pins.When checking, the absence of voltage or its deviation from the nominal value is usually revealed. If the problem is PS_ON, the problem is in the PWM controller. If there is a malfunction with contact + 5VSB, then the problem lies in the electric current conversion device.

It is also useful to check the PWM itself. True, for this you need an oscilloscope. To check, you need to unsolder the PWM and use an oscilloscope to check the contacts by ringing (OPP, VCC, V12, V5, V3.3). For better ringing, the check must be carried out relative to the ground. If the resistance between ground and any of the contacts (on the order of several tens of ohms), then the PWM must be replaced.

Self-repair of the power supply is a rather complicated process, which will require the necessary tools, initial knowledge of BP operationas well as neatness and attention to detail. Nevertheless, each person, with the proper approach, can repair the unit, despite its complex structure. Therefore, it should be remembered that everything is in your hands.

If your computer's power supply fails, do not rush to get upset, as practice shows, in most cases, repairs can be done on your own. Before proceeding directly to the technique, we will consider the block diagram of the power supply unit and provide a list of possible malfunctions, this will greatly simplify the task.

The figure shows an image of a block diagram typical for pulsed power supplies of system units. Image - DIY power supply repair

Switching power supply unit ATX

Indicated designations:

A - power filter unit;
B - low-frequency rectifier with a smoothing filter;
C - cascade of the auxiliary converter;
D - rectifier;
E - control unit;
F - PWM controller;
G - cascade of the main converter;
H - high-frequency rectifier equipped with a smoothing filter;
J - PSU cooling system (fan);
L - output voltage control unit;
K - overload protection.
+ 5_SB - standby power supply;
P.G. - information signal, sometimes referred to as PWR_OK (required to start the motherboard);
PS_On - signal controlling the start of the power supply unit.

To carry out repairs, we also need to know the pinout of the main power connector, it is shown below.

Power supply plugs: A - old (20pin), B - new (24pin)

To start the power supply, it is necessary to connect the green wire (PS_ON #) to any zero black wire. This can be done using a conventional jumper. Note that for some devices the color coding may differ from the standard one, as a rule, unknown manufacturers from China are guilty of this.

It is necessary to warn that turning on impulse power supplies without load will significantly reduce their service life and may even cause damage. Therefore, we recommend assembling a simple block of loads, its diagram is shown in the figure.

Load block diagram

It is advisable to assemble the circuit on resistors of the PEV-10 brand, their ratings: R1 - 10 Ohm, R2 and R3 - 3.3 Ohm, R4 and R5 - 1.2 Ohm. Cooling for resistors can be made from an aluminum channel.

It is undesirable to connect a motherboard as a load during diagnostics or, as some "craftsmen" advise, an HDD and CD drive, since a faulty power supply unit can damage them.

Let's list the most common malfunctions characteristic of pulsed power supplies of system units:

the mains fuse blows;
+ 5_SB (standby voltage) is absent, as well as more or less than the permissible;
the voltage at the output of the power supply (+12 V, +5 V, 3.3 V) is abnormal or absent;
no P.G. signal (PW_OK);
PSU does not turn on remotely;
the cooling fan does not rotate.

After the power supply is removed from the system unit and disassembled, first of all, it is necessary to inspect for the detection of damaged elements (darkening, changed color, violation of integrity).Note that in most cases, replacing a burnt-out part will not solve the problem; a piping check will be required.

Visual inspection allows you to detect "burnt" radioelements

If these are not found, we proceed to the following algorithm of actions:

If a faulty transistor is found, then before soldering a new one, it is necessary to test its entire strapping, consisting of diodes, low-resistance resistances and electrolytic capacitors. We recommend changing the latter to new ones with a large capacity. A good result is obtained by shunting electrolytes using 0.1 μF ceramic capacitors;

Checking the output diode assemblies (Schottky diodes) with a multimeter, as practice shows, the most typical malfunction for them is a short circuit;

Diode assemblies marked on the board

checking the output capacitors of the electrolytic type. As a rule, their malfunction can be detected by visual inspection. It manifests itself in the form of a change in the geometry of the housing of the radio component, as well as traces from the flow of electrolyte.

It is not uncommon for an outwardly normal capacitor to be unsuitable during testing. Therefore, it is better to test them with a multimeter that has a capacitance measurement function, or use a special device for this.

Video: correct repair of an ATX power supply. <>

Note that non-working output capacitors are the most common malfunction in computer power supplies. In 80% of cases, after replacing them, the power supply unit's performance is restored;

Capacitors with disturbed case geometry

the resistance is measured between the outputs and zero, for +5, +12, -5 and -12 volts this indicator should be in the range from 100 to 250 ohms, and for +3.3 V in the range of 5-15 ohms.

In conclusion, we will give some tips for improving the power supply unit, which will make it work more stable:

in many inexpensive blocks, manufacturers install rectifier diodes for two amperes, they should be replaced with more powerful ones (4-8 amperes);
Schottky diodes on channels +5 and +3.3 volts can also be supplied more powerful, but at the same time they must have an allowable voltage, the same or greater;
it is advisable to change the output electrolytic capacitors to new ones with a capacity of 2200-3300 uF and a rated voltage of at least 25 volts;
it happens that instead of a diode assembly, diodes soldered to each other are installed on the +12 volt channel, it is advisable to replace them with an MBR20100 Schottky diode or similar;
if capacities of 1 μF are installed in the piping of key transistors, replace them with 4.7-10 μF, calculated for a voltage of 50 volts.

Such a minor revision will significantly extend the life of the computer power supply.

Video (click to play).

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