Repair of a computer power supply: diagrams for instructions

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

Structural scheme

The figure shows an image of a structural diagram typical of a pulsed PSU of system units.

Indicated designations:

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

Pinout of the main PSU connector

For repairs, we will also need to know the pinout of the main power connector (main power connector), it is shown below.

To start the power supply, you must connect the green wire (PS_ON #) to any zero black. This can be done using a conventional jumper. Note that for some devices the color marking may differ from the standard, as a rule, unknown manufacturers from the Middle Kingdom sin this.

PSU load

It must be warned that inclusion of pulse power supplies no load significantly reduces their service life and can even cause damage. Therefore, we recommend assembling a simple load block; its diagram is shown in the figure.

It is desirable to assemble the circuit on resistors of the PEV-10 brand, their values: R1 - 10 Ohms, R2 and R3 - 3.3 Ohms, R4 and R5 - 1.2 Ohms. Cooling for resistances can be made of aluminum channel.

Connecting a motherboard as a diagnostic load or, as some “craftsmen" advise, HDD and CD drive is undesirable, since a faulty PSU can damage them.

List of possible malfunctions

We list the most common malfunctions characteristic of pulse power supply units of system units:

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

Testing procedure (instruction)

After the power supply is removed from the system unit and disassembled, first of all, it is necessary to produce inspection for detecting damaged elements (darkening, discolored, irregular integrity). Note that in most cases replacing a burned-out part will not solve the problem; a binding check will be required.

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

• check the fuse. Do not trust the visual inspection, but it is better to use the multimeter in the dialing mode. The reason why the fuse is blown may be a breakdown of the diode bridge, the key transistor, or a malfunction of the unit responsible for standby mode;

• check disk thermistor. Its resistance should not exceed 10 Ohms, if it is faulty, we strongly advise against putting a jumper in its place. The pulse current arising in the process of charging capacitors installed at the input can cause a breakdown of the diode bridge;

• we test diodes or a diode bridge on the output rectifier, they should not have a break and a short circuit. If a malfunction is detected, the capacitors and key transistors installed at the input should be checked. The alternating voltage that came to them as a result of the breakdown of the bridge, with high probability, disabled these radio components;

• Checking the input capacitors of the electrolytic type begins with an inspection. The geometry of the body of these parts must not be disturbed. After that, the capacitance is measured. It is considered normal if it is not less than the declared one, and the difference between the two capacitors is within 5%. Also sealed in parallel with the input electrolytes should be checked. varistors and leveling resistances;

• testing of key (power) transistors. Using a multimeter, we check the base-emitter and base-collector transitions (the procedure is the same as with diode test).

If a faulty transistor is found, then before soldering a new one, it is necessary to test its entire wiring, consisting of diodes, low-impedance resistances and electrolytic capacitors. We recommend exchanging the latter with new ones that have a large capacity. Shunting electrolytes with 0.1 μF ceramic capacitors gives a good result;

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

• checking output electrolytic capacitors. 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.

Cases are not uncommon when an externally normal capacitor is unsuitable during verification. Therefore, it is better to test them with a multimeter, which has the function of measuring capacitance, or use a special device for this.

Video: Correct repair of the ATX power supply.
https://www.youtube.com/watch? v = AAMU8R36qyE

Note that non-working output capacitors are the most common malfunction in computer power supplies. In 80% of cases after their replacement, the PSU is restored;

• the resistance between the outputs and zero is measured, 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.

Finalization of BP

In conclusion, we will give some tips for finalizing the PSU, which will make its work more stable:

• in many inexpensive blocks, manufacturers install two-ampere rectifier diodes, they should be replaced with more powerful ones (4-8 amperes);
• Schottky diodes on channels +5 and +3.3 volts can also be put more powerful, but at the same time they must have an allowable voltage, the same or greater;
• it is advisable to exchange the output electrolytic capacitors with new ones with a capacity of 2200-3300 μF and a rated voltage of at least 25 volts;
• it happens that instead of the diode assembly a +12 volt channel is set up of diodes that are welded together, it is advisable to replace them with a Schottky diode MBR20100 or similar;
• if capacitors of 1 microfarad are installed in the key transistor wiring, replace them with 4.7-10 microfarads, designed for a voltage of 50 volts.

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

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