When diagnosing television devices, disproportionately more time is spent on finding a faulty component than replacing it, especially if the search for the defect is carried out on your own and not by a professional TV technician. Of course, it is more logical to entrust the repair to a specialist who has experience and extensive practice in this type of work, but if you have the desire, skills in handling a soldering iron and tester, and the necessary technical documentation in the form of a circuit diagram, you can try to repair the TV at home yourself.
The power supply of a modern TV, be it a plasma panel or LCD, LED TV, is a switching power supply with a given range of output supply voltages and the rated power supplied to the load for each of them. The power board can be made in the form of a separate block, this is typical for small diagonal receivers, or integrated into the television chassis and located inside the device.
Typical symptoms of a malfunction of this unit are the following:
- The TV does not turn on when you press the power switch
- The standby LED is on, but there is no transition to operating mode
- Noise in the image in the form of kinks and stripes, background sound
- There is sound, but no image, which may appear after a while
- It takes several attempts to turn on for normal picture and sound to appear.
Let's look at the circuitry of a standard power supply and its typical faults using the ViewSonic N3260W TV as an example.
To fully view the diagram, you can open it in a new window and enlarge it, or download it to your computer or mobile device
The first thing you should start with is a thorough visual inspection of the board with the device disconnected from the network. To do this, the unit must be removed from the TV by disconnecting the connectors, and be sure to discharge the high-voltage capacitor in the filter - C1. In units of this series of TVs, the electrolytic capacitors of the secondary power supply filters quite often fail. They are easily diagnosed by a swollen top cover. All capacitors whose appearance is questionable must be replaced immediately.
The standby mode unit is made on IC2 (TEA1532A) and Q4 (04N70BF) with elements for stabilizing the 5V output voltage on the optocoupler IC7 and the controlled zener diode ICS3 EA1. Missing or low voltage at the output of this unit, measured on capacitors CS22, CS28, indicates its malfunction. The experience of restoring this section of the circuit shows that the most vulnerable elements are IC2, Q7, ZD4 and Q11, R64, R65, R67, which require checking and replacement if necessary. The performance of the parts is checked by a tester directly on the unit board. In this case, questionable components are soldered off and tested separately to eliminate the influence of neighboring circuit elements on their performance. IC2 chip simply needs to be replaced.
If there is a 5V voltage at the output of the standby circuit, a red LED lights up on the front panel of the TV. Upon command from the remote control or button on the front panel of the TV, the power supply should go into operating mode. This command - Power_ON - in the form of a high potential of about 5V comes to pin 1 of the CNS1 connector, opening the keys on QS4 and Q11. At the same time, supply voltages are supplied to microcircuits IC3 and IC1, putting them into operating mode. To the 8th pin of IC3 directly from the collector of Q11, to the 12th pin of IC1 through the Q9 switch after starting the PFC circuit. The performance of the Power Factor Correction circuit is indirectly determined by an increase in voltage from 310 to 390 volts, measured at capacitor C1. If output supply voltages of 12V and 24V appear, then the main source on IC3, Q1, Q2 operates in normal mode. Practice shows the low reliability of the UCC28051 and LD6598D in critical conditions, when the filtering of secondary sources deteriorates, and their replacement is routine.
Summarizing the experience of repairing television power supplies, it should be noted that the weakest link in their composition is the filter capacitors, which lose their properties and nominal parameters over time. Sometimes the faulty “container” is visible from the swollen lid, sometimes not. The consequences of poor filtering of rectified voltage can be very different: from loss of functionality of the power supply itself, to damage to the inverter elements or software failure of memory chips on the motherboard.
It is very difficult to independently understand all the causes and consequences when repairing the power supply of a modern TV, and to diagnose it correctly without special tools and devices. Our advice in such cases is. This will not put a big dent in your pocket given the current low prices for repairing television equipment and will save time.
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They have always been important elements of any electronic devices. These devices are used in amplifiers and receivers. The main function of power supplies is considered to be to reduce the maximum voltage that comes from the network. The first models appeared only after the AC coil was invented.
Additionally, the development of power supplies was influenced by the introduction of transformers into the device circuit. The peculiarity of pulse models is that they use rectifiers. Thus, voltage stabilization in the network is carried out in a slightly different way than in conventional devices where a converter is used.
Power supply device
If we consider a conventional power supply, which is used in radio receivers, then it consists of a frequency transformer, a transistor, and several diodes. Additionally, the circuit contains a choke. Capacitors are installed with different capacities and their parameters can vary greatly. Rectifiers are usually used of the capacitor type. They belong to the high-voltage category.
Operation of modern blocks
Initially, the voltage is supplied to the bridge rectifier. At this stage, the peak current limiter is activated. This is necessary so that the fuse in the power supply does not burn out. Next, the current passes through the circuit through special filters, where it is converted. Several capacitors are needed to charge the resistors. The unit starts up only after a breakdown of the dinistor. Then the transistor is unlocked in the power supply. This makes it possible to significantly reduce self-oscillations.
When voltage generation occurs, the diodes in the circuit are activated. They are connected to each other using cathodes. A negative potential in the system makes it possible to lock the dinistor. The rectifier start-up is facilitated after the transistor is turned off. In addition, two fuses are provided to prevent saturation of the transistors. They operate in the circuit only after a breakdown. To start feedback, a transformer is required. It is fed by pulsed diodes in the power supply. At the output, alternating current passes through capacitors.
Features of laboratory blocks
The operating principle of switching power supplies of this type is based on active current conversion. There is one bridge rectifier in the standard circuit. In order to remove all interference, filters are used at the beginning and also at the end of the circuit. The pulsed laboratory power supply has conventional capacitors. Saturation of transistors occurs gradually, and this has a positive effect on diodes. Voltage adjustment is provided in many models. The protection system is designed to save blocks from short circuits. Cables for them are usually used in a non-modular series. In this case, the power of the model can reach up to 500 W.
The power supply connectors in the system are most often installed as ATX 20 type. To cool the unit, a fan is mounted in the case. The speed of rotation of the blades must be adjusted in this case. A laboratory-type unit should be able to withstand the maximum load at 23 A. At the same time, the resistance parameter is maintained on average at 3 ohms. The maximum frequency that a switching laboratory power supply has is 5 Hz.
How to repair devices?
Most often, power supplies suffer due to blown fuses. They are located next to the capacitors. Repair of switching power supplies should begin by removing the protective cover. Next, it is important to inspect the integrity of the microcircuit. If no defects are visible on it, it can be checked using a tester. To remove fuses, you must first disconnect the capacitors. After this they can be removed without any problems.
To check the integrity of this device, inspect its base. Burnt fuses have a dark spot at the bottom, which indicates damage to the module. To replace this element, you need to pay attention to its markings. Then you can purchase a similar product in a radio electronics store. Installation of the fuse is carried out only after fixing the condensates. Another common problem in power supplies is considered to be faults with transformers. They are boxes in which coils are installed.
When very high voltage is applied to the device, they cannot withstand it. As a result, the integrity of the winding is compromised. It is impossible to repair switching power supplies with such a breakdown. In this case, the transformer, like the fuse, can only be replaced.
Network power supplies
The operating principle of network-type switching power supplies is based on a low-frequency reduction in the amplitude of interference. This happens thanks to the use of high-voltage diodes. Thus, it is more effective to control the limiting frequency. Additionally, it should be noted that transistors are used at medium power. The load on the fuses is minimal.
Resistors are used quite rarely in a standard circuit. This is largely due to the fact that the capacitor is capable of participating in current conversion. The main problem with this type of power supply is the electromagnetic field. If capacitors are used with low capacitance, then the transformer is at risk. In this case, you should be very careful about the power of the device. The network switching power supply has limiters for peak current, and they are located immediately above the rectifiers. Their main task is to control the operating frequency to stabilize the amplitude.
Diodes in this system partially serve as fuses. Only transistors are used to drive the rectifier. The locking process, in turn, is necessary to activate the filters. Capacitors can also be used as isolation type in the system. In this case, the transformer will start up much faster.
Application of microcircuits
A wide variety of microcircuits are used in power supplies. In this situation, much depends on the number of active elements. If more than two diodes are used, the board must be designed for input and output filters. Transformers are also produced in different capacities, and their dimensions are quite different.
You can solder microcircuits yourself. In this case, you need to calculate the maximum resistance of the resistors taking into account the power of the device. To create an adjustable model, special blocks are used. This type of system is made with double tracks. Ripple inside the board will occur much faster.
Benefits of Regulated Power Supplies
The principle of operation of switching power supplies with regulators is the use of a special controller. This element in the circuit can change the throughput of transistors. Thus, the limiting frequency at the input and output is significantly different. The switching power supply can be configured in different ways. Voltage adjustment is carried out taking into account the type of transformer. Conventional coolers are used to cool the device. The problem with these devices is usually excess current. In order to solve this, protective filters are used.
The power of devices on average fluctuates around 300 W. Only non-modular cables are used in the system. In this way, short circuits can be avoided. Power supply connectors for connecting devices are usually installed in the ATX 14 series. The standard model has two outputs. Rectifiers are used with higher voltage. They can withstand resistance at 3 ohms. In turn, the maximum load of the switching regulated power supply is up to 12 A.
Operation of 12 volt units
Pulse includes two diodes. In this case, filters are installed with a small capacity. In this case, the pulsation process occurs extremely slowly. The average frequency fluctuates around 2 Hz. The efficiency of many models does not exceed 78%. These blocks are also distinguished by their compactness. This is due to the fact that transformers are installed with low power. They do not require refrigeration.
The 12V switching power supply circuit additionally involves the use of resistors marked P23. They can withstand only 2 ohms of resistance, but this is enough power for a device. A 12V switching power supply is used most often for lamps.
How does the TV box work?
The operating principle of switching power supplies of this type is the use of film filters. These devices are able to cope with interference of various amplitudes. Their choke winding is synthetic. Thus, high-quality protection of important components is ensured. All gaskets in the power supply are insulated on all sides.
The transformer, in turn, has a separate cooler for cooling. For ease of use, it is usually set to silent. These devices can withstand maximum temperatures of up to 60 degrees. The operating frequency of the TV switching power supply is maintained at 33 Hz. At subzero temperatures, these devices can also be used, but much in this situation depends on the type of condensates used and the cross-section of the magnetic circuit.
Models of 24 volt devices
In 24-volt models, low-frequency rectifiers are used. Only two diodes can successfully cope with interference. The efficiency of such devices can reach up to 60%. Regulators are rarely installed on power supplies. The operating frequency of the models does not exceed 23 Hz on average. Resistors can only withstand 2 ohms. Transistors in models are installed with the marking PR2.
To stabilize the voltage, resistors are not used in the circuit. The 24V switching power supply filters are of the capacitor type. In some cases, dividing species can be found. They are necessary to limit the maximum frequency of the current. To quickly start a rectifier, dinistors are used quite rarely. The negative potential of the device is removed using the cathode. At the output, the current is stabilized by blocking the rectifier.
Power sides on diagram DA1
Power supplies of this type differ from other devices in that they can withstand heavy loads. There is only one capacitor in the standard circuit. For normal operation of the power supply, the regulator is used. The controller is installed directly next to the resistor. No more than three diodes can be found in the circuit.
The direct reverse conversion process begins in the dinistor. To start the unlocking mechanism, a special throttle is provided in the system. Waves with large amplitude are damped by the capacitor. It is usually installed of the dividing type. Fuses are rarely found in a standard circuit. This is justified by the fact that the maximum temperature in the transformer does not exceed 50 degrees. Thus, the ballast choke copes with its tasks independently.
Models of devices with DA2 chips
Switching power supply microcircuits of this type are distinguished from other devices by their increased resistance. They are used mainly for measuring instruments. An example is an oscilloscope that shows fluctuations. Voltage stabilization is very important for him. As a result, the device's readings will be more accurate.
Many models are not equipped with regulators. Filters are mainly double-sided. At the output of the circuit, transistors are installed as usual. All this makes it possible to withstand a maximum load of 30 A. In turn, the maximum frequency indicator is at around 23 Hz.
Blocks with installed DA3 chips
This microcircuit allows you to install not only a regulator, but also a controller that monitors fluctuations in the network. The resistance of the transistors in the device can withstand approximately 3 ohms. The powerful switching power supply DA3 can handle a load of 4 A. You can connect fans to cool the rectifiers. As a result, the devices can be used at any temperature. Another advantage is the presence of three filters.
Two of them are installed at the input under the capacitors. One separating type filter is available at the output and stabilizes the voltage that comes from the resistor. There are no more than two diodes in a standard circuit. However, a lot depends on the manufacturer, and this should be taken into account. The main problem with power supplies of this type is that they are not able to cope with low-frequency interference. As a result, it is impractical to install them on measuring instruments.
How does the VD1 diode block work?
These blocks are designed to support up to three devices. They have three-way regulators. Communication cables are installed only non-modular ones. Thus, current conversion occurs quickly. Rectifiers in many models are installed in the KKT2 series.
They differ in that they can transfer energy from the capacitor to the winding. As a result, the load from the filters is partially removed. The performance of such devices is quite high. At temperatures above 50 degrees they can also be used.
How to repair and modify a Chinese-made 12-volt switching power supply
I want to start with the fact that I came into my hands with several burnt out and already “repaired” 220/12 V power supplies by someone. All the units were of the same type - HF55W-S-12, therefore, having entered the name in the search engine, I hoped to find a circuit . But apart from photographs of the appearance, parameters and prices for them, I found nothing. Therefore, I had to draw the circuit myself from the board. The diagram was drawn not to study the principle of operation of the power supply, but solely for repair purposes. Therefore, the network rectifier is not drawn, I also did not saw the pulse transformer and I do not know where the tap is made (start-end) on the 2nd winding of the transformer. Also, C14 -62 Ohm should not be considered a typo - there are markings on the board for an electrolytic capacitor (+ is shown in the diagram), but everywhere in its place there were resistors with a nominal value of 62 Ohms.
When repairing such devices, they need to be connected through a light bulb (100-200 W incandescent lamp, in series with the load), so that in the event of a short circuit in the load, the output transistor does not fail and the tracks on the board do not burn out. And your household will feel safer if the lights in the apartment don’t suddenly go out.
The main malfunction is the breakdown of Q1 (FJP5027 - 3 A, 800 V, 15 MHz) and, as a consequence, the breakage of resistors R9, R8 and the failure of Q2 (2SC2655 50 V\2 A 100 MHz). They are highlighted in color in the diagram. Q1 can be replaced with any transistor suitable for current and voltage. I installed BUT11, BU508. If the load power does not exceed 20 W, you can even install J1003, which can be found on the board from a burnt-out energy-saving lamp. One block was completely missing VD-01 (Schottky diode STPR1020CT -140 V\2x10 A), I installed MBR2545CT (45 V\30 A) instead, which is typical, it does not heat up at all at a load of 1.8 A (we used a 21 car lamp W\12 V). And within a minute of operation (without a radiator), the original diode heats up so much that it is impossible to touch it with your hand. I checked the current consumed by the device (with a 21 W lamp) with the original diode and with the MBR2545CT - the current (consumed from the network, I have a voltage of 230 V) dropped from 0.115 A to 0.11 A. The power decreased by 1.15 W, I believe that this is exactly how much was dissipated on the original diode.
There was nothing to replace Q2 with, so I found the C945 transistor at hand. I had to “power up” it with a circuit with a KT837 transistor (Figure 2). The current remained under control and when comparing the current with the native circuit on 2SC2655, there was an even reduction in power consumption with the same load at 1 W.
As a result, with a load of 21 W and when operating for 5 minutes, the output transistor and rectifier diode (without a radiator) heat up to 40 degrees (slightly warm). In the original version, after a minute of operation without a radiator, they could not be touched. The next step to increasing the reliability of blocks made according to this scheme is to replace the electrolytic capacitor C12 (prone to drying out of the electrolyte over time) with a conventional non-polar, non-electrolytic one. The same nominal value of 0.47 µF and a voltage of at least 50 V.
With such characteristics of the power supply, you can now safely connect LED strips without fear that the efficiency of the power supply will worsen the efficiency of LED lighting.
If you have repaired a UPS, then you have probably encountered this situation: all faulty elements have been replaced, the remaining ones seem to have been checked, but you turn on the TV and... bam... and everything has to start all over again! There are no miracles in radio engineering, and if something doesn’t work, there’s a reason for it! Our task is to find her!
The UPS is the most unreliable component in modern radio devices. This is understandable - huge currents, high voltages - after all, all the power consumed by the device passes through the UPS. At the same time, let’s not forget that the amount of power supplied by the UPS to the load can change tens of times, which cannot have a beneficial effect on its operation.
Most manufacturers use simple UPS designs. This is understandable. The presence of several levels of protection can often only complicate repairs and have virtually no effect on reliability, since the increase in reliability due to an additional protection loop is compensated by the unreliability of additional elements, and during repairs we have to spend a long time figuring out what these parts are and why they are needed. Of course, each UPS has its own characteristics, differing in the power supplied to the load, the stability of the output voltages, the range of operating mains voltages and other characteristics that play a role during repairs only when you need to choose a replacement for a missing part.
It is clear that when making repairs it is advisable to have a diagram. Well, if it’s not there, simple TVs can be repaired without it. The operating principle of all UPSs is almost the same, the only difference is in the circuit designs and types of parts used.
I use a technique developed over many years of repair experience. More precisely, this is not a technique, but a set of mandatory actions for repairs, proven by practice.
The proposed method assumes that you are at least somewhat familiar with how a TV works. For repairs, you need a tester (avometer) and, preferably, but not necessarily, an oscilloscope.
So, let's repair the power supply.
They brought you a TV or yours got damaged.
* Turn on the TV, make sure that it is not working, that the standby indicator is not on. If it is on, then the problem is most likely not in the UPS. Just in case, you will need to check the horizontal scan supply voltage.
* Turn off the TV and take it apart.
* External inspection of the TV board, especially the area where the UPS is located. Sometimes swollen capacitors, burnt resistors, etc. may be found. It will be necessary to check them in the future.
* Carefully review the soldering, especially the transformer, key transistor/microcircuit, chokes.
* Check the power circuit: check the power cord, fuse, power switch - if there is one, chokes in the power circuit, rectifier bridge. Often, with a faulty UPS, the fuse does not burn out - it simply does not have time. If the key transistor breaks through, the ballast resistance is more likely to burn out than the fuse. It happens that the fuse burns due to a malfunction of the posistor, which controls the demagnetizing device (demagnetization loop). Be sure to check for a short circuit the terminals of the mains power filter capacitor without desoldering it, since in this way you can often check for breakdown the collector-emitter terminals of a key transistor or microcircuit if a power switch is built into it. Sometimes power is supplied to the circuit from the filter capacitor through ballast resistors, and if they break, it is necessary to check for breakdown directly at the switch electrodes.
* Take a moment to check the remaining parts of the unit - diodes, transistors, some resistors. First, we check without desoldering the part; desoldering it only when there is a suspicion that the part may be faulty. In most cases, such a check is sufficient. Ballast resistors often break. Ballast resistances have a small value (tenths of an Ohm, units of Ohm) and are designed to limit pulse currents, as well as for protection as fuses.
* We need to see if there are any short circuits in the secondary power circuits - to do this, we check the terminals of the capacitors of the corresponding filters at the outputs of the rectifiers for short circuits.
After completing all checks and replacing faulty parts, you can perform a live test. To do this, instead of the mains fuse, we connect a 150-200 Watt 220 Volt light bulb. This is necessary so that the light bulb protects the UPS in case the malfunction is not resolved. Disconnect the degaussing device.
Turn it on. There are three options:
1. The light flashed brightly, then went out, and a raster appeared. Or the standby mode indicator lights up. In both cases, you need to measure the voltage supplying the drain scanner - it varies for different TVs, but not more than 125 Volts. Often its value is written on the printed circuit board, sometimes near the rectifier, sometimes near the TDKS. If it is too high to 150-160 Volts, and the TV is in standby mode, then switch it to operating mode; some TVs allow the voltage to be too high at idle (when the horizontal scan does not work). If the voltage is too high in operating mode, check the electrolytic capacitors in the power supply only by replacing them with a known good one. The fact is that often electrolytic capacitors in a UPS lose their frequency properties and cease to perform their functions at the generation frequency, despite the fact that when tested by a tester using the charge-discharge method, the capacitor seems to be in good order. The optocoupler (if there is one) or the optocoupler control circuit may also be faulty. Check whether the output voltage is regulated by internal regulation (if any). If it is not adjustable, then you need to continue searching for faulty parts.
2. The light flashed brightly and went out. Neither the raster nor the standby mode indication appeared. This indicates that the UPS does not start. It is necessary to measure the voltage on the surge protector capacitor, it should be 280-300 Volts. If it is not there, sometimes they put a ballast resistor between the mains rectifier bridge and the capacitor. Check the power supply and rectifier circuits again. If the voltage is too low, one of the diodes of the mains rectifier bridge may be broken or, what is more common, the mains power filter capacitor may have lost capacity. If the voltage is normal, then you need to once again check the rectifiers of the secondary power supplies, as well as the starting circuit. The triggering circuit for simple TVs consists of several resistors connected in series. When testing a circuit, you need to measure the voltage drop across each of them, measuring the voltage directly at the terminals of each resistor.
3. The light is on at full brightness. Turn off the TV immediately. Recheck all items. And remember - there are no miracles in radio engineering, it means you missed something somewhere, you didn’t check everything.
95% of the malfunctions fit into this diagram, but there are more complex malfunctions when you have to rack your brains. For such cases, you cannot write methods and you cannot create instructions.
Problems and defects of power supplies can be completely different - from complete inoperability to permanent or temporary failures. As soon as you start repairing the power supply, make sure that all contacts and radio elements are visually in order, the power cords are not damaged, the fuse and switch are working, and there are no short circuits to ground. Of course, the power supplies of modern equipment, although they have common principles of operation, are quite different in their circuitry. Try to find a diagram for the power supply, this will greatly simplify and speed up repairs; remember that many power supply diagrams with a list of typical faults can be found in the service manual for this equipment.
When starting repairs, you must be able to use a multimeter, and preferably an oscilloscope, and, of course, have tin and rosin.
A problematic power supply becomes the cause of unstable operation of the device, manifesting itself either as periodic errors in operation, or spontaneous glitches, or even the failure of one or another unit in the equipment.
At the first signs of unstable operation of the power supply, it is necessary to begin repairing and diagnosing the power supply as soon as possible. And the first action that you should take immediately after opening the device case is to vacuum the power supply, and then carefully inspect all the radio components and their connections.
Carefully inspect all electrolytic capacitors to see if any are swollen. And it will be best if you check each of them. Even if a capacitor looks great, that doesn't mean it hasn't lost capacity or experienced a dramatic increase in ESR. There are small attachments for multimeters and ready-made ones that will help even without removing it from the circuit. If you wish, you can assemble such a device yourself.
Another problem that may arise and require repair of the power supply is unstable operation and ripple of the supply voltage caused by poor filtering. They are easy to see on the oscilloscope screen. Small pulsations can be ignored, but strong noises will require repairs. But the issue of ripple is acute only in switching power supplies used in TVs, monitors, and computers and is not at all relevant for some simple devices.
Let's look at the different types of power supplies and the problems that arise in them in more detail.
Switching power supplies are the most unreliable element in modern household appliances and devices. This is logical - high currents, high voltages - after all, all the power that the circuit consumes passes through the switching power supply. At the same time, keep in mind that the power supplied by the power supply to the load can change dozens of times during operation, which in itself is not very good.
Most companies use simple switching power supply circuits. This is understandable: firstly, it is cheaper, and secondly, there is a high probability that your device will fail after a few years of operation.
When repairing a switching power supply, it is advisable to have a circuit diagram. Well, if it’s not there, simple TVs can be repaired without it. The operating principle of all pulse generators is almost the same, the only difference is in the circuit designs and types of parts used. You can download a huge range of monitors and video equipment from us.
Let's get started repairing a power supply using the example of a UPS from a TV.
Turn on the TV, make sure that it does not turn on, that the standby LED is not lit. If it lights up, then the problem is most likely not in the power supply.
Turn off the TV and disassemble it
Visual inspection of the board, especially the place where the switching power supply is located
Carefully inspect the solder joints, especially at the transformer
Check the power cord, fuse, power switch, chokes and bridge rectifier. Then, diodes and zener diodes and, if possible, microcircuits. First, carry out the check without unsoldering the radio component from the circuit; unsolder it only when you suspect it.
Often, when a switching power supply breaks down, the fuse does not have time to burn out. In the case of a burnt-out key transistor, also check the ballast resistance. A fuse due to which is used in CRT televisions to control the demagnetizing device. Be sure to test the terminals of the mains power filter capacitor with a multimeter for a short circuit, but just do not unsolder it from the circuit, since you can also check a key transistor or a microcircuit with a built-in power switch for breakdown with one measurement.
Ballast resistors often break. Because they have a very low resistance (tenths of an Ohm, units of Ohm) and are used to limit pulse currents, as well as for protection
We need to see if there are short circuits in the secondary power circuits - to do this, we check for short circuits the capacitor terminals of the corresponding filters at the outputs of the rectifiers
After completing all these steps and replacing the defective parts, you can perform a live test. But before replacing the mains fuse, install a 150-200 Watt light bulb (or light bulbs connected in series). The load will protect your switching power supply if the problem is not completely resolved.
There are three options:
The light bulb lights up brightly, and then goes out, and a raster appears. Or the standby LED lights up. It is necessary to measure the voltage of the waste sweep. If it is too high, check the electrolytic capacitors, but only by replacing them with exactly good ones. Also, optocouplers (if they are present in the circuit) or its control circuits may not work. can be read here.
If the light flashed brightly and went out. There is no raster or indication. This means that the switching power supply does not start. It is necessary to measure the voltage on the surge protector capacitor and make sure that it is about 280-300 Volts. If it is not there, ring the power circuit and rectifier again. If it is lower, check the diode bridge or the filter capacitor has lost capacity. If the voltage is 280-300 volts, check the rectifiers of the secondary power supplies, as well as their starting circuits.
The light is very bright. Turn off the TV immediately. Check everything again
Typically, defects that occur in switching power supplies fall into this troubleshooting algorithm, but sometimes more complex problems occur. There is no technique for such cases, just use your brain.
Let's take a look at the possible options for why the power supply could burn out and what problems this could cause for the rest of the electronics of the system unit? Very often, blocks burn during voltage surges; in this case, first of all, you need to ring the input circuits of the inverter or filter; sometimes damaged elements can be recognized simply by visual inspection.
If the ATX power supply in a personal computer does not show signs of performance: the fans do not spin, the motherboard does not start, disconnect it and remove it from the system unit for repair.
So, before you start repairing the power supply, you need to understand for sure that the computer does not work only because of it. Only after this can you begin to remove the power supply from the case. For those who are doing this for the first time, I recommend photographing their connection before disconnecting the cables from the motherboard.
In order to be able to turn on the computer power supply for repair purposes you need to connect a load to it according to the diagram:
If there are no traces of burning or other noticeable defects. We start repairs with fuse checks. If it burns out, connect an incandescent lamp of 100 Watts to it, as in the case of repairing a switching TV power supply. If there is a short circuit, it will flash brightly, and thereby signal to us about the breakdown of the diode bridge or capacitors.
Now you need to check all output voltages of the power supply.
To check the +/-5 V and +/-12 V circuits, measure their resistance with the power supply turned off (+5 V is the red wire, and +12 is the yellow wire, the black wire is ground). If the resistance is less than 100 Ohms, there is most likely a breakdown of the diodes in the rectifier bridge. The breakdown of rectifier diodes most often manifests itself as a low buzzing sound. Check the -5 V/-12 V lines in the same way.
More difficult make sure the PWM controller on the TL493 chip is working, TL494, TL495. Their data and pinouts are available in . Start by measuring the supply voltage of the chip. If this voltage is missing, check the external circuits and, if they are in order, replace the microcircuit.
Using a multimeter, measure the reference voltage on the microcircuit, it should be +5 V. If the voltage does not match, check the resistor dividers connected to this circuit.
At pin 5 of TL493, TL494, TL495 there should be a sawtooth voltage ripple with an amplitude of about 3 V and a frequency of 1 to 50 kHz, which can be viewed using an oscilloscope. If they are not there, check the capacitor at leg 5, and the resistor 6. If they are working, change the microcircuit.
It remains to check the signals at the output of the PWM controller. If pulses with clear edges and an amplitude of about 2-3 V are observed, the microcircuit is operational. Otherwise, the transistors in the high-voltage switch circuit are broken.
It would also be a good idea to check the windings of the transformers.
There is also such a typical defect: The computer power supply turns on spontaneously. The fans are spinning, but the computer does not turn on. The cause of the defect, in most cases, is a breakdown of the standby voltage stabilizer of the power supply, which generates the standby voltage +5V. Without seeing it at startup, the system is simply not able to pass the initial stage of self-testing.
If you are in the mood to assemble a load circuit, you can simply close the PS-ON and COM contacts. The figure below shows two versions of the pin arrangement on the computer power supply cable.
But before you close the PS-ON and COM contacts together, you need to check for the presence of a “standby” voltage of +5V at the “+5VSB” contact; it is usually purple. To do this, you need to connect the power supply to a 220 volt network, take a multimeter, switch it to “voltmeter” mode, then connect the negative probe to one of the COM contacts, and the positive one to +5VSB. The multimeter should show the presence of +5V. If there is no voltage, then it is necessary to disassemble the power supply and check the circuit using this power supply.
If there is “standby” voltage, then you can safely bridge the PS-ON and COM contacts, and after applying 220V power, begin checking the remaining available voltages.
If the absence of one or more of them is detected, then you can begin to disassemble the power source.
After disassembling, first clean the power supply unit from dust. After cleaning, visually inspect the board, especially the capacitors, they really like to swell there. It looks like this:
If you find such capacitors with swollen tops, feel free to change them. This malfunction is the most common, and almost anyone who knows how to hold a soldering iron with their hands, and not with their appendages from one place, can eliminate such a minor nuisance. The main thing is to remember that all electrolytic capacitors have polarity, so do not confuse their terminals.
Repair of power supplies - Zalman ZM500-GS does not turn on with a malfunction.
Having connected the power supply to the network and checked it with a tester for computer power supplies, I was convinced that it was completely inoperable. The mains fuse turned out to be burnt out. By connecting a 100-watt incandescent lamp instead. In working condition under load, it should light up (at the moment of charging the mains capacitors), and then go out a little. In standby mode, when the power supply consumption is low, the lamp may light for a short time and then go out. This behavior should be repeated cyclically.
Turning on the power, the lamp lit up brightly, indicating a short circuit or high current consumption in the primary circuits. Having connected the tester, I made sure that all output voltages were present on the ATX connector. This is already buzzing, having torn off the thermal insulation from a radio element similar to a capacitor, I saw a burnt one underneath it. Replacing it with a new power supply began to work correctly.
They gave me a Chinese transformer power supply, model HKA-12100EC-230, but it turned out not to work. If you believe the designation on it, it produces a current of up to 1 A. This is exactly what I need, so I decided to disassemble and try to repair this Chinese thing.
Educational film in Russian, revealing the technology for repairing an ATX computer power supply