Faults in the switching power supply with PWM. Do-it-yourself quick repair of switching power supplies

A typical ATX power supply circuit is shown in the figure. Structurally, it is a classic pulse unit on the 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 consider the structure of the ATX power supply in more detail. Its first element is
:

Its task is to convert AC current 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 overloading 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 lead to malfunctioning 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.

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 pulse converter is an PWM controller... This abbreviation has already been mentioned several times, but not 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 corresponding frequency, which, after the isolation transformer, are smoothed by the 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.

They have always been important elements of any electronic device. These devices are used in amplifiers and receivers. The main function of power supplies is considered to be the reduction of the limiting 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, the 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

Considering regular block power supply that is used in radios, it consists of a frequency transformer, a transistor, and several diodes. Additionally, there is a choke in the circuit. Capacitors are installed with different capacities and can vary greatly in parameters. Rectifiers are used, as a rule, of the capacitor type. They belong to the high-voltage category.

Work of modern blocks

Initially, voltage is supplied to the bridge rectifier. At this stage, the peak current limiter is triggered. This is necessary so that the fuse in the power supply does not blow. Further, the current passes through the circuit through special filters, where it is converted. Several capacitors are required to charge the resistors. The launch of the node occurs only after the 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, diodes in the circuit are activated. They are interconnected by cathodes. The negative potential in the system makes it possible to lock the dinistor. Facilitation of starting the rectifier is carried out after locking the transistor. Additionally provided To prevent saturation of the transistors, there are two fuses. They work in the circuit only after a breakdown. A transformer is required to trigger feedback. Pulse diodes feed it in the power supply unit. At the output, alternating current flows through the capacitors.

Features of laboratory units

The principle of operation of switching power supplies of this type is based on active current conversion. One bridge rectifier is provided in the standard circuit. In order to remove all interference, filters are used at the beginning as well as at the end of the chain. Pulse capacitors laboratory unit food has the usual. Saturation of transistors occurs gradually, and this has a positive effect on diodes. Voltage regulation is provided in many models. The protection system is designed to save blocks from short circuits. The 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 of the ATX 20 type. To cool the unit, a fan is mounted in the case. The speed of rotation of the blades must be regulated in this case. The laboratory-type unit should be able to withstand the maximum load at a level of 23 A. At the same time, the resistance parameter is on average maintained at around 3 ohms. The limiting frequency that a pulsed laboratory power supply has is 5 Hz.

How do I repair devices?

Most often, power supplies suffer from blown fuses. They are located next to the capacitors. You should start repairing switching power supplies by removing the protective cover. Next, it is important to inspect the integrity of the microcircuit. If defects are not visible on it, it can be checked using a tester. To remove the fuses, you must first disconnect the capacitors. Then they can be removed without any problems.

To check the integrity of this device, inspect its base. Burnt fuses at the bottom have a dark spot, which indicates damage to the module. To replace this element, you need to pay attention to its labeling. Then in the electronics store, you can purchase a similar product. Installation of the fuse is carried out only after the condensation has been fixed. Another common problem in power supplies is considered to be faults with transformers. They are boxes in which the coils are installed.

When the voltage is applied to the device very high, then they will not withstand. 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 principle of operation of switching power supplies of the network type is based on a low-frequency reduction in the amplitude of interference. This is due to the use of high voltage diodes. Thus, it is more effective to control the limiting frequency. Additionally, it should be noted that medium power transistors are used. The load on the fuses is minimal.

Resistors are rarely used in the standard circuit. This is largely due to the fact that the capacitor is able to participate in the conversion of current. 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 mains 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.

The diodes in this system partially function as fuses. Only transistors are used to start the rectifier. The locking process, in turn, is required to activate the filters. Capacitors can also be used as a decoupling type in the system. In this case, the start of the transformer will be much faster.

Application of microcircuits

The microcircuits in power supplies are used in a variety of ways. In this situation, a lot depends on the number of active elements. If more than two diodes are used, then the board must be sized for input and output filters. Transformers are also produced in different capacities, and their dimensions are quite different.

You can do the soldering of microcircuits yourself. In this case, you need to calculate the limiting 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. The ripple inside the board will be much faster.

Benefits of regulated power supplies

The principle of operation of switching power supplies with regulators is to use a special controller. This element in the circuit can change the bandwidth of the transistors. Thus, the limiting frequency at the input and at the output is significantly different. You can configure the switching power supply in different ways. Voltage regulation 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 it, protective filters are used.

The power of the devices on average fluctuates around 300 watts. Only non-modular cables are used in the system. Thus, short circuits can be avoided. The power supply connectors for connecting devices are usually installed in the ATX 14 series. The standard model has two outputs. Rectifiers are used with high voltage. They are able to withstand resistance at the level of 3 ohms. In turn, the pulse regulated power supply unit accepts the maximum load up to 12 A.

The operation of blocks for 12 volts

Pulse includes two diodes. In this case, the filters are installed with a small capacity. In this case, the pulsation process is extremely slow. The average frequency fluctuates around 2 Hz. The efficiency of many models does not exceed 78%. These blocks also differ in their compactness. This is due to the fact that transformers are installed with low power. At the same time, they do not need cooling.

The scheme of a 12V switching power supply additionally implies the use of resistors marked P23. They are able to withstand only 2 ohms of resistance, but such a power is enough for a device. A 12 V switching power supply is used most often for lamps.

How does the TV box work?

The principle of operation of switching power supplies of this type consists in the use of film filters. These devices are able to cope with interference of varying amplitudes. They have synthetic winding. Thus, high-quality protection of important nodes is provided. All gaskets in the power supply are insulated from 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 the maximum temperature up to 60 degrees. The operating frequency of the switching power supply for TVs 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.

24 volt device models

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. Resistance resistors can only handle 2 ohms. Transistors in models are installed with PR2 marking.

To stabilize the voltage, resistors are not used in the circuit. Filters 24V switching power supply is of capacitor type. In some cases, you can find dividing species. They are necessary to limit the limiting frequency of the current. To quickly start the rectifier, dinistors are rarely used. The negative potential of the device is removed using the cathode. At the output, the current is stabilized due to blocking of the rectifier.

Power supply sides on the DA1 diagram

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 near the resistor. There are no more than three diodes in the circuit.

The direct reverse conversion process begins in the dinistor. A special throttle is provided in the system to start the unlocking mechanism. High amplitude waves are damped at the capacitor. It is installed usually of the dividing type. Fuses are rare in the 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.

Device models with DA2 microcircuits

Microcircuits of switching power supplies of this type, among other devices, are distinguished 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 instrument 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 ordinary. All this makes it possible to withstand the maximum load at the level of 30 A. In turn, the indicator of the limiting frequency is at around 23 Hz.

Blocks with installed DA3 microcircuits

This microcircuit allows you to install not only a regulator, but also a controller that monitors fluctuations in the network. The resistance transistors in the device are capable of withstanding approximately 3 ohms. Powerful switching power supply DA3 with a load of 4 A copes. You can connect fans to cool the rectifiers. As a result, the devices can be used at any temperature. Another advantage is the availability of three filters.

Two of them are installed at the inlet under the capacitors. One crossover filter is available at the output and stabilizes the voltage that comes from the resistor. There are no more than two diodes in the 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 devices.

How does the VD1 diode unit work?

These blocks are designed to support up to three devices. They have three-way regulators. Communication cables are installed only non-modular. Thus, the conversion of the current is fast. Rectifiers in many models are installed in the KKT2 series.

They differ in that the energy from the capacitor is able to transfer to the winding. As a result, the filter load is partially relieved. The performance of such devices is quite high. At temperatures over 50 degrees, they can also be used.

Depending on the causes and types of breakdowns that have occurred, different types of tools may be required, it is imperative to have:

• a set of screwdrivers with various types of working tips and sizes;
• insulating tape;
• pliers;
• knife with a sharp blade;
• soldering machine, solder and flux;
• braid designed to remove unnecessary solder;
• tester or;
• tweezers;
• nippers;

In the most difficult cases, when it is not possible to establish the exact cause of the problem, an oscilloscope may be needed.

Repair of basic faults

After carrying out diagnostics, and identifying the causes of incorrect operation, you can start repairing it:

1. Dust trapped inside the power supply can simply be removed with a conventional household vacuum cleaner.
2. If the reason was a faulty fuse, then you need to purchase a new part, which is available in all relevant stores. After that, the old element is removed and the new fuse is soldered. If this sequence of actions did not help, and the power supply did not work, then it remains to send it to the workshop for diagnostics using professional types of equipment, or simply purchase a new device.
3. If the problem was in the capacitors or, then the malfunction is corrected according to the same algorithm: new parts are acquired and soldered into the circuit instead of old elements.
4. If the problem was the throttle, then it is not necessary to replace it, since this element can be repaired using a fairly easy technique. The choke is removed from the power supply, after which it will need to be disassembled and start winding up the burnt wire, while it is important to carefully count the winding turns. Then you need to pick up a similar wire with an equal diameter and wind it instead of the damaged conductor, making the same number of turns that was wound. After completing these steps, the throttle is installed back into place and, if everything was done correctly, the device should function.
5. Thermistors cannot be repaired, they are simply replaced with new elements, most often this is done together with fuses.
6. For prevention, during the repair, you can remove the cooler from the device and lubricate it with machine oil, and then install it in place.
7. If cracks are found on the board surface, which have damaged the connection of the contacts, then they must be closed with soldering. In the same way, any breakdown of contacts in a resistor, inductor or.

Device

Power supplies of this type are essentially a kind of voltage stabilizers, the device of which looks like this:

1. Mains rectifier is one of the main elements that is necessary to smooth out the resulting ripple. Also, it is required to maintain the charge of the filtering capacitors in the switched on mode and the continuous transfer of electricity to the load if the voltage in the main supply network falls below the permissible parameters for operation. Its design includes special types of filters that allow you to suppress most of the arising noise.
2. Voltage transformer, the main components of which are the converter and the controller of the control device.
3. Converter It also has a complex structure, which includes a pulse-type transformer, an inverter, a number of rectifiers and stabilizers that provide a secondary feed and voltage supply to the load. The inverter is necessary to change the shape of the constant output voltage, which after the conversion process becomes an alternating voltage with a rectangular shape. The presence of a transformer operating on high frequencies with a value above 20 kHz, due to the need to maintain the operating state of the inverter in an auto-generator mode, as well as to obtain a voltage that is used to feed the controller, load circuits and a number of protective circuits.
4. Controller performs the functions of controlling the transistor switch, which is part of the inverter. In addition, it stabilizes the parameters of the voltage supplied to the load and protects the device as a whole from possible overloads and unwanted overheating. If the power supply has an additional function that provides remote control of the device, then the controller is also responsible for its implementation.
5. Power supply controller this type consists of a number of functional units, such as a source that provides uninterrupted power supply; protective system; pulse duration modulator; a logic circuit for signal processing and a generator of a special type of voltage intended for supply to transistors located in the converter.
6. In most modern models, there are optocouplers used as decoupling. They are gradually replacing transformer types of isolation, this is due to the fact that they take up less free space and have the ability to transmit signals in a much wider frequency spectrum, but at the same time require a significant number of intermediate amplifiers.

Major malfunctions and their diagnostics

Sometimes switching power supplies break down and their malfunctions can be of a very different nature, but there are a number of similar cases, on the basis of which a list of the most common types of malfunctions was compiled:

1. Unwanted ingestion dust devices, especially construction dust.
2. Fuse failure, most often this problem is caused by another malfunction - burnout of the diode bridge.
3. No output voltage with a functional and serviceable fuse. This problem can be caused by various reasons, most often they are a breakdown of a rectifier diode, or a burnout of a filter choke in the low-voltage region of the circuit.
4. Failure of capacitors, most often this happens for the following reasons: loss of capacity, leading to poor quality filtering of the voltage at the output and an increase in the level of operating noise; excessive increase in series resistance parameters; short circuit inside the device or rupture of internal leads.
5. Lost contact connectionswhich is most often caused by cracks in the board.

If the power supply fails for any reason, then before carrying out any troubleshooting work on your own, it is necessary to conduct a thorough diagnosis to identify their causes.

Depending on different situations, this procedure has its own characteristics:

1. Inspect the power supply generally for the presence of accumulated dust in it, which may be the reason for its incorrect operation.
2. Check the main board for the presence of cracks on its surface.
3. Visual inspection the main power supply board allows you to determine the status of the fuses. It will be quite simple to notice a breakdown; this element of the device will swell or completely collapse in the event of a breakdown. It is also recommended to immediately carry out a comprehensive check of the power bridge, filter capacitor and all power switches.
4. If the fuse is in good condition, then it is necessary to check the choke and electrolyte capacitors, malfunctions are also easily detected by a visual method by the resulting deformations or swelling. It is more difficult to diagnose a diode bridge or individual diodes, they will need to be removed from the circuit and separately checked using a tester or multimeter.
5. Condenser test It is also carried out by a visual method, since the resulting overheating could melt the electrolyte and destroy their bodies, or with the help of a special device designed to measure the level of their capacity, if no external faults were detected.
6. Inspect the thermistorwhich is subject to frequent breakdowns due to power surges or overheating. If its surface turns black, and it itself collapses from light touches, it means that the cause of the malfunction is in it.
7. Check contacts all remaining elements (resistor, transformer, inductor) for possible connection failures.

Additionally, when diagnosing or repairing switching power supplies, it is recommended to follow the following tips:

1. Self-repair such devices is a rather complex process that requires certain skills and knowledge, even if detailed instructions are available. Therefore, if there is no self-confidence, it is better to contact a qualified technician so as not to cause even more serious damage to the power supply.
2. Before starting any action with the switching power supply, it must be disconnected from the mains. At the same time, pressing the corresponding key on the device itself does not guarantee complete safety during repairs, therefore, it is necessary to disconnect the power cord.
3. After the power supply has been completely de-energized, it is necessary to wait about 10-15 minutes before starting any work. This time is required to completely discharge the capacitors on the board.
4. If soldering is required, then they must be carried out extremely carefully, since overheating of the soldering point can cause delamination of the tracks, and there is also a risk of their closure with solder. Best of all, soldering machines with a power parameter in the range of 40-50W are suitable for these purposes.
5. Collecting the power supply after the completion of the repair, it is allowed to be performed only after a careful examination of the soldering points, in particular, it is required to check the closure with solder between the tracks.
6. It is recommended to provide a switching power supply high-quality ventilation and cooling, which will protect it from dirt and overheating, which minimizes possible breakdowns. Also, the ventilation openings on the device must not be blocked.

Industrial power supplies often fail, sometimes even high-quality and expensive samples. In this case, an ordinary person most often throws away and acquires a new one, but the cause of the breakdown may be insignificant, and for the radio amateur, such devices are of considerable interest in terms of studying and the possibility of returning to working capacity. Despite the fact that devices that cost a lot of money are often thrown away.

We suggest users to consider a simple repair of a stabilized pulse-type power supply based on a flyback generator with feedback by current and voltage, which, in addition to stabilization, allows for overload protection. The unit is powered from an alternating current with a voltage of 100 to 240 Volts and a frequency of 50/60 Hertz and produces a constant voltage of 12 Volts 2 Amperes.

The malfunction described here is quite common in power supplies of this type and has the following symptoms: the output voltage periodically appears and disappears with a certain frequency, which is visually observed as flashes and extinguishing of the output power indicator LED:

If the indicator LED is not installed, then a similar symptom can be detected with a dial-up voltmeter by connecting it to the output of the power supply. In this case, the voltmeter needle will periodically deviate to a certain value and come back (maybe not completely). This phenomenon is observed due to the operation of the device's protection, when the voltage or current at certain points exceeds the permissible value.

This can happen both with a short circuit and with an open circuit. A short circuit most often occurs during the breakdown of capacitors or semiconductor radioelements such as diodes or transistors. A break can be observed both in semiconductors and resistors. In any case, first of all, you should visually inspect the printed circuit board and the radio elements installed on it.

Diagnostics of the power supply before repair

It is best to do visual diagnostics using a magnifying glass:

Power supply repair step by step with photo

The resistor could burn out when the rated power dissipation was exceeded for a long time. The burned out resistor was soldered out, and its seat was cleaned:

At the moment, according to a quick visual inspection, one can distinguish a good mains and output filter, a well-thought-out circuitry for controlling a power transistor and a good stabilization of the output voltage. The physical design of the device is also on high level, installation is rigid and smooth, the soldering is clean, precision radioelements are used. All this allows us to obtain a high quality device with precisely specified parameters and characteristics.

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Semiconductors and capacitors in high-voltage circuits are especially dangerous, which, in the event of a breakdown, can lead to irreversible consequences for the entire device if it is turned on repeatedly without revealing a complete list of damaged components. With correct and careful diagnostics, in most cases everything ends well and the breakdown can be eliminated by replacing the damaged parts with the same serviceable ones or similar in value and parameters.

Video instructions for repairing a switching power supply:

Switching power supplies are the most unreliable component in modern radio devices. This is understandable - huge currents, high voltages. The UPS passes all the power consumed by the device. At the same time, let's not forget that the amount of power supplied by the UPS to the load can change tenfold, which cannot have a beneficial effect on its operation.

Most manufacturers use simple schemes switching power supply, it is understandable. The presence of several levels of protection often only complicates the repair and practically does not affect reliability, since the increase in reliability due to an additional protection loop is compensated by the unreliability of additional elements, and during repair it takes a long time to figure out what these parts are and why they are needed.

Of course, each switching power supply has its own characteristics, which differ in the power supplied to the load, the stability of the output voltages, the operating range mains voltages and other parameters that play a role during repair only when you need to choose to replace a missing part.

It is clear that it is desirable to have a circuit during repairs. Well, if it is not there, simple TVs can be repaired without it. The principle of operation of all switching power supplies is practically the same, the only difference is in circuit solutions and types of parts used.

• How to fix ?

So, step-by-step instruction repair of a switching power supply:

1. We turn on the TV, make sure that it does not work, that the standby indicator is off. If it is on, then most likely the matter is not in the power supply. Just in case, you will need to check the line scan supply voltage.
2. We turn off the TV, disassemble it.
3. We conduct an external examination of the TV board, especially the area where the power supply is located. Sometimes swollen capacitors, burnt resistors, and more can be found. It will be necessary to check them in the future.
4. We carefully look at the soldering, especially the transformer, key transistor / microcircuit, chokes.
5. We check the power circuit: we ring the power cord, fuse, power switch (if any), chokes in the power circuit, rectifier bridge. Often, when the UPS is faulty, the fuse does not burn out - it simply does not have time. If the key transistor breaks through, the ballast resistance will burn out rather than the fuse. It happens that the fuse burns due to a malfunction of the posistor, which controls the demagnetizing device (demagnetizing loop). Be sure to check the filter capacitor leads for a short circuit. mains supply, without soldering it, since in this way it is often possible to check for breakdown the conclusions of the collector - the emitter of the 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 in the event of a breakdown, it is necessary to check for breakdown directly on the key electrodes.
6. We check the rest of the block details - diodes, transistors, some resistors. First, we check without soldering the part, we solder it only when there is a suspicion that the part may be faulty. In most cases, this check is sufficient. Ballast resistances are often cut off. Ballast resistors have a small value (tenths of Ohm, units of Ohm) and are designed to limit impulse currents, as well as for protection as fuses.
7. We look for short circuits in the secondary power circuits - for this we check the capacitor leads of the corresponding filters at the rectifier outputs for a short circuit.

We include. At this stage, three options are possible:

1. The light bulb flashed brightly, then went out, a raster appeared. Or the standby indication came on. In both cases, it is necessary to measure the voltage supplying the horizontal scan - it is different 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 overestimated to 150-160 Volts, and the TV is in standby mode, then put it into working mode. In some TVs, it is allowed to overestimate the voltage at idle (when the line scan does not work). If the voltage is too high in operating mode, check the electrolytic capacitors in the power supply only by replacing it with a known good one. The fact is that often electrolytic capacitors in a UPS lose their frequency properties and at the generation frequency cease to perform their functions, despite the fact that when checked with a tester by the charge-discharge method, the capacitor seems to be working. The optocoupler (if any) or the optocoupler control circuit may also be faulty. Check if the output voltage is internally regulated (if any). If it is not regulated, then you must continue to search for faulty parts.
2. The bulb flashed brightly and went out. Neither a raster nor an indication of the standby mode appeared. This indicates that the switching power supply does not start. It is necessary to measure the voltage across the capacitor of the mains filter, it should be 280-300 Volts. If it is not there, sometimes they put a ballast resistance between the bridge of the mains rectifier and the capacitor. Check the supply and rectifier circuits again. If the voltage is too low, one of the diodes of the mains rectifier bridge may be cut off or, more often, the capacitor of the mains supply filter has lost its capacity. If the voltage is normal, then you need to check the rectifiers of the secondary power supplies, as well as the starting circuit. The trigger circuit for simple TVs consists of several resistors in series. When testing a circuit, you need to measure the voltage drop across each of them by measuring the voltage directly at the terminals of each resistor.
3. The light is on at full brightness. Switch off the TV immediately. Check all items again. And remember - there are no miracles in radio engineering, it means that you missed something somewhere, did not check everything.

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