Spots on the screen. Repairing the kinescope demagnetization system

Among modern color CRT TVs, a malfunction is quite common. posistor in the kinescope demagnetization circuit.

Externally, a posistor malfunction can manifest itself as follows:

Such a malfunction sometimes misleads people, which leads to the misconception that the TV picture tube is faulty. In fact, the kinescope is completely intact, just highly magnetized.

Magnetization of the kinescope may appear if the TV has not been disconnected from the power supply for a long time, i.e. The device was working for a long time or was in standby mode. As a result, under the influence of the Earth’s magnetic field, a special plate was magnetized inside the kinescope; it is called a shadow mask.

Thanks to this mask, three electron beams are projected onto the phosphor layer of the screen: red, blue and green. Naturally, if it is magnetized, then this introduces distortion and the rays are brought together incorrectly. Because of this, areas of unnatural color rendition appear on the screen.

How does the degaussing circuit work in CRT TVs?

In practice, two demagnetization schemes are used. One uses a two-terminal posistor, and the other uses a three-terminal one. The difference is small, but it is there. Let's look at both schemes.

If you don’t know what a posistor is, then read the page about thermistors and their varieties.

In color picture tube TVs with small screen diagonals (21 inches or less), the picture tube demagnetization circuit is implemented according to a fairly simple scheme. Take a look.

The circuit consists of a posistor (PTC) and an inductor ("loop"). It is designated as L1. Coil L1 is a kind of electromagnet. Thanks to it, magnetization is removed from the kinescope mask.

Every time you turn on the TV, a fairly significant current begins to flow through the coil, with an amplitude of about 10 amperes and a mains frequency (50 Hz). This current in the coil generates an electromagnetic field. It demagnetizes the kinescope mask. In order for the electromagnetic field to fade smoothly and quickly, a posistor (PTC) is installed in series with the coil. Let me remind you that at room temperature, in the so-called “cold” state, its resistance is low and is only 18 ~ 24 Ohm.

Under the influence of a large surge of current, it instantly heats up and its resistance increases sharply. As a result, the current in the coil ("loop") decreases, and, consequently, the electromagnetic field that was required to demagnetize the kinescope. That's all, the kinescope is demagnetized.

Further, while the TV is working or simply “resting” in standby mode, the posistor in the demagnetization circuit is in a “heated” state and limits the current in the demagnetization coil L1 to a minimum. This continues until the TV is disconnected from the 220V network and the posistor cools down. The next time you turn on the TV, it will work again together with the demagnetization loop.

This demagnetization circuit works only when the 220 V network is directly turned on. If the TV was not disconnected from the 220 V network for a long time, for example, it was in standby mode, then naturally the demagnetization circuit will not work when turned on.

Therefore, it is recommended to periodically, at least once a week, completely turn off the TV (using the Power or simply turn off the mains power by unplugging the plug from the socket). This way we will allow the posistor to cool down.

A demagnetization circuit that uses a three-terminal posistor is also very common. Take a look.

As you can see, there is a lot in common with the scheme that we saw earlier. It works in a similar way. When you turn on the TV, a large current begins to flow through the 2nd posistor and the demagnetization coil L1. Next, the resistance of the posistor increases sharply, and the current in the circuit drops sharply.

Also, at the moment of switching on, current begins to flow (blue arrow) through the 1st posistor. At the initial moment, its resistance is high and equal to approximately 1.3 ~ 3.6 kOhm. The posistor heats up and its resistance increases. Subsequently, the weak current only warms it up, and, consequently, the second posistor, which is structurally installed next to it. Thanks to this heating, the residual current that flows through the 2nd posistor is reduced after the demagnetization loop has triggered. This eliminates “background”, weak magnetization.

It is worth noting that higher quality TVs use a circuit with a three-terminal posistor.

I also note that on more expensive and widescreen CRT TVs, the degaussing circuit turns on automatically every time it is turned on. Even if the TV was in “sleep”, the so-called standby mode.

Let's look at troubleshooting a kinescope demagnetization circuit using the example of repairing a color TV DAEWOO KR21S8 .

Initially the TV did not turn on.

After an external inspection of the electronic board and replacing the mains fuse with a new one, an attempt was made to turn on the TV. The mains fuse burned out again, indicating a short circuit in the circuits of the switching power supply.

After measuring the resistance in the electronic circuit, it turned out that a failed posistor was to blame for the short circuit. The posistor had low resistance in operating condition, as a result of which a short circuit circuit was formed, consisting of the posistor itself and the demagnetization loop coil. This led to the blowing of the mains fuse.

After disconnecting the demagnetization coil connector from the main board and reinstalling the protective fuse, the TV began to turn on and work properly.

The connector for connecting the demagnetization loop coil on the board is indicated by the inscription D/G COIL (from D e G aussing - demagnetization).

Replacing a posistor

Whether the posistor is working or not can be determined by external inspection. If you open the posistor cover, there will be two “tablets” inside (in the case of a three-terminal posistor). If both are intact, the posistor is usually in good condition. If one of the “pills” has cracks, broken pieces and burns on the surface, then in most cases the posistor is damaged.

It is also worth noting that for three-terminal posistors, one “tablet” has a resistance in the region of 18 ~ 24 Ohms. It is connected in series with the demagnetization loop. The second “tablet” is usually smaller in size, but its resistance at room temperature is 1.3 ~ 3.6 kiloOhms (i.e. 1300 ~ 3600 Ohms). This “tablet”, or rather the PTC thermistor, plays the role of a heater for the main posistor.

A two-terminal posistor has a resistance at room temperature of 18 ~ 24 ohms. It is not difficult to verify this by measuring the resistance with a conventional multimeter.

PTC resistors are marked differently, but many of them are interchangeable. Structurally, they differ little from each other.

If you don’t have the necessary posistor at hand, you can select one using this advice from TV experts.

We measure the resistance of the demagnetization loop and select a posistor with a similar resistance. For example, if the loop resistance is 18~20 Ohms, then we take a posistor with a resistance of 18 Ohms. In a three-terminal posistor, only one section is low-impedance, the one that is connected in series with the loop. It needs to be measured. The marking of many posistors indicates the resistance of the loop for which the posistor is intended. For example, the MZ73-18RM posistor is 18 ohms and is suitable for a loop with a resistance of 18 ohms.

Purely technically, a faulty posistor can simply be removed from the board, the TV will work without a demagnetization circuit, but over time the kinescope will be magnetized and multi-colored spots will appear on the screen. At first, the spots will be invisible and appear in the corners of the screen. In the future, the entire kinescope will be covered in rainbow stains.

As a rule, this is how the defect appears when the TV turns on, but there are colored spots on the screen. In this case, the posistor simply does not work, has a high resistance, or passes a small current through the coil, which causes magnetization of the kinescope.

Demagnetization of a kinescope after replacing a posistor.

If the kinescope not very magnetized , then the magnetization can be removed in a simple way.

After replacing the posistor, it is necessary to perform the procedure of turning on and off the TV several times with breaks 15 – 20 minutes. Breaks between switching on are necessary so that the posistor cooled down and its resistance decreased. If this is not done, the posistor will have a high resistance, and no current will flow through the demagnetization coil.

Usually the on/off procedure needs to be repeated 5 -7 times until the color spots completely disappear.

At strong magnetization kinescope, you should use an external demagnetization loop.

The magnetization of the picture tube in modern TVs can be easily checked using a simple operation. You need to go to menu settings TV and enable the option "Blue screen" . If this option is enabled, then when the antenna is disconnected or when the received signal is weak, the screen is filled with blue instead of ripples. After enabling the option "Blue screen" , turn off the receiving antenna. The screen should turn blue. If there are multi-colored spots on the blue background, then the screen is magnetized. The photograph shows a color TV with a faulty posistor in the demagnetization circuit. There is a red spot on most of the TV screen. It is clear that with such a malfunction, the image on the screen will be reflected unnaturally.

After replacing the faulty posistor and the demagnetization procedure that was described, there is a clear blue field on the screen. This indicates that the magnetization of the kinescope has been removed.

And finally, a couple of examples for novice radio mechanics. Application of a two-terminal and three-terminal posistor. Examples are taken from actual circuit diagrams of televisions.

DEGAUSSING COIL - this is the very coil or “loop” of demagnetization.

Serial connection of a two-terminal posistor and a demagnetization loop (Rolsen C2121, EX-1A chassis).

Switching on a three-terminal posistor in the demagnetization circuit (AIWA TV-C141).

Hi all!

Quite often, in the practice of repairing CRT TVs, there is such a malfunction as the appearance of color spots on the screen or seemingly unreasonable blowing of a protective fuse.

Colored spots , mainly, are formed in the corners of the kinescope and do not appear simultaneously, but over a certain period of time. It may seem that the manifestation of such a malfunction tells us about failure kinescope , but I hasten to reassure you, kinescope not to blame here and is fully functional. This “spotty” image indicates demagnetization or magnetization screen our TV.

If the TV was not turned off from the network for a long time, but was turned off using the remote control (it was in standby mode), then magnetization of the kinescope may occur. The fact is that in most CRT TVs, the demagnetization system starts working when the TV is turned on to the network, and if the device is constantly connected to the network, then demagnetization does not occur when the TV is turned on from the remote control.

The principle of the demagnetization system is as follows: when you turn on the “network” button on the TV, voltage begins to flow to the posistor, which, in turn, powers the kinescope demagnetization loop located on its bandage, i.e. on the back of the screen. When the TV is demagnetized, the posistor limits the power supply to the loop. And so on every time you turn on the TV. And if your device is constantly in standby mode, i.e. turns on and off only from the remote control, then the power is on posistor and the power supply is supplied continuously (this can be observed by looking at the LED on the TV panel) and the demagnetization system is constantly disabled. That is why it is recommended to disconnect the TV from the 220 V network at least once a week.

« posistor is an ordinary thermistor that changes resistance depending on temperature. In a cold state, the resistance of the posistor is very small (5 - 15 Ohms), in a heated state it is more than 10 kOhms. The posistor is connected directly to the TV's power circuit in series with the demagnetization loop. When the TV is connected to the network, the resistance of the posistor is low and current flows through it to the demagnetization loop. After heating, the posistor gives greater resistance, which prevents the passage of voltage to the loop. The design of posistors may differ, but they are all interchangeable.”

This malfunction can also appear if the posistor itself fails. If you turn your TV off and on from the network several times, and the spots do not disappear, then this indicates a failure of the posistor, which should be replaced.

Another option in which a posistor may be to blame is when the mains fuse burns out. The power supply is in good condition. In the posistor, in this case, when voltage is applied to it, a short circuit occurs and, accordingly, the entire voltage supply to the TV is short-circuited. As a result, the protective fuse blows.

Replacement posistor

Replacing a posistor is not particularly difficult, nor does it require any special knowledge.

You need to unscrew the back cover of the TV, pull out the board on which the radio components are located and find the plug for turning on the demagnetization loop. As a rule, a posistor is located directly next to this plug. The failed part must be desoldered and a new one or a known good one must be soldered in its place.

That's all, actually!

If you have any questions or have any suggestions or comments, you can put them in the comments.

Many owners of outdated TV models are wondering: how can you demagnetize a TV at home? People encounter this problem when the device starts to malfunction, showing a distorted picture. This problem is inherent in most cathode ray tube devices and is called “magnetization.”

The main reason why a kinescope becomes magnetized is the prolonged presence of electronic devices in close proximity to it. In modern homes, this situation is not uncommon: microwave ovens, computers and telephones are present in almost every apartment. It’s not uncommon for a TV to be placed on a microwave without thinking about it, huh? If your device has been damaged by regular exposure to electromagnetic field– don’t rush to take it to the workshop. You can solve the problem yourself. Depending on how strongly magnetized your TV is, you may be able to fix it using one of two options:

• activate the built-in magnetization protection;
• use a device called a throttle.

Important: never use a permanent magnet to demagnetize the picture tube.

Activating built-in protection

Each CRT TV has built-in protection against screen magnetization - the so-called demagnetization loop. To put it into action, just turn off the device for a while and let the loop do its job.

You need to know that this loop starts working only when the device is disconnected from the power supply. The thing is that when the power is turned on, voltage is supplied to the posistor continuously, and it cannot limit the supply of energy to the demagnetization loop. In this case, the demagnetization system remains inactive. Experts recommend periodically disconnecting the TV from the electrical network to prevent possible problems with the picture tube.

When choosing this option, it is worth considering that the loop cannot cope with the strong magnetization of the kinescope. In this case, you will have to resort to the second method.

Demagnetization using a choke

How to demagnetize a TV at home using a choke? First, you need to prepare the device for this process:

• disconnect the TV from the network;
• remove all electrical devices from the demagnetization site.

After the above steps, turn on the inductor to the power supply and begin circular movements in a spiral, bringing it closer to the center of the kinescope. By performing similar manipulations, move the throttle away from the TV screen to a sufficient distance and turn off the device.

Important: the entire demagnetization process should not take you more than 40-50 seconds, otherwise you can damage the TV picture tube.

Homemade choke

How to make a choke at home

You can assemble an electric magnet at home using an electric wire with a plug, an iron arc and a winding.

Take iron arc and connect it to the electrical network by connecting to a 220V wire. Isolate the resulting device and plug it into the power supply. The sequence of actions is no different from the process of demagnetization with a special choke.

If you followed the instructions in the article exactly, but your device for watching TV channels still shows a poor-quality image or, a possible reason is kinescope shadow mask shift. This malfunction cannot be repaired, and the only solution to the problem is to buy a new picture tube or TV. Now there are many models on the market, and in order not to make a mistake when purchasing, you need to know. The optimal solution would be one working on .

Sometimes a magnetized tool is useful - for example a screwdriver, the screw will not fall off. And when a file, tap, drill, or pliers are magnetized, this is not very good, rather even very bad in terms of the adhesion of metal filings and their subsequent removal. This article will discuss the topic of how you can make a demagnetizer with your own hands and using improvised means.

And so, let's go. To begin with, I will talk about demagnetizers, the components for which I managed to find in my supplies. At the end of the article I will give several more options for the design of the demagnetizer.

A degausser is essentially an electromagnet. If you apply a constant voltage to its coil, then a constant magnetic field will appear in it, and if it is alternating, then an alternating field will appear, which will demagnetize the metal.

I took the kinescope demagnetization loop:

I rolled it once:

And he folded it two:

As a result, we get a demagnetizer coil that is ready for use. But due to the small working area and strong heating, I connected another loop in series:

In order not to burn the coil or forget to turn it off, we connect the whole thing via a push-button and a fuse:

Such a coil is good for demagnetizing a large tool, but using it to demagnetize drills and taps will be inconvenient, so I made the second option - small and neat.

In this version, I used a solenoid from a reel-to-reel tape recorder connected through a transformer.

How to use degaussers:

To demagnetize, you need to apply to the coil AC voltage, corresponding to the coil, then insert the part inside the solenoid and hold it there for several seconds, then remove it, without turning off the power.

Where to get the reel:

Almost any reel will do. The main thing to remember is that the coil must match the voltage, for example, if we connect a solenoid from a tape recorder at ~220V, it will burn out, but if we connect a kinescope demagnetization device at ~12V, there will be no effect. Usually the data is written on the reel itself, and if not, then Google the name.

You can use a transformer - disassemble the core, wind up the secondary, and connect the primary to the network. The effect will be the same. There are transformers wound on a ring - such modifications do not require.

The coil is located in the electromagnetic bell, the retractor of the car starter relay. Lots of options...

You can also wind the coil yourself. Here are the data: Solenoid frame 80 mm long. The internal diameter of the frame is 30-35 mm. Along the edges of the frame there are cheeks with a diameter of 80 mm and a thickness of 5-6 mm. The solenoid winding is approximately 1000 turns of PEL or PEV wire with a diameter of 0.7-0.9 mm. The resistance of such a winding will be about 8 ohms. This coil is designed for a voltage of 10-15 volts.

Winding data for various electromagnets can be found online.

Conclusion from the above:

— We connect the coil designed for 220 volts directly to the network. A coil designed for example at 110 volts can be connected directly to the network, but only for a short time. We connect the coil designed for 12 volts through a transformer.

— We power the coil with alternating voltage

— When demagnetizing, first remove the tool from the coil, and only then turn off the power. Otherwise, the metal may not be demagnetized.

This series of articles is devoted to the repair of computer equipment that is used in office work and everyday life. The published materials are intended for engineers and radio amateurs. More details about the design of various computer components can be found in the book “Personal Computer Hardware. Self-instruction manual by Valentin Solomenchuk, which was published by the publishing house BHV-St. Petersburg in 2003. Starts in Nos. 12-20.

Monitor demagnetization system

Surprisingly, even advanced users have the opinion that only televisions have a demagnetization system, but monitors do without it. In fact, monitors are just “simplified” televisions, so they necessarily have a demagnetization loop, which regularly eliminates the effect of earthly magnetism on the iron parts of the picture tube. Accordingly, if there is a demagnetization coil and elements that control it, then malfunctions may occur - traditional for all equipment that uses color vacuum picture tubes.

In Fig. Figure 1 shows how the demagnetization loop is attached to a vacuum kinescope. Usually it is held on the neck of the picture tube by four plastic brackets and one or two springs. The demagnetization loop can be twisted as shown in Fig. 1, but this is not a rule. More often it is laid in the form of a regular loop between the monitor body and the metal band of the kinescope. In Fig. Figure 2 shows an option for laying the demagnetization loop. The demagnetization loop is made of enameled copper wire used for winding transformers. The number of turns in the loop is from 50 to 100. To protect against electrical breakdowns, the demagnetization loop is insulated with at least two layers of insulation. Usually the first layer is a cut polyethylene tube, over which electrical tape is wound. Please note that in fig. 1 also shows a grounding wire, which is designed to connect the graphite aquadag applied to the glass neck of the picture tube with the common wire of the monitor’s electrical circuit. Aquadag and the kinescope anode form a high-voltage capacitor, which smoothes out the ripple of the accelerating voltage of 24 kV. If, after repairing the monitor, the aquadag does not have electrical contact with the rest of the electronics, then electrical discharges between the aquadag and various elements of the monitor are possible in the monitor. In severe cases, for example, transistors and microcircuits may fail, not to mention the fact that a person may be injured by a high voltage discharge.

In Fig. Figure 3 shows a schematic diagram of the monitor demagnetization system, which uses a microprocessor to control service functions. To supply voltage to the demagnetization loop DGC1, relay RY1 is used. Limiting the operating time of the demagnetization system, for example, 5-10 periods of mains voltage, is done using the thermistor THP1. In simpler monitors with analog control, a relay is not installed.

When the monitor is turned on, the demagnetization loop DGC1 is connected to the 220 V circuit through the thermistor THP1, which in the initial state (cold) has a small resistance. The current passing through the demagnetization loop can reach 1-3 A. The thermistor, which is a plate of pressed semiconductor material, heats up when current passes, which leads to an increase in resistance and a decrease in the current through the demagnetization loop. When there is no relay RY1 to turn off the demagnetization circuit, a small current flowing through the thermistor THP1 keeps it hot.

Troubleshooting

The demagnetization circuit is connected immediately after the balun transformer T1. Accordingly, a short circuit in the demagnetization coil or a breakdown on the metal band of the kinescope leads to the burnout of fuse F1. In both cases, the degaussing loop DGC1 must be replaced. But it should be noted that without seriously disassembling the monitor, that is, removing the kinescope board and disconnecting all the wires leading from the kinescope to the printed circuit boards, it is impossible to replace the demagnetization loop.

If colored spots are visible on the kinescope screen, then the first thing to check is whether the thermistor THP1 is heating up. The warm body of the thermistor THP1 indicates that the fault is most likely a short circuit of part of the turns of the demagnetization coil DGC1. When the body of the thermistor THP1 is cold, you need to check the entire demagnetization circuit with a tester. The following malfunctions are most likely: the wire in the demagnetization coil has broken, the thermistor THP1 has burned out, the contacts of the RY1 relay have burned out, or there is no control signal from the microprocessor. In some cases, especially when the monitor has been previously repaired, it is necessary to check whether the demagnetization loop is connected to the connector on the printed circuit board.

When repairing the demagnetization system, the following should be taken into account: the demagnetization loop can be used from a domestic color TV, and a burnt thermistor can often be replaced with ST15-2-220V, which is used in TVs of the 3USTST type. In the latter case, only one section of the domestic analogue should be connected.

Since the monitor demagnetization system is similar to that used in color TVs, analogues from imported color TVs and VCRs can be used to replace the relay and thermistor. From the above, it is clear that repairing the demagnetization system, despite the simplicity of its electrical circuit, is most often associated with big problems when disassembling the monitor and searching for analogues to replace burnt out elements. Therefore, in many cases, you can limit yourself to only turning off the demagnetization circuit, and to eliminate colored spots on the monitor screen, periodically demagnetize the kinescope using a separate demagnetization loop. Practice shows: if the monitor is used at one workplace and is not constantly rotated, then it is enough to demagnetize the kinescope once a week or month.

To make a separate demagnetization loop, you should use a loop from a domestic TV. It is safest to use a demagnetization loop from an old tube (type ULPTsT-58.61) or thyristor (type UPIMTs-61) color TV. These TVs have better insulation in the degaussing loop than newer TV models. And this is important, since we must not forget that you have to hold the demagnetization loop in your hands. To power the demagnetization loop, you should not connect it directly to a 220 V network - this is life-threatening, and the loop gets very hot when turned on for more than 10 seconds. It is better to take a step-down transformer, for example TP-30-2, which can also be used to power a soldering iron with a voltage of 24-30 V.

When demagnetizing the kinescope, the supply voltage is supplied to the demagnetization loop only when it is no closer than 1 m from the monitor, since at a shorter distance the kinescope can be further magnetized. To demagnetize, the loop is smoothly brought to the monitor screen. It moves in a circular motion for 10-20 seconds near the screen of the switched on monitor, on which distortions in colors and line shapes will be observed. The demagnetization loop should be turned off only after it has been smoothly removed from the monitor at a distance of at least 1 m.

In conclusion, we note that in cases where the control microprocessor is faulty, replacing it is associated with significant difficulties, since you have to look for the same microcircuit, and this poses a big problem when repairing a monitor from a little-known company.