Buy a loop for demagnetizing picture tubes. Spots on the screen. We repair the kinescope demagnetization system. Conclusion from the above

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.

Hi all. An LG 21FU3RG-Z3 TV was brought in for repairs with stains on the screen.

Such malfunctions occur quite often, and repairs consist of replacing just one part, the so-called posistor.

I would like to immediately clarify one very significant nuance. Before starting repairs, be sure to ask the owner if the TV has been dropped.. It very often happens that after a fall, the kinescope mask (the fine mesh inside the kinescope) gets knocked down, which can never be restored, and all this can only be cured by replacing the kinescope. In my case, there were no falls, so it can be repaired.

So, let's begin. Having disassembled the TV, I began to look for a posistor.

This is usually located near the degaussing loop connector. To be on the safe side, I rang the loop with a tester, the result showed 5 ohms, which is within normal limits.

The posistor in this TV is used on 3 legs.

I usually buy 2-leg posistors, since it seems to me that they are universal, and I use them in all TVs.

Below is a diagram of a demagnetization loop with a 3-leg posistor.

To install a posistor on 2 legs, we connect it in series with the demagnetization loop as in the diagram, without using one terminal.

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.

Owners of old CRT TVs from time to time are faced with the fact that colored spots or stripes appear on their screens, which distort the image to a certain extent. Such defects indicate that it is necessary to demagnetize the shadow mask of the kinescope.

Reasons for magnetization

The shadow mask of a cathode ray tube (hereinafter referred to as CRT or kinescope) is a structural element of a kinescope made in the form of a raster grid. As a rule, it is made of invar (an alloy of iron and nickel), which has a small coefficient of thermal expansion.

The shadow mask in the CRT design ensures that electron beams of blue, green and red colors are projected onto the phosphor layer. While the TV is operating, the electromagnetic field created by its components (transformer, speakers, etc.) magnetizes the mask, which is why the electron beams may not converge correctly. This in turn contributes to the appearance of distortions on the screen.

To avoid this effect, picture tubes are equipped with a special device that should remove magnetization from the shadow mask. It includes a kinescope demagnetization loop and a posistor.

Reference! A posistor is a thermistor whose resistance increases when heated. They are also called PTC thermistors (PTS - Positive Temperature Coefficient, from English - positive resistance coefficient).

Defects caused by strong magnetization of the mask do not indicate failure of the kinescope and can be easily eliminated by the TV owner with his own hands. It should be remembered that the functional unit pre-installed in the television receiver, which prevents the magnetization of the mask, is activated only when the TV is turned off.

Experts divide the reasons that lead to magnetization of the CRT mask into two groups:

• defects that appear as a result of long-term use of the TV and failure of parts of the demagnetization system;
• malfunctions caused by external factors.

Preset demagnetization circuit

In the general case, the demagnetization process is carried out due to the smooth disappearance of the alternating magnetic field. In this case, the scheme this process works as follows.

1. After turning on the television receiver alternating current passes through the demagnetization loop about 10A.
2. In this loop a magnetic field arises, with the help of which the shadow mask is demagnetized.
3. The smooth disappearance of the existing magnetic field is ensured using a posistor, which instantly heats up under the influence of alternating current. It leads to increasing its resistance and decreasing the current in the loop demagnetization. As a result, the magnetic field gradually disappears, and the mask is demagnetized.
4. While the TV is operating or in “standby” mode, a heated posistor limits the current passing through the demagnetization loop. In this case, the demagnetization system does not work. Only When you disconnect the TV from the AC mains, the posistor will cool down.
5. The next time you turn on the TV, the kinescope demagnetization system will work again.

On a note! In TVs with picture tubes, the diagonal size of which does not exceed 21″, two-terminal posistors are used. In TV receivers with big screen circuits with both two posistors and a three-terminal device are used.

Such models allow you to turn on the demagnetization system automatically and reduce the background bias of the kinescope during its operation.

Self-demagnetization of the mask

In order for the kinescope to demagnetize when image distortions become visually noticeable, it is usually enough to turn off the television receiver for a few minutes (10-15). But if this does not help, you can go in two ways: the first is to change the faulty parts of the pre-installed factory circuit, the second is to remove the magnetization with a special external device. But first you should check what the real reason is defective picture: in the magnetic field of the CRT or other malfunction.

Checking the magnetization of the kinescope

Modern CRT TVs (Samsung, LG, Philips, etc.) can be checked for the presence of magnetization of the shadow mask. To do this, you need to go to the settings menu of the TV receiver and select the option “ Blue screen" However, for the screen to turn blue, you need disable external antenna. If multi-colored spots are visible on a plain background, then the kinescope mask is magnetized.

Replacing a posistor

The appearance of small color spots on the kinescope screen is most often caused by a failure of the posistor. The defect is eliminated by replacing the faulty unit. You can find out how much a part costs at your nearest radio repair shop.

Advice! If you don’t have a working posistor at hand, you can simply remove the faulty one from the board. In this case, the TV will begin to broadcast the picture without distortion, but the mask will quickly be magnetized again - first in the corners of the screen, and soon rainbow stains will appear on its entire surface. Therefore, this method can be used exclusively as a temporary measure - until a new posistor is soldered in.

After installing a working posistor, it is recommended to demagnetize the kinescope. To do this, you need to turn the TV on and off several times. In this case, the interval between cycles should be at least 15 minutes., which will allow the new unit to cool down. Moreover, if the mask is slightly magnetized, the demagnetization process will be successful and the color spots will disappear from the screen. If the stains remain, then you can demagnetize the TV at home only using an external device.

Demagnetization by an external device

To demagnetize a highly magnetized CRT mask (PC monitor or TV screen) yourself, it is best to use a purchased inductor or a homemade inductor.

Important! Only CRT TVs can be demagnetized using the above external devices. The operation of modern television receivers equipped with plasma panels or LCD displays is organized on completely different principles.

To demagnetize a kinescope using a choke, you need to perform a number of operations:

• remove all electrical devices that can affect the magnetic field of the TV (speaker systems, power supplies, etc.);
• turn on the television receiver (monitor) and let it warm up for 10 minutes;
• place the throttle parallel to the screen at a distance of 2 m from the TV and turn it on;
• slowly approach the TV receiver, making circular movements with the throttle, the diameter of which must be gradually reduced;
• approaching the minimum distance for 3-5 seconds. make several circular movements with the throttle around the perimeter of the screen;
• slowly, without stopping the circular movements, move away from the TV to the starting position;
• turn the throttle perpendicular to the screen and turn it off.

After this, rainbow stains and color spots will disappear. The entire procedure should not last more than 15-30 seconds to prevent the throttle from overheating. At the same time, when performing the above operations, you do not need to pay attention to the strong distortion of colors on the TV screen, as well as to the sounds made by the throttle (buzzing, howling, etc.).

Making a throttle yourself

If it is not possible to purchase a factory device, then you can make a demagnetization choke yourself. To do this, you need to take a mandrel with a diameter of 100-200 mm and wind 600 to 900 turns on it with PEL-2 wire. Wire diameter - 0.15-0.8 mm. Having removed the resulting winding from the mandrel, connect a power cord with a standard plug to it, and then carefully insulate it with electrical tape. And to make using the device convenient, there is a power button on the cord. You can see the process visually in the photo:

Demagnetization with other available means

You can demagnetize a kinescope mask using other available means, for example:

• a magnetic starter coil designed for an alternating current voltage of 220-380 V;
• an iron in which the soleplate is heated by a spiral;
• electric razor;
• a switched-on electric drill with a powerful neodymium magnet attached to the drill;
• powerful pulse soldering iron.

The procedure remains unchanged, and the main condition for obtaining a positive result is the magnetic field power sufficient for demagnetization.

Attention! A permanent magnet cannot be used to demagnetize a picture tube. Why this is dangerous can be seen in the video (starting from the 2nd minute)

So, demagnetizing a shadow mask is not difficult at home, but for this you need to have, if not special equipment, then at least knowledge of the peculiarities of EMF electrical engineering. But if it was not possible to remove rainbow stains and color spots from the TV screen using the methods described above, then most likely the shadow mask in the kinescope has moved. It is impossible to repair this defect; in this case, you will need to change the kinescope itself or purchase a new TV.

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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 electronic circuit It turned out that a faulty 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).