External demagnetization loop. Ways to self-demagnetize the TV

Hello everyone. An LG 21FU3RG-Z3 TV with spots on the screen was brought in for repair.

Such malfunctions are quite common, and the repair consists in replacing only one part, the so-called posistor.

I would like to stipulate at once one very significant nuance. Before starting the repair, be sure to ask the owner if the TV has fallen... It often happens that after a fall, the picture tube mask (a small mesh inside the picture tube) gets lost, which can never be restored, and all this is treated only by replacing the picture tube. In my case, there were no falls, so it can be repaired.

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

It is usually located next to the demagnetization loop connector. For reliability, I rang the loop with a tester, the result showed 5 ohms, which is within normal limits.

The posistor in this TV is used for 3 feet.

I usually buy 2-foot posistors, as 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, while not using one terminal.

Even an inveterate color-blind person can notice that greenish blurry spots have appeared along the edges of the CRT of his favorite box. As the candidate of technical sciences, associate professor of the Institute of Electronic Control Machines Vladimir Rodionov explained to us, there are only two reasons for this mysterious phenomenon: the magnetization of the kinescope or the shift of the mask * (note by the telemaster Funtik: “The mask of the kinescope is, brother, such a plate with vertical slots, which carries out color separation To move it, you need to give a good kick with your foot on the apparatus, it doesn't matter - on the tube or on the body. If you hit it successfully, the colors will float "). Magnetization can be easily treated, but shifts cannot. Usually the screen turns green due to the proximity to sources of magnetic radiation: speakers, tape recorder ... Any devices with an unshielded transformer or an electric motor are dangerous. Even if you are used to using an electric shaver, looking instead of a mirror in a TV set, this can lead to magnetization. Moreover, Vladimir assures that a slight magnetization of the kinescope is also possible if the TV is moved from the bathroom to the toilet, since when the location is changed, the effect on the kinescope of the Earth's magnetic field also changes.

The first way

To eliminate low magnetization in all modern boxes, an internal demagnetization loop is provided, which is taken to work after pressing the "on" button. The loop will "come to life" only after you completely de-energize the TV set, and then plug it back into the network. It is advisable that the TV remained without 220 volts for several minutes (and for some models - hours). This is the simplest and least interesting method of kinescope treatment. But there is another.

Another way

“If the built-in loop fails, you will need a demagnetizing choke,” says Vladimir. The choke can be borrowed from a friend who is a radio amateur or politely asked for a while in any TV workshop, waving a pistol.

Remove from the screen all devices that can disturb the magnetic field, insert the choke into the outlet and bring it to the CRT, holding it perpendicularly. Now turn the choke so that the plane of the turns is parallel to the glass, and for 3-4 seconds make circular movements (while dancing is allowed) near the screen as close as possible, grabbing the edges. Be prepared for the throttle to howl and colorful devils running after it on the screen. Then again turn the throttle perpendicular to the screen, gently move it away from the TV for three meters and only then turn it off. (“If these subtleties are not followed, the magnetic field will be disturbed again,” warns Vladimir.)

// published on 5/19/2003, # 21 (206)

This article series focuses on repairing computer technology, which is used in office work and everyday life. The published materials are designed for engineers and radio amateurs. More details about the design of various computer nodes can be found in the book “Hardware of personal computers. Self-study guide ”by Valentin Solomenchuk, which was published by the publishing house“ BHV-Saint Petersburg ”in 2003. Beginning in №№ 12-20.

Monitor demagnetization system

Surprisingly enough, even advanced users are of the opinion that only TVs have a demagnetizing system, but monitors do without it. In fact, monitors are just "simplified" TVs, so they necessarily have a demagnetization loop, which regularly eliminates the effect of terrestrial magnetism on the iron parts of the CRT. Accordingly, if there is a demagnetizing coil and the elements that control it, then malfunctions may appear - traditional for all equipment in which colored vacuum tubes are used.

In fig. 1 shows how the demagnetization loop is attached to the vacuum tube. Usually it is held on the neck of the tube by means of four plastic brackets and one or two springs. The demagnetizing 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 case and the metal tube of the picture tube. In fig. 2 shows a variant of laying the demagnetization loop. The demagnetizing 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. Typically, the first layer is a cut polyethylene tube over which electrical tape is wound. Note that in fig. 1 also shows a ground wire, which is designed to connect the graphite aquadag applied to the glass throat of the picture tube with a common wire. electrical circuit monitor. Aquadag and the picture tube anode form a high-voltage capacitor, which smooths out the pulsation of the accelerating voltage of 24 kV. If, after the repair of 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 that a person may be damaged by a high voltage discharge.

In fig. 3 shows a schematic diagram of a monitor demagnetization system that uses a microprocessor to control service functions. The RY1 relay is used to supply voltage to the DGC1 demagnetization loop. The limitation of the operating time of the demagnetization system, for example, 5-10 periods of the mains voltage, is carried out using the THP1 thermistor. In simpler monitors with analogue control, the relay is not installed.

At the moment the monitor is turned on, the DGC1 demagnetization loop is connected to the 220 V circuit through the THP1 thermistor, which in the initial state (cold) has a small resistance. The current passing through the demagnetization loop can reach 1-3 A. A 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 current through the demagnetization loop. When there is no RY1 relay that turns off the demagnetizing circuit, a small current flowing through the THP1 thermistor keeps it warm.

Troubleshooting

The demagnetizing circuit is connected immediately after the balun transformer T1. Accordingly, a short circuit in the demagnetizing coil or a breakdown on the metal band of the kinescope leads to burnout of the fuse F1. In both cases, the DGC1 demagnetization loop must be replaced. But it should be noted that without serious disassembly of the monitor, that is, removing the kinescope board and disconnecting all the wires leading from the kinescope to printed circuit boards, it is not possible to replace the demagnetization loop.

If colored spots are visible on the CRT screen, then the first thing to check is whether the THP1 thermistor heats up. The warm body of the THP1 thermistor indicates that the malfunction is most likely in a short circuit of part of the turns of the DGC1 demagnetizing coil. With a cold body of the THP1 thermistor, it is necessary to check the entire demagnetization circuit with a tester. The following malfunctions are most likely: there was a wire break in the demagnetizing coil, the THP1 thermistor burned out, the contacts of the RY1 relay are burnt, 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 demagnetizing 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-out thermistor can quite often be replaced with ST15-2-220V, which is used in TVs of the 3USCT 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, analogs from imported color TVs and VCRs can be used to replace the relay and thermistor. From the foregoing, it is clear that the repair of the demagnetization system, despite the simplicity of its electrical circuit, is most often associated with big problems when disassembling the monitor and searching for analogs to replace burnt elements. Therefore, in many cases, you can limit yourself only to turning off the demagnetizing circuit, and to eliminate color spots on the monitor screen, periodically demagnetize the kinescope using a separate demagnetizing loop. Practice shows: if the monitor is operated at one workplace, it 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 lamp (type ULPCT-58.61) or thyristor (type UPIMTs-61) color TV. These TVs have better insulation for the demagnetization loop than newer TVs. And this is important, because one must not forget that the demagnetizing loop has to be held in hand. To power the demagnetizing loop, do not connect it directly to the 220 V network - it is life-threatening, and the loop gets very hot when it is 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 to the demagnetizing loop is supplied only when it is at least 1 m from the monitor, since at a shorter distance, you can additionally magnetize the kinescope. For demagnetization, the loop is gently brought to the monitor screen. In circular motions, it moves for 10-20 seconds around the screen of a switched-on monitor, on which distortions of colors and line shapes will be observed. The demagnetization loop should be turned off only after it is 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, its replacement is fraught with significant difficulties, since you have to look for the same microcircuit, and this is a big problem when repairing a monitor from a little-known company.

Sometimes a magnetized tool is useful - for example a screwdriver, the screw will not come off from it. And when the file, tap, drill, pliers are magnetized - this is not very good, rather even very bad in terms of 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 from improvised means.

And so, let's go. To begin with, I will tell you about the demagnetizers, the components for which I managed to find in my stocks. At the end of the article, I will give a few more options for the demagnetizer.

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

I took the CRT demagnetization loop:

I folded it once:

And folded it two:

As a result, we get a demagnetizer coil, which is already ready for work. But due to the small working area and strong heating, I connected one more loop in series:

In order not to burn the coil or forget to turn it off, we connect the whole thing through a button without latching 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 baboon tape recorder, connected through a transformer.

How to use demagnetizers:

To demagnetize, you must feed to the coil AC voltage corresponding to the coil, then insert the part inside the solenoid and hold it there for a few seconds, then remove, without turning off the power.

Where to get the reel:

Almost any coil will do. The main thing to remember is that the coil must correspond to the voltage, for example, if we connect the solenoid from the tape recorder at ~ 220V, it will burn, and if we connect the device for demagnetizing the kinescope at ~ 12V, then there will be no effect. Usually the data is written on the coil itself, and if not, then we 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 included in the electromagnetic chime, the solenoid relay of the car starter. There are a lot of options ...

You can also wind the coil yourself. Here is the data: Solenoid frame 80 mm long. The inner diameter of the frame is 30-35 mm. Along the edges of the frame, cheeks with a diameter of 80 mm and a thickness of 5-6 mm. Solenoid winding - about 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. Such a coil is intended for a voltage of 10-15 volts.

The winding data of various electromagnets can be found on the net.

Conclusion from the above:

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

- Powering the coil alternating voltage

- When demagnetizing, first remove the instrument from the coil, and only then turn off the power. Otherwise, the metal may not demagnetize.

From time to time, owners of old CRT TVs are faced with the fact that colored spots or stripes appear on their screens, which to some extent distort the image. Such defects indicate that it is necessary to demagnetize the shadow mask of the picture tube.

Reasons for magnetization

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

The shadow mask in the CRT design ensures the projection of blue, green and red electron beams onto the phosphor layer. During operation of the TV, the electromagnetic field created by its nodes (transformer, speakers, etc.) magnetizes the mask, which can cause the electron beams to converge incorrectly. This in turn contributes to the appearance of distortion on the screen.

To avoid this effect, picture tubes are equipped with a special device that must remove the 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 (PTC - Positive Temperature Coefficient, from English - positive coefficient of resistance).

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

The reasons that lead to the magnetization of the CRT mask, experts divide into two groups:

defects resulting from prolonged operation of the TV and the failure of parts of the demagnetization system;
malfunctions caused by external factors.

Preset demagnetization circuit

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

After turning on the TV receiver an alternating current flows through the demagnetization loop value of the order of 10A.
In this loop a magnetic field appears, with the help of which the shadow mask is demagnetized.
The smooth disappearance of the existing magnetic field is ensured by means of a posistor, which, under the action of alternating current heats up instantly. This leads to an increase in its resistance and a decrease in the current strength in the loop demagnetization. As a result, the magnetic field disappears smoothly and the mask is demagnetized.
While the TV is working or is in "standby" mode, the heated PTC resistor limits the current passing through the demagnetization loop. In this case, the demagnetization system does not work. Only when the TV is disconnected from the AC power supply, the PTC thermistor will cool down.
The next time you turn on the TV, the kinescope demagnetization system will work again.

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

Such models allow you to include a demagnetization system in automatic mode 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 TV 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 it follows check what is the real reason defective picture: in the magnetic field of the CRT or other malfunction.

Checking the magnetization of the CRT

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

Replacing the posistor

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

Advice! If there is no good posistor at hand, then you can simply drop the faulty one out of the board. In this case, the TV will begin to broadcast the picture without distortion, but the mask will quickly magnetize again - first in the corners of the screen, and soon rainbow stains will appear on its entire surface. That's why this way can be used exclusively as a temporary measure - until a new posistor is soldered.

After installing a working posistor, it is recommended to demagnetize the kinescope. To do this, the TV must be turned on and off several times. In this case, the interval between cycles should be not less than 15 minutes., which will allow the new unit to cool down. Moreover, if the mask is slightly magnetized, then the demagnetization process will be successful and the color spots from the screen will disappear. If the stains remain, then you can demagnetize the TV at home only with the help of an external device.

Demagnetization with an external device

To demagnetize a highly magnetized CRT mask (PC monitor or TV screen) yourself, it is best to use a commercially available choke 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 the kinescope using a choke, a number of operations must be performed:

remove all electrical devices that can affect the magnetic field of the TV (speaker systems, power supplies, etc.);
turn on the TV receiver (monitor) and let it warm up for 10 minutes;
place the choke 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;
coming up to the minimum distance, within 3-5 seconds. make several circular movements with the throttle around the perimeter of the screen;
slowly, without stopping circular movements, move away from the TV to the starting position;
turn the throttle perpendicular to the screen and turn it off.

After that, iridescent streaks and color spots will disappear. The whole procedure should not last more than 15-30 seconds in order to prevent strong heating of the throttle... At the same time, 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 choke with your own hands

If it is not possible to purchase a factory device, then a demagnetization choke can be made by hand. To do this, you need to take a mandrel with a diameter of 100-200 mm and wind on it from 600 to 900 turns with a PEL-2 wire. Wire diameter - 0.15-0.8 mm. Having removed the resulting winding from the mandrel, a power cord with a standard plug is connected to it, and then carefully insulated with electrical tape. And to use the device conveniently, a power button is located on the cord. Visually, the process can be seen in the photo:

Demagnetization by other means at hand

You can demagnetize the picture tube mask using other available means, for example:

a coil of a magnetic starter designed for an alternating current voltage of 220-380 V;
an iron in which the sole is heated by a spiral;
electric shaver;
the included electric drill with a powerful nodymium magnet fixed on the drill;
powerful pulse soldering iron.

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

Attention! A permanent magnet cannot be used to demagnetize the CRT. You can see how dangerous it is 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 features of EMF electrical engineering. But if it was not possible to remove rainbow stains and color spots from the TV screen using the above methods, then, most likely, the shadow mask in the CRT has moved. It is impossible to repair this defect; in this case, you will need to change the picture tube directly or purchase a new TV.