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11-14 The effect of magnetization on the screen and the demagnetization file: (Degauosing)

The magnetic fields close to the television set, whether from inside the device or from an external source - such as iron blocks, induction coils, motors, as well as geomagnetism, may cause the deflection of the electronic beam heading from the projector towards the screen. It leads to a weakening of the purity of the color and the appearance of dull color spots. A magnetic barrier of iron or steel is usually installed around the cone of the screen valve to reduce the effect of magnetic fields on the beam, and a demagnetization coil is placed around it, and it is fed from a circuit in the device called the demagnetization circuit (Fig. 11-48). When the device is turned on, a large current (6) reaches the coil, and this current decreases within 5 seconds until its intensity becomes about (2 milliamps). This task is carried out by a special resistance whose resistance value changes with the change of voltage and it is called (VDR) with another resistance whose resistance value changes with the change of temperature and it is called (PTC).

Automatic degaussing circuit:

In this circuit, Figure (11-49), the two resistors (X2X1), which have a positive thermal factor (PTC), are small in resistance when the device is turned on (their temperature is normal). A current of great strength passes through them, and a strong magnetic field is generated in the coil. The coil current passes through the voltage-related resistance (VDR), which is also a small resistance, as its resistance is inversely proportional to the voltage applied to it.

With the passing of the magnetizing current, the temperature of the two thermistors rises and its resistance increases, which weakens the current strength, and thus the voltage applied to the VDR decreases and its resistance increases.
also .

As a result, the current of the magnetizing coil decreases a lot until it reaches about 20 milliamps within five seconds, and it requires the return of the resistors to their first state about
(1 minute). The demagnetization file removes the effects of magnetic fields when changing the location of the device or the approach of iron blocks or devices that generate magnetic fields. Note: We may see a changing color spot on the screen as a result of the magnetization of the screen, and this spot may not disappear when the device is turned off and then turned on directly. Therefore, it is necessary to turn off the device and wait for a period of not less than 15 minutes before restarting it, so that this magnetization is effectively removed by the effect of the internal coil.

Degaussing with an external coil:

In rare cases, it may require removing the magnetization with an external coil, which is a coil with a diameter of about (30 cm) that is fed with alternating current (220) volts and generates a strong magnetic field. This coil is connected to a long wire by means of a cutter. As for the inner wire of the coil, it is a wire insulated with varnish, such as winding wires for motors and transformers, with a diameter of 4-45 dzim and the number of its turns not less than (600-650 turns).

How to demagnetize the external coil:

​​​​​​The TV is placed in the place designated for it. The file is connected to a general AC (220) volt outlet, the circuit breaker is closed, then the file is placed parallel to the surface of the screen, and it is moved in a circular motion up and down to wipe all the screen interface. After about 30-60 seconds, we slowly remove the coil from the device, to a distance of no less than 2 meters, and put it on the ground, then cut off the current. This operation can be performed while the TV set is on or without it, and it is forbidden to cut off the current while the coil is near the screen, because this magnetizes the screen more and distorts its colors. Remove the coil and finish the degaussing process
15- Dangers of electrocution with the voltage of the screen valve:

​​​​​​The screen valve elevator is fed with high voltage, which in some devices may reach (33 kV) through a well-insulated shielded cable that connects the high voltage unit to an opening in the back wall of the screen. A capacitor with a capacity of 1500 pF is formed between the conductive layer inside the screen and the conductive layer outside the screen, and the insulator is between these two balanced conducting surfaces, and this capacitor is charged with high voltage and retains this charge for a long time after the device is turned off. Especially in places of high voltage and the screen, and be careful of the possibility of dangerous electric shock. Therefore, it is preferable to discharge the electric charge in the screen with the grounded metal in the device.

Screen Burst Dangers:

Exposing the screen to shock or stress may cause the screen to explode and its glass to scatter dangerously. It is worth noting that this screen is very large, making it subject to an external pressure of about (8100 kg), so the screen must be carried, moved, dismantled and installed with extreme caution, and it is preferable to wear protective glasses and gloves while disassembling and installing the screen. It is recommended that they be placed and dealt with in closed containers with an appropriate warning attached to them when storing or transporting them. Several methods have been adopted to reduce the risk of screen explosion, namely:

1- Strengthening the glass surface of the screen, especially that which the viewer can see


2- Surrounding it from the side with a taut metal belt.

3- Filling the space between the glass and the belt with special solid and strong materials that prevent the release of glass fragments if subjected to stress or breakage. This is called the armored or bonded screen.

Figure (850-11) and (B-50-11) show these methods used for protection and prevention in the screen.