Television sets that have picture tubes in their design have long been replaced by plasma and liquid crystal devices. However, there are people in whose homes you can still see these devices. Due to their long service life, they often fail, therefore, despite the development of technology, the repair of CRT TVs is still a popular service.
Content
- CRT device
- Principle of operation
- Possible problems
- TV repair services
CRT device
The role of the main part in an old-style television receiver is played by a cathode ray tube (CRT), called a kinescope. Its principle of operation is based on electronic emission. The mechanism of such a tube includes:
- electronic guns;
- focusing and deflecting coils;
- anode outlet;
- a shadow mask for separating color images;
- phosphor layer with different glow zones.
A picture tube made of glass is coated with a discrete phosphor inside. The coating consists of triads - a collection of three points, each of which corresponds to red, blue and green.
A point in the triad takes on a beam emanating from a particular electron gun and begins to emit light of varying intensities. To achieve the required shade, special metal gratings of the shadow, slot or aperture type are built into the tube design.
Principle of operation
For the image to appear on the TV screen, the beam emitted by the electron gun must successively touch all points in the direction from left to right and from top to bottom, causing them to glow. The speed of the beam propagation across the screen should be up to 75 times per secondotherwise the dots will fade out. If the speed is reduced to 25 times per second, it will cause the picture to flicker.
In order for the rays that have touched the phosphor coating to be reflected from it, a system consisting of four coils is attached to the neck of the picture tube. The magnetic field created on them contributes to the reflection of the rays in the desired direction. Individual luminous points are added into a single image under the action of control signals. A specific sweep is responsible for each direction of the beam:
- the lowercase provides a straight horizontal stroke;
- personnel is responsible for vertical movement.
In addition to straight paths, there are zigzag (from the upper left to the lower right corner of the monitor) and reverse moves. Signals with off brightness are responsible for movement in the opposite direction.
The main technical characteristic of a CRT is the frame rate, measured in hertz. The higher it is, the more stable the image will be. The vertical frequency multiplied by the number of lines displayed in one frame determines the line frequency parameter in kilohertz. Depending on the image formatting method (interlaced or interlaced), even and odd lines can appear in turn or immediately during one frame scan period.
Another important parameter is size of phosphor dots. It affects the clarity of the displayed image. The smaller the dots, the better. For the picture on the screen to be of high quality, the distance between them should be 0.26-0.28 mm.
In black and white televisions, the CRT screen is completely covered with a phosphor that emits only white light. An electronic searchlight, fixed in the neck of the tube, forms a thin beam that scans the screen line by line and contributes to the glow of the phosphor. The intensity of this glow is controlled by the strength of the video signal, which contains all the information about the image.
Possible problems
During the operation of the CRT TV, various problems may occur. The reason for their occurrence lies in the breakdown of parts of the electron-beam mechanism.
Failure of the power supply unit will result in the device not turning on. To check its operability, you must first turn off the line scan cascade, which acts as a load, then solder a household lamp into the circuit. The lack of light in the lamp indicates that the power supply is defective.
Identification of problems in line scanning is carried out using the same lamp. Its constant glow signals a malfunction of the output transistor. In a normal state, the lamp should flash and go out.
With a luminous horizontal stripe, you should pay attention to the scan of the frames. To restore its work, you will need to lower the brightness level, thereby protecting the phosphor layer. Additionally, you need to check the serviceability of the master oscillator and the output stage. It should be borne in mind that their operating voltage is in the range of 24-28 volts.
The complete absence of glow can most often be caused by problems with the power supply of the kinescope. During the diagnostic process, you will need to check the filament and the voltage level on it. If the integrity of the thread is not compromised, then the way out is winding the winding. In this case, replacing the transformer is not required.
If there is a problem with the color block and video amplifier, the sound disappears. The opposite situation, when there is no picture in the presence of sound, means there is a problem in the low-frequency amplifier. If the picture disappears along with the sound, then the reason should be looked for in a malfunctioning radio channelstarting the video processor and tuner.
TV repair services
To troubleshoot a television receiver on your own, you must have the appropriate knowledge about the structure and operation of the kinescope. If there is no such knowledge, it is best to contact qualified specialists. Finding a company that repairs CRT TVs is not difficult.
Most of these firms provide customers with a convenient repair method (in a workshop or at home) and free diagnostics. Experienced technicians quickly diagnose the problem and fix it using quality parts recommended by TV manufacturers and modern equipment. All work carried out is guaranteed. All problems arising during the warranty period are eliminated free of charge
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