Full Form of CRT – Cathode Ray Tube
CRT stands for Cathode Ray Tube is a class of vacuum electronic devices that use a stream of electrons concentrated in the form of a single beam or beam of rays. They are controlled according to intensity (current) and position in and interact with a fixed spatial target (screen) of the device.
It is also known as Electron Beam Instruments(EBT). The main scope of EBT is the conversion of optical information into electrical signals. And the reverse conversion of the electrical signal into optical — for example, into a visible television image.
X-ray tubes, photocells, photomultiplier, gas-discharge devices and receiving-amplifying electron tubes ( beam tetrodes, vacuum indicators, lamps with secondary emission, etc.). With the beam, current form is not included in the class of electron-beam devices.
The electron beam device consists of at least three main parts:
- An electronic searchlight forms an electron beam (or a beam of rays, for example, three beams in a color picture tube) and controls its intensity (current).
- The deflecting system controls the spatial position of the beam (its deviation from the axis of the searchlight).
- The target (screen) of the receiving EBT converts the beam energy into the light flux of the visible image. The target of a transmitting or storage ELP accumulates spatial potential relief read by a scanning electron beam.
Transmitting electron beam devices convert the optical image into an electrical signal.
- Dissector (tube of instant action) – historically the first type of transmitting tube used for astronomical observations. In devices of industrial automation and for scanning documents.
- The iconoscope is historically the first type of transmitting television tube.
- Ortikon, superorthicon, vidicon – basic types of transmission tubes, used in television prior to the transition to solid-state transducers.
- Specialized devices, such as a monoscope, are a tube for conversion into an electrical signal of a single (hence the device name). The image formed inside the tube during the manufacturing process – as a rule, a test table.
Receiving electron-beam devices convert the electrical signal into an optical (visible) image:
- Oscillographic tube – ELP with electrostatic beam deflection, used to visualize the shape of electrical signals.
- The kinescope is a receiving tube of a television system with a magnetic deflection system and horizontal scanning of the image.
- Quantoscope (laser kinescope) is a kind of kinescope whose screen is a matrix of semiconductor lasers pumped by an electron beam. Quantoscopes are used in image projectors.
- Indicator cathode-ray tube – a receiving tube of a radar system with a magnetic deflecting system and a circular sweep. As well as a variety of specialized indicators, sign-generating tubes, etc.
- Sign-generating (sign-printing) tubes (character, taypotron and their analogs).
- A memory tube records information on a spatial target, stores it for a specified time. And (in reading tubes) reproduces or reads it with an electron beam. Various tubes of this subclass were used both for storing, processing and reproducing optical images. As well as the binary storage devices of early computers.
- A cadoscope is a cathode ray tube with a visible image, intended for adjusting the units of scanning. And focusing the beam in equipment using cathode ray tubes without a visible image (graphekons, monoscopic, potential scopes). The cadoscope has a pinout and reference dimensions similar to the cathode ray tube used in the apparatus. Moreover, the main CRT and cadoscope are selected by parameters with very high accuracy and are supplied only with a kit. When setting up, instead of the main tube, an cadoscope is connected.
History of CRT
The cathode ray tube, or CRT, was invented in 1897 by Carl Ferdinand Braun. He was a German but was not used until the creation of the first televisions in the late 1940s. The first television was created on January 26, 1926, by John Logie Baird. Although the CRTs used in modern monitors had many modifications that allowed them to improve the quality of the image, they still use the same basic principles.
The first version of the cathode ray tube was a diode of the cold cathode, actually a modified tube Crookes with a layer of phosphor on the front. This tube is sometimes called the Braun tube. The first version that used a hot cathode was developed by JB Johnson and HW Weinhart of the Western Electric company.
Some screens or televisions that use cathode tubes can accumulate static electricity, harmless. On the front of the tube, which can involve the accumulation of dust, which reduces the quality of the image. Cleaning is necessary (with a dry cloth or a suitable product, since some products can damage the antireflective if it exists).
- Electromagnetic fields: Although there is no proof of this, some believe that the electromagnetic fields emitted during the operation of the cathode tube can have biological effects. The intensity of this field is reduced to irrelevant values one meter away. And in any case, the effect is more intense on the sides of the screen than in front of it.
- Risk of implosion: When too much pressure is exerted on the tube or it is struck, an implosion due to the internal vacuum may occur. The explosions that are sometimes seen in movies and television are not possible. In the tubes of modern televisions and monitors, the front is much thicker. Several layers of glass and plastic sheets are added so that it can withstand collisions and implodes. The rest of the tube and in particular the neck are very delicate. In other tubes, such as oscilloscopes, there is no screen reinforcement, instead, a plastic sheet is used as protection. The cathode tube has to be handled with attention and competence. In particular, it must be avoided by lifting it by the neck and always holding it by the points indicated by the manufacturer.
- Toxicity: In older pipes and in some modern ones, toxic substances were used in their manufacture such as cadmium, phosphorus, barium, etc. Currently, they have since replaced by safer. The implosion or in any case the breakage of the glass causes the dispersion of these materials. In the elimination and recycling of the tubes, it is also necessary to take into account the presence of lead in the glass, which is very polluting.
- Blinking: The effect of flickering is not exclusive to vacuum tubes. It is also observed in flat screens although in these it is common to find systems to reduce it. The conventional TV signal consists of 25 images per second in the PAL system and 30 images in the NTSC system. With interlacing, it is possible to reduce the flicker by dividing each image in two. One with the even lines and one with the odd lines shown one after the other increasing the frequency to 50/60 Hz. This continuous flicker is what causes dizziness and visual discomfort when we watch television for too long. In some sensitive people, it can even trigger epileptic seizures. Some models of televisions solve this problem by storing the signal in a memory. And repeating each complete image without interlacing several times. The most widespread system in PAL is the 100 Hz that repeats each image 4 times and significantly reduces the flicker. The primitive systems of 100 Hz announced an increase in quality. But when using primitive analogical/digital converters with little sampling and quantification the image quality was noticeably smaller. The digitization method tried to use as little memory as possible since memory was very expensive by then. The cheapening of integrated memory circuits and the advancement of electronics in general. They have achieved that in the market we can find 200 Hz screens that make the flicker imperceptible maintaining the quality of the signal.
- High voltage: To direct the beam in the cathode ray tubes, very high voltages (tens of thousands of volts) are used. These voltages can remain in the device for a period of time after switching it off and disconnecting it from the mains. Therefore, you should avoid opening the monitor or television if you do not have adequate technical training.
- X Rays: The interaction between the electrons bouncing on the fluorescent screen of the CRT produces, well adjusted, small doses of X-rays. If the anode voltage exceeds the maximum recommended by the manufacturer of the CRT. The dose will be higher than the dose allowed by law.
Tube failures and their associated components
Fundamental problems that originate:
The aging or exhaustion of the tube will cause a loss of contrast and definition. CRT rejuvenators can be used, which can prolong (for a short time) the almost useful life of the CRT. Another way is to increase the supply voltage of the filaments to achieve more emission of the cathodes, which only accelerates the aging process.
Due to movements while the TV is running, some of the three strands are usually cut, with the consequent variation of the colors represented on the screen. There are several techniques to recover the tube, achieve the contact of the cut filament.
In cases of falls or blows, the Ampoule seems intact, but microcracks cause the entry of air into the unit which is proven in several ways:
- When the TV is energized, electric arcs of a violet color are produced inside what we call the neck of the tube. This sometimes, in some TV, causes the overload produced, stop the source, turning off the TV. Another way to detect if the TRC has entered the air or is gaseous is to connect only the terminal of the anode. And one of the cables of the tester or multimeter, place one end of the latter to amass potential. And with the other Approach, do not touch, just approach, to the base of the neck and observe the high voltage arcs that will jump to the approximate point.
- Televisions, computer monitors and video monitors.
- The oscilloscopes, spectroscopes and other measuring instruments.
- The radars.
The cathode ray tube is composed mainly of two parts:
- Electron gun: is responsible for emitting the electron beam, necessary for the operation of the tube.
- Screen coated with phosphor: the coating of the when in contact with the electrons, causes a light reaction.
The electron gun has two main components: an anode and a cathode. Both are electrodes, the first is positively charged and the second negatively. When the cathode is heated, it emits radiation towards the anode. Because this beam of electrons follows after passing the anode, to be after it with the screen coated with phosphorus, causes a light reaction.
In order to display color images on the screen, it will be covered with thousands of pixels formed by three different materials. Each one, reacting with the electron beam, will generate one of the three colors on which the color monitoring systems are based: green, blue and red. In order to activate each color, the barrel has three electron beams. So that they provoke the reaction in one point of each color at a time, thus generating the desired color.
Based on this, we see that the tube can only illuminate one pixel at a time, so to generate the image. The electron beam sweeps the entire screen starting with the upper left corner until completing that line and moving to The next. For us to perceive the moving image and not notice the sweep. The barrel has to perform the full sweep of the screen in less time than the retinal persistence of the human eye. So, for our eye, the entire image is generated at the same moment.
Most frequent failures
There is no image, a single primary color predominates (Red, Green or Blue ), and fine diagonal lines are observed that are repeated every few centimeters. There are very different possibilities of the origin of this fault:
- One of the final color transistors (the color that we see on the screen) is defective or no longer receives voltage (approximately 180 Volts in the collector).
- The cathode of that color has been short-circuited with the filament. In this case, a winding of approximately 3 to 4 turns must be carried out in the Fly-back core. And, prior to cutting the printing tracks feeding the filament of the tube, feed the latter with the winding carried out. In this way, the GND potential is isolated from the filament, is the same as the one taking the cathode. No matter what it since at its ends there will be about 6 volts generated by the winding we have made.
- A very problematic component in the RGB is the Electrolytic Capacitor between 1 uF and 10 uF that filters the voltage of 180 Volts that is needed in this sector. The color drips to the right, the image leaves a wake as if navigating to the screen from the right. And when you have doubts, the first thing to do is replace it. Moreover, since there is a temperature in this zone due to the collector resistance of the RGB amplifier transistors. The latter’s sheath shrinks sharply, revealing that it can be dry.
- Another fault is a severe deterioration in the focus of the image, which often leads to thinking about the potentiometer, which is responsible for regulating this tension. In the TVs that bring the Focus and Screen controls integrated into the same Fly-back. It is very rare that this control deteriorates, there will be no other option but to replace the whole unit. On older TVs, it was more common to find deteriorated Focus potentiometers. But there is a fault that is usually presented very hidden and is the socket connecting to the crotch of the TRC. Socket contacts often become hygroscopic, which is sometimes only seen as a greenish sulfate. This is very frequent to happen, so we must control it every time we see blurring in the image.
- When there is a predominance of a certain color over others or lack of color. First, try to establish that the three filaments are lit, then those that have an oscilloscope control that the three color signals reach the RGB amplifiers, and those that do not. Have that instrument control the tensions at various points of the amplifiers, which are similar in all three. If everything is correct and the defect continues, the emissions of the three guns must be regulated until they are balanced.
Only the most vivid colors corresponding to the image on a generally dark background are observed.
There are designs in which the RGB amplifiers come on the one hand the signals of color difference (RY, BY, GY) and on the other side the luminance signal Y. Within the amplifiers, there is a simple algebraic sum that results from the colors to attack the color cannons. But when the transistor that acts as a buffer for the luminance deteriorates we find the mentioned phenomenon.
Observations and Measurements
In cases of total obscuration of the image and the presence of sound, it never hurts to have a visual inspection to verify that the filaments are on. There may be an intermittent failure of momentary darkening, which is usually due to poor welds in the feeding of the same.
Also visually check the connection of the drivers that come from the Fly-back, which are, Grid Tension 2 or G2 and Focus voltage.
Once the visual check has been made, check the voltage of Grid 2, which should oscillate between 300 Volts. And 500 Volts, depending on the CRT model used by the TV. A mismatch in excess in this tension can cause a very strong brightness with loss of contrast and appearance of fine diagonal lines every few centimeters. A mismatch will cause a lack of brightness very remarkable. Despite placing the main control of brightness to the maximum.
The practical and simple method to adjust the emission of the RGB guns
- Place the TV in Service mode, with the key that everyone usually has and eliminate the raster leaving a bright horizontal line.
- Lower the tension of G2 or Screen (if necessary) with the corresponding potentiometer that is in the Fly-back, until the point where the line disappears. Well to the limit, but not appear.
- Begin to regulate the preset of the colors, which in the serigraphy appear as Bias R, Bias B, and Bias G, as follows: Move forward until the line of color that we are activating appears. And when this happens we go back a little bit, until the limit disappears, do not go back too far, only to the limit.
- Do the same for the two remaining cannons.
- We pass the key to normal mode.
- Readjust if necessary the voltage of G2.
- Place the Color or Saturation control to a minimum, where we have an image in White and Black
- If we do not observe an exact Black and White image, that is, some color tonality remains. We will tweak the presets of Drive G and B (they are the two remaining presets in the adjacencies) until we obtain a perfect monochrome vision.
Cathode tubes (CRT), which have been very popular in much of the twentieth century, are practically in disuse since the late 2000s (first in computer monitors, and then in televisions). Since little by little Flat screens (LCD) replace the cathode tube screens.
These new types of LCD screens have many advantages, such as A comparatively smaller size. Possible dimensions are larger or smaller. Most extreme high / wide relationships. Concave, flat or convex screen shapes, there are even flexible screens. Lower health risks, do not leave trails on the screen in case of rapid change of images (only LCD and LED screens) and lower energy consumption.
They also have some disadvantages, such as a black color shown very clear (liquid crystal (LCD), since they use a backlight). In certain cases, a high response time. On non-concave screens, colors are not displayed uniformly (unless a filter is used to darken the center of the screen). Since in the CRTs the width of the diagonally seen glass is compensated with electronic circuits that change the illumination across the width and height of the screen.