According to ABBREVIATIONFINDER.ORG, CRT stands for Cathode ray tube. They are glass vacuum tubes inside which an electron gun emits a current of electrons guided by an electric field towards a screen covered with small phosphorescent elements.In the cathode ray tube, an electron gun produces and confines a beam of electrons that sends towards a screen covered with luminescent material, so that when electrons collide with it, it emits light whose intensity or brightness is proportional to the quantity and speed of the incident electrons. In other words, the kinetic energy of the electron beam is transferred to the screen material and converted into light energy. Between the electron gun and the screen there is a deflector system made up of coils placed outside the tube, to deflect the electron beam horizontally and vertically. Unlike the magnetic deflection system used in television, oscilloscopes employ electrostatic deflection, deflecting the electron beam through horizontal and vertical squares placed inside the tube. The brightness can be varied, if the density of the electron beam is varied by means of a control grating, whose action is similar to that of a triode or other valves such as the tetrode or the pentode.
The cathode ray tube, or CRT, was developed by Karl Ferdinand Braun, a German scientist, in 1897 but was not used until the creation of the first televisions in the late 1940s.. Although the CRTs used in modern monitors underwent many modifications that allowed them to improve image quality, they still use the same basic principles. The first version of the cathode tube was a cold cathode diode, actually a modification of the Crookes tube with a phosphor coating on the front. This tube is sometimes called a Braun tube. The first version to use a hot cathode was developed by JB Johnson and HW Weinhart of Western Electric. This product was marketed in 1922.
The TRC is divided into neck, hood, screen.
In the neck we find the filament as the first component, which is responsible for heating the cathode, which is the second component, after this the control grid (G1), the screen grid (G2) and finally the grid of focus (G3) commonly called focus, all of these have the specific function of accelerating the electron beam so that it hits the surface of the phosphor with which the point is illuminated on the CRT screen.
Inside the hood is the anode, which attracts the electrons generated in the cathode and makes them crash at high speed against the screen that is covered with phosphor, which when hit by the electrons generates a bright light, same that will form the images. On the outside in the area of the hood it is covered with a black paint called ACUADAC which is the hub of the TRC in this way a capacitor is formed between the hub (acuadac) and the internal part of the tube, the glass acts as a dielectric. Between the neck and the bell on the outer side are the deflection coils (yoke), it has the function of deflecting the electrons producing a sweep over the entire surface of the screen.
The shadow mask is located on the screen, which is responsible for ensuring that each color cannon (RGB) excites only the corresponding color points. This can be of the conventional type or Wega Trinitron type used by Sony televisions Finally, the phosphor of red, green, blue colors is located, which will serve to form the color images going from black to white simply with the combination of the three primary colors.
The electron gun is made up of a cathode, a negatively charged metal electrode, and one or more anodes (positively charged electrodes). The cathode emits the electrons attracted to the anode. The anode acts as an accelerator and concentrator for the electrons, creating a stream of electrons directed at the screen. A magnetic field guides the electrons from right to left and from top to bottom. It is created with two electrified plates X and Y (called baffles) that send current in a horizontal and vertical direction, respectively.
This screen is covered with a thin layer of phosphorescent elements, called phosphors, which emit light by excitation, that is, when electrons hit them, thus creating an illuminated point called a Pixel. The activation of the magnetic field causes the electrons to follow a scanning pattern, going from left to right and then moving down to the next row once they have reached the end.
The human eye is unable to visualize this scan due to persistence of vision. Try waving your hand in front of your screen to check this phenomenon: You will see multiple hands at once! Combined with the firing or cessation of the electron gun, the scan tricks the eyes into believing that only a few pixels on the screen are illuminated.
Cathode tubes are becoming outdated as LCD and plasma displays are gradually replacing cathode tube displays. These new types of screens have some advantages, such as a reduced size and lower energy consumption, although they also have disadvantages, such as the black color is shown very clearly (by the back light), the response time is high compared to CRTs., and does not display colors uniformly (if the screen is made to show a single color, it is not uniform and it looks darker around the edges of the Monitor and clearer in the center). Although the response time is decreasing, which allows that some models (below 12 ms) can be used for purposes such as action video games, without having to suffer trails in the visualization of fast movements, which even the present was an important brake on the use of these screens in computers, although at present it has a quite high price compared to CRTs, especially in televisions.
- Most of the televisions and computer monitors
- Oscilloscopes, spectroscopes, and other measurement instruments
Failures of the tube and its associated components
Fundamental problems that originate:
- Aging or exhaustion of the tube will cause a loss of contrast and definition. CRT rejuvenators can be used, which can extend (for a short time) the near life of the CRT. Another option is to increase the supply voltage of the filaments to achieve more emission from the cathodes, which only accelerates the aging process.
- Due to movements while the TV works, some of the three filaments tend to be “cut”, with the consequent variation of the colors represented on the screen. There are various techniques to retrieve the tube, achieve contact with the cut filament.
- In the case of falls or blows, the “Blister” appears intact, but microcracks cause air to enter the unit, which is verified in several ways:
- a) When the TV is energized, electric arcs of a violet color are produced within what we call “the neck” of the tube. This sometimes, in some TVs, causes the overload produced to stop the source, turning off the TV. Another way to detect if the CRT has entered air or “it is gaseous” is to connect only the terminal of the Anode and with one of the cables of the tester or multimeter, place one end of the latter to a ground potential and with the other it approach, do not touch, just approach, the base of the neck and observe the high tension arcs that will jump to the approximate point.
Most frequent failures
- There is no image, a single primary color (Red, Green or Blue) predominates, and fine diagonal lines are observed that are repeated every few centimeters. There are two very different possibilities of the origin of this failure:
- a) One of the final color transistors (the color that we see on the screen) is defective or has stopped receiving voltage (approx. 180 Volts in collector). b) 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 made in the core of the Fly-back and before having cut the printing tracks that feed the tube filament, start feeding the latter with the winding carried out. In this way, the filament is isolated from the GND potential, becoming the same one that the cathode takes, no matter what it is, since at its ends there will be about 6 volts generated by the winding that we have made.
- A very problematic component in RGB amplifiers is the Electrolytic Capacitor of between 1 uF and 10 uF that filters the 180 Volt voltage that is needed in this sector. The color drips to the right, the image leaves a trail as if it were navigating to the screen from the right, and when in doubt, the first thing to do is replace it. Furthermore, as there is temperature in this area due to the collector resistances of the RGB amplifying transistors, the sheathing of the same is contracted sharply, revealing that it may be “dry”.
- Another failure that produces a severe deterioration in the focus of the image, which often leads to think about the potentiometer, which is responsible for regulating said voltage. In TVs that have the Focus and Screen controls integrated in the same Fly-back, it is very rare that said control deteriorates, there will be no other choice but to replace the complete unit. In older TVs it was more common to find damaged Focus potentiometers. But there is a fault that is usually very hidden and it is the connection socket to the “shorts” of the TRC. Socket contacts often become “hygroscopic”, which only sometimes looks like greenish sulfate. This is very frequent to happen, so we must control it every time we observe blurring in the image.
- When there is a predominance of a certain color over others, or lack of a color, first try to establish that the three filaments are on, then those that have an oscilloscope check that the three color signals reach the RGB amplifiers, and those that do not. have that instrument control the voltages 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 brightest colors corresponding to the image are seen on a generally dark background.
There are designs in which RGB amplifiers get the color difference signals (RY, BY, GY) on the one hand and the luminance Y signal on the other. Inside the amplifiers, a simple algebraic sum is produced that results in the colors to attack the color guns, but when the transistor that acts as a buffer for the luminance deteriorates, we find the aforementioned phenomenon.
Observations and Measurements
- In cases of total darkening of the image and the presence of sound, a visual inspection to verify that the filaments are lit is never too bad. There may be an intermittent failure of momentary darkening, which is usually due to bad welds in the supply of the same.
- Also visually check the connection of the conductors that come from the Fly-back, which are, Grid 2 or G2 voltage and Focus voltage.
- Once the visual check is done, check the voltage of Grid 2, which should range between 300 Volts and 500 Volts, depending on the CRT model that the TV uses. An excess mismatch in this tension can cause us a very strong brightness with loss of contrast and the appearance of fine diagonal lines every few centimeters. A deficiency mismatch will cause a very noticeable lack of brightness, despite setting the main Brightness control to maximum.
Convenient and easy method of adjusting the emission of RGB guns
- Put the TV in Service mode, with the key that everyone generally has and removes the raster leaving a bright horizontal line.
- Lower the voltage of G2 or Screen (if necessary) with the corresponding potentiometer found on the Fly-back, to the point where the line disappears. Well at the limit, but not appear.
- Start to regulate the color preset, which in the silkscreen appear as Bias R, Bias B and Bias G, as follows:
- Advance until the line of the color that we are activating appears and when this happens we go back a little, until the limit where it disappears, we do not go back too much, only to the limit.
- Do the same for the two remaining guns.
- We pass the key to normal mode.
- Readjust if necessary the voltage of G2.
- Place the control color and saturation to the 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 tone has remained, we will retouch the presets of Drive G and B (they are the two remaining presets in the adjacencies) until we obtain a perfect monochrome vision.