CRT (Cathode Ray Tube) are glass vacuum tubes in which an electron gun radiates an electron current directed by an electric field towards a screen covered with a small phosphorescent element.
What is CRT Monitor in Computer?
In the cathode ray tube, an electronic ball produces and limits an electron beam that it sends towards a screen covered with luminescent material so that when the electrons collide, it emits light whose intensity or brightness is proportional to the amount and speed of the event electrons.
In other words, the kinetic energy of the electron beam is converted into light energy by transferring it to the screen material.
Between the electronic ball and the sieve, there is a deflection system consisting of coils placed outside the pipe to deflect the electronic beam horizontally and vertically.
Unlike the magnetic deflection system used on television, oscilloscopes use electrostatic deflection and deflect the electron beam using horizontal and vertical squares placed inside the tube.
The brightness can be changed by changing the intensity of the electron beam by using a control grid whose action is similar to a triode or other valves such as tetrode or pentode.
The cathode ray tube or CRT monitor was developed in 1897 by a German scientist, Karl Ferdinand Braun, but was not used until the creation of the first televisions in the late 1940s.
Although they have many modifications that allow CRTs used in modern monitors to improve image quality, they still use the same basic principles.
The first version of the cathode ray tube was a cold-cathode diode, actually, it is a kind of phosphor coating in front of the Crookes tube.
This tube is sometimes called the Braun tube. The first version using the hot cathode was developed by J. B. Johnson and H. W. Weinhart from the Western Electric Society. This product was released in 1922.
CRT; neck, headline, the screen is divided.
On the neck, we find the filament as the first component responsible for the heating of the second component, the cathode, then the control grid (G1), the display grid (G2) and finally the Focus (G3), which are generally called focal, all of which have the special function of accelerating the electron beam, thus, it reaches the surface of the phosphor where the spotlighting on the CRT screen is produced.
Inside the hood is the anode, which attracts electrons produced at the cathode and collapses at high speed against the phosphor-coated screen, which, when shot by the electrodes, produces equally bright light to create images.
Outside, in the hood area, it is covered with black paint, ACUADAC, which is the center of the CRT. In this way, a condenser forms between the core (Ecuador) and the inside of the tube, the glass acts as a dielectric.
There are deflection coils (yoke) between the neck and the bell on the outside, it has the function of deflecting the electrons from the sieve and makes a sweep on the entire surface of the screen.
The screen has a shadow mask that is responsible for ensuring that each color ball (RGB) only alerts the corresponding color points.
This may be the traditional type used by Sony televisions or the Wega Trinitron type. Finally, there is a phosphor of red, green, blue colors that will serve to create color images from black to white with a combination of three primary colors.
How Does It Work?
The electron gun consists of a cathode, a negatively charged metal electrode, and one or more anodes (positively charged electrodes).
The cathode emits electrons attracted to the anode. The anode acts as an accelerator and concentrator for electrons and creates an electron flow directed to the screen.
The magnetic field directs electrons from right to left and from top to bottom. It is created by two electric X and Y plates that send the current in horizontal and vertical directions, respectively.
This screen is covered with a thin layer of phosphorescent elements called phosphors that emit light through excitation, that is, when electrons hit them, thereby forming a light spot called Pixel.
Activation of the magnetic field causes the electrons to follow a scan pattern by landing from left to right after reaching the end and then to the next line. The human eye cannot visualize this sweep due to constant vision. With the firing or stopping of the electron gun, the sweep deceives the eyes, believing that only a few pixels on the screen are illuminated.
Since plasma and LCD screens are gradually replacing cathode tube screens, we can say that cathode tubes fashion is completely over now.
These new display types have some advantages such as small size and lower power consumption, but they have disadvantages as black color is very clearly shown, the response time is high compared to CRT and does not show colors evenly.
While the response time is decreasing, it allows for fast movements, which are an important element for use in computers, and for some models, such as action, video games. It currently has a fairly high price compared to CRTs, especially on televisions.
CRT technology has been widely used in television and computer monitors, oscilloscopes, spectroscopy, and other measuring devices and radars.
Malfunctions and Related Components
The aging or end of the tube causes loss of contrast and definition. CRT rejuvenates can be used, which can extend the close life of CRT.
Another option is to increase the supply voltage of the filaments only to achieve more emissions from the cathodes that speed up the aging process.
Due to the movements that occur while the TV is running, some of the three filaments are usually cut off, and as a result, the colors displayed on the screen change. There are various techniques to save the tube, to contact the cut filament.
In the event of a fall or impact, the Bulb looks solid, but microcracks cause air to enter the unit, which is verified in several ways.
When energizing the TV, violet springs are produced inside what we call the neck of the tube. This sometimes causes the overload generated on some TVs to stop the source and turn off the TV.
Another way to detect if the air enters the CRT or whether it is in the gaseous state is simply to connect the terminal of the anode and one of the cables of the tester or multimeter, approach one end of the second to a potential mass, and what another approach, not just touch, but near the neck base and about the end observe the high tension springs that will jump.
Most Common Faults in CRTs
CRT monitors are dominated by a single primary color, and in case of a breakdown, thin cross lines are repeated every few centimeters. There are two different reasons for this problem:
One of the color tip transistors is faulty or has stopped receiving voltage.
The cathode of this color is short-circuited with the filament.
In this case, approximately 3 to 4 turns of winding should be made in the core of the Fly-back, and before cutting the print marks that feed the tube filament, feed the second with the wrapped winding.
In this way, the potential for GND is isolated for the filament, the cathode becomes the same as the area because regardless of what it is, it will be about 6 volts produced by the winding made at the ends.
A very problematic component in RGB amplifiers is the Electrolytic Capacitor between 1 uF and 10 uF, which filters the voltage of 180 Volts needed in this industry.
The color drips to the right, the image leaves a trace as if it were navigating from the right, and the first thing to do in such a problem is to change it.
Also, since there is a temperature in this area due to the collector resistors of RGB amplifier transistors, the sheath when it can be the same shows that it shrinks sharply and can be dry.
Another error that causes a serious distortion in the focus of the image, which leads to the consideration of the potentiometer, which is usually responsible for the regulation of said voltage.
On TVs with Spot and Display controls integrated into the same Return, the disruption of the control in question is very rare, there will be no other option than to replace the entire unit.
On older TVs, it was more common to find broken Spot potentiometers. However, there is usually a very hidden malfunction, and this is the cartridge connection socket of the CRT. Socket contacts often become hygroscopic, and this is only sometimes seen as a greenish sulfate.
This is very common, so we should check the blur every time we see the image.
When a particular color has superiority over others or lack of color, first try to determine if three filaments are turned on, then those with oscilloscopes can check that the three color signals reach RGB amplifiers and do not have this instrument. You can also check the voltages at various points of the amplifiers, which are similar in all three.
If everything is correct and the defect continues, it should be adjusted until the emissions of the three balls are balanced. Usually, only the most vivid colors corresponding to the image are observed on a dark background.
RGB amplifiers have designs that receive color difference signals (R-Y, B-Y, G-Y) on the one hand and brightness signal Y on the other.
Observations and Measurements
In the event that the image is completely darkened and the sound is present, it never performs a visual inspection to verify that the filaments are clear.
Often this may be an intermittent momentary dimming error due to bad sources in the power supply.
You can also visually check the connection of conductors from Back Voltage, Mains Voltage 2 or G2 and Focus voltage.
After visual inspection, you can check the mains 2 voltage, which should be between 300 Volts and 500 Volts depending on the CRT model used by the TV.
Excessive mismatch in this tension can result in very strong brightness, with loss of contrast and the appearance of thin diagonal lines every few centimeters.
A deficiency mismatch will result in a very pronounced lack of brightness, although it sets the main Brightness control to maximum.
It is a practical and simple method to adjust the emission of RGB weapons.
You can put the TV in service mode with a switch, leaving a bright horizontal line that everyone generally owns and lifts.
You can lower the G2 or Display voltage to the point where the line disappears with the corresponding potentiometer on Reversing.