Principles Of A CRT (Cathode Ray Tube): There are three main sections in the operation of a CRT: The electron gun, the deflection system, and the screen. The electron gun begins with what is often called the "heater". It is a filament of metal (usually tungsten, which is the same material used in everyday household light bulb filaments). This filament heats up a cathode, which results in a production of electrons. The reasons why this happens are more related to physics than electrical engineering, and therefore are beyond the scope of this document to fully explain, but suffice it to say that when something increases in temperature, the atoms that comprise the material move around faster, and this makes their electrons more prone to flying off into space. This phenomenon is what physicists call "thermionic emission", but you can just think of it as the tendency of metal to throw off electrons when it gets hot. There are also three (sometimes four) important anodes between the heater and the rest of the CRT assembly. These anodes are commonly called the "grids". The first grid (often labeled G1) is the "control grid", the second (G2) is the "screen grid" (also known as the "cutoff"), and the third (G3) is the "focus grid". The control grid (G1) control the brightness of the CRT. It is generally placed directly on top of the cathode. If there is no voltage on G1, the electrons can flow freely from the cathode. If there is some negative voltage on G1, the electrons from the cathode are repelled somewhat, and the screen appears darker. The greater the voltage on G1, the darker the screen becomes, because more electrons reaching the screen equals a brighter image, and less electrons hitting the screen equals less brightness. The screen grid (G2), also known as the cutoff, serves to "push" the electrons so they travel more quickly toward the screen, in much the same way as blowing in one end of a tube will force something out the other end. Although electrons go through the screen grid before the focus grid, it seems that in some systems, there will actually be another anode which serves this same purpose *after* the focus grid, and either or both of these anodes may be referred to as the "accelerating" anodes. The screen grid (or accelerating anode(s)) actually uses a positive voltage to pull the electrons along. The focus grid (G3), appropriately enough, controls the focus (concentration) of the electron beam. This is required to make the beam create a small, sharp point on the screen. If the beam were not in focus, it would create a fuzzy blob on the screen instead. Some CRT-using equipment (such as oscilloscopes) have a focus knob on them, allowing the user to adjust the voltage of the focus grid, and thus change the focus of the beam. The focus grid applies a strong (usually a few hundred volts, although it may be as much as a few thousand) negative charge to the electrons passing through it. Because the electrons are repelled by a negative charge, they will compress into a small, tight line in the center of the focus grid. If there is a fourth grid in the CRT, it will control the astigmatism of the beam; Astigmatism is a refraction error which produces effects similar to an out-of-focus display. The deflection system consists of two sets of electromagnets, one for vertical positioning, one for horizontal. Sometimes coils are used, and sometimes plates are used. Together, they are called the "yoke" of the CRT. These serve to control where the electron beam is aiming. The electrons in the beam are sensitive to magnetism, and can be moved by subjecting them to an electromagnetic field. The vertical plates control the vertical position of the electron beam, and the horizontal plates control the horizontal position of the electron beam. The screen is probably the simplest of the three stages described here; It is simply a plate of glass, coated on the inside with phosphor. When a part of this coating is struck with the electron beam, the phosphors become "excited", and they produce light. The chemical composition of the phosphor determines what color of light it will produce. To make a colour screen, phosphors of red, green, and blue are clustered together in one "pixel" on the screen. The electron beam must strike each phosphor individually. The strength of the electron beam determines how brightly the phosphor will glow. It should be mentioned that the screen has a plate of some metal (often aluminum) underneath the coating of phosphors; This plate is given a very strong positive charge, on the magnitude of several thousand volts. This positive charge pulls the electrons strongly toward the screen. A cathode ray tube typically has a set of connector pins at the back of the tube, which you are meant to hook wires up to so you can control the tube. Typical tube pin functions are: Heater Grids (G1, G2, G3) Cathodes (One in a monochrome CRT, three in a colour CRT) Deflection (One pair for vertical, another pair for horizontal) The heater pins, of which there are typically two, are simply the power connections. They power the filament which generates the CRT's electrons. The grid pins control the level of voltage in the CRT's "grids". In a monochrome CRT, there is only one cathode control pin. In a colour CRT, there are three, one for each of the light's primary colours (red, green, and blue). These pins are basically used as digital inputs, rather than analog. They are turned on when that particular cathode is meant to shine, and turned off when it is not meant to. They are used in conjunction with the deflection control pins to control what colors are placed on the screen, and where. The deflection pins are the only pins which are often not mounted at the back of the tube; In tubes which do not include these pins as part of the regular pinout, you should find the pins upon the neck of the tube, mounted upon the actual yoke. There are four yoke-control pins total, two for the horizontal position, and two for the vertical position. Changing the voltage across these pins changes the vertical or horizontal position of the electron beam on the screen. In CRTs which are used to control a video display such as a computer monitor or a television, the yoke-control pins receive a "sawtooth" type of waveform, which is one characterized by a gradual climb or dive, followed by a rapid return to the starting point. It receives its name from the fact that it resembles the teeth of a saw. It looks something like this: / / / / /| /| /| / / | / | / | / / |/ |/ |/ / / / / A video CRT creates a picture on the screen by "painting" a horizontal line from left to right, beginning with a line at the top of the screen and moving down, line by line, until the entire screen has been painted one horizontal line at a time. When the whole screen has been filled, the electron beam moves back to the upper-left corner of the screen and begins again. Because the voltage on the horizontal yoke control (and thus, the horizontal position of the electron beam on the screen) is directly controlled by the voltage on the horizontal control pins of the CRT, the voltage is steadily increased to slide the beam across the screen, then the voltage is suddenly dropped to bring the beam back to the left side of the screen for the next line. Similarly, the yoke's vertical control voltage is also a sawtooth, although it rises much more slowly; It only increases when a line has been drawn, and only returns to its original level when the whole screen has been painted and it's time to move the beam to the top of the screen again. (Notice that I mentioned "video CRTs" using sawtooth waves, to distinguish from CRTs used in things like oscilloscopes, in which the point is not to fill the whole screen, but simply to trace a line.) Remember, a CRT uses high levels of voltage, and it can retain lethal voltage for days or even weeks after it has been turned off. If you are planning to work on a CRT, you *must* first drain it of its stored electricity. This makes CRTs especially dangerous for novice electricians to work on. Be absolutely sure that you know what you are doing and that you have drained the CRT before you attempt to work on it. If you are going to try and measure the high voltages used in CRTs with a voltmeter, there are special high-voltage probes which you should use, because even high-end voltmeters usually cannot measure more than 1,000 volts. These high-voltage probes scale down a voltage from kilovolts to the equivalent number of millivolts. For example, if you measure 20 kilovolts with the probe, your meter will read 20 millivolts.