Com 226 comp trouble shooting II theory book
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| Inside The CRT Before reading about CRT problems, you should have an understanding of the color CRT itself. Figure 27-11 shows a cross-section of atypical color CRT. To produce an image, electron beams are generated, concentrated, and directed across a phosphor-coated face. When electron beams (which are invisible) strike phosphor, light is liberated—this is the light you see from the CRT. The color of light is determined by the particular phosphor chemistry. Notice that there are three electron guns in the color CRT abeam for red, abeam for green, and abeam for blue. Electron beams start with a heater wire. When energized, the heater becomes extremely hot this is the glow you see in the CRT neck. The heat from a heater warms its corresponding cathode, and a barium tip on the cathode begins boiling off electrons. Ordinarily, electrons would simply boil off into a big, clumsy cloud. But because electrons are negatively charged, they will be attracted to any large positive potential. A moderate positive potential (+500 V or soon the screen grid starts accelerating the electrons down the CRT’s neck, while the control grid voltage limits the electrons—effectively forcing the unruly cloud into abeam. Once electrons pass the screen grid, a high positive potential on the CRT anode (anywhere from 15 to 30 kV) rockets the electrons toward the CRT face. The beam is still rather wide, so a focus grid applies another potential that concentrates the beam. All this is very effective at generating narrow, high-velocity electron beams. But unless you want to watch a big, bright spot in the middle of the CRT, there has to be some method of tracing the beams around the CRT face. Beam tracing is accomplished through the use of deflection magnets placed around the CRT neck—these magnets (actually electromagnets) are heavy coils of wire where the CRT funnel meets the neck. Four electromagnets are in this deflection assembly two opposing electromagnets direct the beam in the vertical direction, and another set of opposing magnets direct the beam in the horizontal direction. Using electrical signals from the monitor’s raster circuits, an electron beam can trace across the entire CRT face. Another element of the CRT that you should understand is called the shadow mask. A shadow mask is basically a thin metal sheet with a series of small holes punched into it. Some CRTs use a mask of rectangular openings referred to as an aperture grille or slot mask. Both types of mask perform the same purpose—to ensure that electron beams strike only the color phosphors of the intended pixel. This is a vital element of a color monitor. Ina monochrome monitor, the CRT is coated with a single homogeneous layer of phosphor if stray electrons strike nearby phosphor particles, a letter or line might simply appear to be a bit out of focus. Fora color CRT, however, stray electrons can cause incorrect colors to appear on nearby pixels. Masks help to preserve color purity. Color purity is also aided by a purity magnet, which helps correct fine beam positioning. A convergence magnet helps ensure that all three electron beams meet (or converge) at the shadow mask. Of course, grids, heaters, and cathodes are all located inside the glass CRT vessel. Electrical connections are made through a circular arrangement of sealed pins in the neck. Table 27-2 explains the designations for each pin. Remember that the high-voltage anode is attached directly to the CRT in the top right part of the glass funnel. Also remember that some CRT designs might use additional pins. Download 1.89 Mb. Share with your friends:
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