Com 226 comp trouble shooting II theory book
Download 1.89 Mb. View original pdf
|
| Identifying CRT Problems CRTs enjoy along, reliable working life because there are really no moving parts—merely a set of stationary metal elements. However, the arrangement of grids and cathodes are located in very close proximity to one another. Physical shocks can dislodge elements and cause sudden short circuits. Eventually, regular use will alter the physical dimensions of cathodes and grids (resulting in the development of a slower, more gradual short-circuit). The stress of regular wear can also cause open circuits in the heater, cathode, or grid. When considering a CRT replacement, you should remember that the CRT is typically the most expensive part of the monitor. For larger monitors, the CRT becomes an even larger percentage of the monitor’s overall cost. In many cases, the cost fora replacement CRT approaches the original cost of the entire monitor. As a consequence, you should carefully evaluate the economics of replacing the CRT versus buying anew monitor outright. Symptom 27-1. Heater opens in the CRT Each time the heater runs, it expands. When the CRT turns off, the heater cools and contracts again. This regular thermal expansion and contraction might eventually fatigue the heater and cause it to open. You will see this as a complete loss of the corresponding color. Because heaters are all tied together electrically, there is noway to measure a particular heater directly, but you might see only two glowing heaters in the CRT neck, instead of three. An open heater cannot be recovered, and the only available alternative is to replace the CRT itself. Symptom 27-2. Heater shorts to a cathode in the CRT This is not as strange as it might seem at first. To heat a cathode effectively, the heater must be extremely close to the cathode—especially to the barium element that actually liberates the electrons. Overtime, the heater might develop accumulations of corrosion, which might eventually cause the heater to contact the cathode. In theory, this should never happen because the inert low-pressure gasses inside the CRT should prevent this. But in actuality, some small amount of oxygen will still be present in the CRT, and oxidation might occur. A shorted heater will cause the electron gun to fire at full power—in effect, the electron gun will be stuck on The image will appear saturated with the color of the defective electron gun. For example, if the blue heater shorts to the cathode, the image will appear to be saturated with blue. You will also likely see visible retrace lines in the image. You can verify this problem by removing all power from the monitor, removing the video drive board from the CRTs neck, and measuring the resistance between a heater wire and the suspect cathode. For the CRT pinout listed in Table 27-2, you could check the blue cathode by measuring resistance between the KB and H (or H) pins. Ideally, an infinite resistance should be between the heaters and cathodes. If there is measurable resistance (or a direct short- circuit, you have found the problem. If the resistance measures infinity, as expected, you might have a defect on the video drive board. Symptom 27-3. Cathode shorts to the control grid in the CRT A cathode can also short- circuit to the control grid. Often, corrosion flakes off the cathode and contacts the control grid. When this happens, the control grid loses its effectiveness, and the corresponding color will appear saturated. This symptom will appear very much like a heater short. Fortunately, you should be able to verify this problem with your meter by measuring resistance between the control grid and the suspect cathode. Ideally, an infinite resistance should be between the control grid and all cathodes. If you read a measurable resistance (or a direct short-circuit), chances are good that you’re facing a cathode-to-control grid short. Symptom 27-4. One or more colors appear weak This is a common symptom in many older CRTs. Overtime, the barium emitter in your cathodes will wear out or develop a layer of ions (referred to as cathode poisoning), which inhibit the release of electrons. In either case, the afflicted cathode will lose efficiency, resulting in weakened screen colors. Typically, you might expect all three cathodes to degrade evenly overtime and they will—but by the time the problem becomes serious enough for service, you will usually notice one color weaker than the others. Try increasing the gain of the afflicted signal on the video drive board. If the cathode is indeed afflicted, increasing signal gain should not have a substantial effect on the color brightness, and you should consider replacing the CRT. Symptom 27-5. CRT phosphors appear aged or worn Phosphors are specially formulated chemicals that glow in a particular color when excited by a high-energy electron beam. Typically, phosphors will last for the lifetime of the monitor, but age and normal use will eventually reduce the sensitivity of the phosphors—for old CRTs, you might see this as dull, low-contrast colors. Perhaps a more dramatic problem occurs with phosphor burn which occurs when a monitor is left on displaying the same image fora very long period of time. If you turn the monitor off, you can seethe latent image burned onto the CRT as a dark shadow. In both cases, there is noway to rejuvenate phosphors, so the CRT will have to be replaced. You can advise customers to prolong the life of their CRT by keeping the brightness at a minimum and using a screensaver utility, if an image will sit unchanging fora longtime. Symptom 27-6. The CRT suffers from bad cutoff (aka. bad gamma) On a CRT, color linearity is a function of the cathode’s ability to adjust the level of electron emission—in other words, beam intensity must be linear across the entire range of the Download 1.89 Mb. Share with your friends:
|