Com 226 comp trouble shooting II theory book
Download 1.89 Mb. View original pdf
|
| 924 Monitor Troubleshooting Video signal (e.g., 0 to 20 V or 0 to 50 V. As cathodes age, however, they tend to become nonlinear. When this happens, images tend to be too black and white rather than display a smooth transition of colors. Technicians often refer to this as a gassy CRT, which is actually a CRT gamma problem. In addition to cathode wear, control grid failure can adversely affect beam intensity. Symptom 27-7. The control grid in the CRT is open The control grid is used to limit the beam intensity produced by a cathode by applying a potential on the grid. Occasionally, you will find that a control grid might open. In that case, there is no longer a potential available to control the beam intensity, and the beam will fire at full intensity. At first glance, you might think this is a cathode-to-control grid short or a heater-to-cathode short. But if you can’t find a short with your multimeter, the control grid is probably open, and the CRT will have to be replaced. Symptom 27-8. The CRT screen grid is open The screen grid plays an important role in image brightness by accelerating the electron beam toward the CRT phosphors. If the screen grid opens, no potential will be available to begin accelerating the beam. This will result in a very dark image—even with the screen voltage at maximum. You might think this is a control-to-screen grid short, but if you can’t find the short with your multimeter, the screen grid is probably open, and the CRT will have to be replaced. Symptom 27-9. The CRT focus grid is open A focus grid assembly concentrates electron beams into narrow pinpoints by the time the beam reaches the shadow mask. Typically, a focus control is located around the flyback transformer. If the focus grid fails, the image will appear highly distorted, and the focus adjustment will have no effect. When a focus grid fails, the entire CRT will have to be replaced. Symptom 27-10. The control grid shorts to the screen grid in the CRT The same flakes of oxidation that can short a cathode to the control grid can also short the control grid to the screen grid. The screen grid starts accelerating the electrons toward the CRT face. If the screen grid is shorted, it will reduce the energy imparted to the electrons in effect, a shorted screen grid will significantly reduce the overall image brightness (even with the brightness at maximum. In extreme cases, the image might disappear entirely. You can measure the screen grid voltage at G, which typically runs from 250 to 750 V in normal operation. If the voltage is low (even with the screen grid control at maximum, power down the monitor, remove the video drive board from the monitor’s neck, restart the monitor, and measure the screen voltage again. If the screen voltage returns to normal, you can be confident that the screen grid is shorted. If screen voltage remains low, you might have a fault in the screen voltage circuit. You can also verify a short between the control and screen grids by powering down the monitor and measuring resistance between the G and G pins on the CRT neck. Ideally, it should bean infinite resistance. Download 1.89 Mb. Share with your friends:
|