CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [317]
Before we begin in earnest, I want to give you a note of warning about the inside of a traditional monitor. I will discuss what can be repaired and what requires more specialized expertise. Make no mistake—the interior of a monitor might appear similar to the interior of a PC because of the printed circuit boards and related components, but the similarity ends there. No PC has voltages exceeding 15,000 to 30,000 V, but most monitors do. So let’s get one thing perfectly clear: Opening up a monitor can kill you! Even when the power is disconnected, certain components retain a substantial voltage for an extended period of time. You can inadvertently short one of the components and fry yourself—to death. Given this risk, certain aspects of monitor repair lie outside the necessary skill set for a normal PC support person, and definitely outside the CompTIA A+ certification exam domains! I will show you how to address the problems you can fix safely and make sure you understand the ones you need to hand over to a monitor shop.
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CAUTION The inside of a CRT has very high voltage components. These can, literally, kill you. Be careful!
The inside of the display screen has a phosphor coating. When power is applied to one or more of the electron guns, a stream of electrons shoots towards the display end of the CRT (see Figure 19-2). Along the way, this stream is subjected to magnetic fields generated by a ring of electromagnets called a yoke that controls the electron beam’s point of impact. When the phosphor coating is struck by the electron beam, it releases its energy as visible light.
When struck by a stream of electrons, a phosphor quickly releases a burst of energy. This happens far too quickly for the human eye and brain connection to register.
Figure 19-2 Electron stream in the CRT
Fortunately, the phosphors on the display screen have a quality called persistence, which means the phosphors continue to glow after being struck by the electron beam. Too much persistence and the image is smeary; too little and the image appears to flicker. The perfect combination of beam and persistence creates the illusion of a solid picture.
Essentials
Refresh Rate
The monitor displays video data as the electron guns make a series of horizontal sweeps across the screen, energizing the appropriate areas of the phosphorous coating. The sweeps start at the upper-left corner of the monitor and move across and down to the lower-right corner. The screen is “painted” only in one direction; then the electron guns turn and retrace their path across the screen, to be ready for the next sweep. These sweeps are called raster lines (see Figure 19-3).
Figure 19-3 Electron guns sweep from left to right.
The speed at which the electron beam moves across the screen is known as the horizontal refresh rate (HRR), as shown in Figure 19-4. The monitor draws a number of lines across the screen, eventually covering the screen with glowing phosphors. The number of lines is not fixed, unlike television screens, which have a set number of lines. After the guns reach the lower-right corner of the screen, they turn off and point back to the upper-left corner. The amount of time it takes to draw the entire screen and get the electron guns back to the upper-left corner is called the vertical refresh rate (VRR), shown in Figure 19-5.
Figure 19-4 Horizontal refresh rate
Figure 19-5 Vertical refresh rate
The monitor does not determine the HRR or VRR; the video card “pushes” the monitor at a certain VRR and then the monitor sets the corresponding HRR. If the video card is set to push at too low a VRR, the monitor produces a noticeable flicker, causing eyestrain and headaches for users. Pushing the monitor at too high a VRR, however, causes a definite distortion of the screen image