57 MONITOR TESTING AND ALIGNMENT
CONTENTS AT A GLANCE
Before You You Begin Testing vs. alignment Know the warranty Getting from here to there High-voltage cautions The mirror trick Making an adjustment
Test and an d Procedures High-voltage test and regulation Screen control Focus Dynamic pincushion
Horizontal phase Horizontal and vertical centering Horizontal and vertical size (height and width) Horizontal and vertical linearity Static convergence Dynamic convergence Color purity Color drive Cleaning and vacuuming
Further Study
When set-up and ventilated properly, computer monitors are notoriously rugged devices. The CRT itself enjoys enjoys a reasonably long life span. By their very nature, CRTs CRTs are remarkably tolerant to physical abuse and can withstand wide variations in power and signal voltages. However, even the best CRT and its associated circuitry circuitry suffers eventual degradation with age and use. Monitor operation can also be upset when major sub-assemblies sub-assemblies are replaced, such as circuit boards boards or deflection assemblies. assemblies. Maintenance and alignment alignment procedures are available to evaluate a monitor’s performance and allow you to keep it 1476
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working within its specifications. specifications. This chapter illustrates illustrates a comprehensive set of procedures that can be performed to test and adjust the monitor’s monitor’s performance. Keep in mind that this chapter uses test patterns generated with the commercial utility “MONITORS” available on the companion CD. The MONITORS utility is commercial software that is included on the companion CD in encrypted format. To use MONITORS, you’ll need to purchase the unlock code. See the order form at the back of this book for more ordering information.
Before Before You You Begin Begin Adjusting a monitor is a serious matter and it should not be undertaken without careful consideration. A myriad of adjustments are found on the main circuit board—any of which can render a screen image unviewable if adjusted improperly. The delicate magnets and deflection assemblies around the CRT’s neck can easily be damaged or knocked out of alignment by careless handling. In short, aligning a monitor can do more harm than good unless you have the patience to understand understand the purpose of each procedure. procedure. You also need to have a calm, methodical approach. The following points might help to keep you out of trouble.
TESTING VS. ALIGNMENT The difference between monitor testing and monitor alignment is distinct. Testing is a lowlevel operation. Testing is also unobtrusive, unobtrusive, so you can test a monitor at any point point in the re pair process. By using the companion software software with your PC, you can test any monitor that that is compatible with your video adapter. Testing is accomplished by displaying a test pattern on the monitor. After observing the condition condition of the pattern, you can usually deduce the monitor’s fault area very quickly. Once the monitor is working (able to display a steady, full-screen full-screen image), you can also use test patterns to evaluate the monitor’s current state of alignment. Alignment is a high-level operation—a task that is performed only after the monitor has been completely repaired. repaired. Because alignment requires requires you to make adjustments to the monitor circuits, the monitor monitor circuits must be working properly. After all, alignment does no good if a fault is preventing the monitor from displaying an image in the first place. Proper alignment is important to ensure that the monitor is displaying images as accurately as possible.
KNOW THE WARRANTY A warranty is a written promise made by a manufacturer that their product will be free of most problems for some period of time. For monitors, typical warranties cover parts and la bor for a period of one year from the date of purchase. The CRT itself is often covered up to three years. Before even touching the monitor, you should check to see if the warranty is still in force. Monitors that are still under warranty should be sent back to the manufacturer for repair. This book does not advocate voiding voiding any warranty and the reasoning reasoning is very simple—why spend your time and effort to do a job that the manufacturer will do for free?
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You paid for that warranty when you bought the monitor. monitor. Of course, most monitors in need of service are already out of warranty. Only three exceptions might prompt you to ignore a warranty. warranty. First, the warranty might already be void if you purchased the monitor monitor used. Manufacturers typically typically support the warranty only for the original purchaser (the individual that returns the warranty card). Third-party claims are often refused, but it might still be worth a call to the manufacturer’s service manager just to be sure. Second, any warranty is only as good as the manufacturer. A manufacturer that goes out of business is not concerned with supporting your monitor, although reputable manufacturers that close their doors will turn over their service operations to an independent repair house. house. Again, a bit of detective work work might be required to find out if the ultimate service provider will will honor the unit’s warranty. Finally, you might choose to ignore a warranty for organizations with poor or unclear service performance. Call the service provider and ask about the turn-around time and return procedures. procedures. If they “don’t know” or you can’t get a straight answer, chances are that your monitor is going to sit untouched for quite a while.
GETTING FROM HERE TO THERE Monitors require special care in moving moving and handling. A monitor is typically a heavy device (most of the weight being contributed by the CRT and chassis). chassis). Back injury is a serious concern. If you must move the monitor between locations, remember remember to lift from the knees and not from the back—it’s back—it’s hard to fix a monitor while you’re you’re in traction. When carrying the monitor, keep the CRT screen toward your chest with your arms wrapped carefully around the enclosure to support the weight. weight. Large monitors (larger than than 17") are particularly unwieldy. unwieldy. You are wise to get another person’s help when when moving such bulky, expensive devices. If the monitor is to be used in-house, in-house, keep it on a roll-around cart so that you will not have to carry it. When transporting a monitor from place to place in an automobile, the monitor should be sealed in a well-cushioned box. If an appropriate box is not to be had, place the monimonitor on a car seat that is well-cushioned with soft foam or blankets—even an old pillow or two will do. Sit the monitor on its cushioning face down because that will will lower the monitor’s center of gravity and make it as stable as possible. possible. Use twine or thin rope to secure the monitor so it will not shift in transit. When shipping the monitor to a distant location, the monitor should be shipped in its original container and packing materials. materials. If the original shipping material material has been discarded, purchase a heavy-gauge cardboard cardboard box. The box dimensions should be at least least 4" to 6" bigger in every dimension than the monitor. Fill the empty space with plenty of foam padding or “bubble-wrap” that can be obtained from any full-service stationary stationary store. The box should be sealed and reinforced reinforced with heavy-gauge box tape. tape. It does not pay to skimp here—a monitor’s weight demands a serious level of protection.
HIGH-VOLTAGE HIGH-VOLTAGE CAUTIONS It is also important to remind you that a computer monitor uses very high voltages for proper operation. Potentially lethal lethal shock hazards exist within the monitor monitor assembly— both from ordinary ac line voltage, as well as from the CRT anode voltage developed by the flyback transformer. You must exercise extreme caution whenever the monitor’s monitor’s outer housings are removed.
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THE MIRROR TRICK Monitor alignment poses a special problem for technicians—you must watch the adjustment that you are moving while also watching the display to see what effect the adjustment is having. Sure, you could watch the display display and reach around the back of the monitor, but given the serious shock hazards that exist with exposed monitor circuitry, that is a very unwise tactic (you would place your personal safety at risk). Monitor technicians use an ordinary mirror placed several feet feet in front of the CRT. That way, you can watch inside the monitor as you make an adjustment, then glance up to see the display reflected in the mirror. If a suitable mirror is not to be found, ask someone someone to watch the display for you and relate what is happening. Ultimately, the idea is that you should never take your eyes off of your hand(s) while making an adjustment.
MAKING AN ADJUSTMENT Monitor adjustments are not difficult to make, but each adjustment should be the result of careful consideration consideration rather than a random, haphazard “shot in the dark.” The reason for this concern is simple—it simple—it is just as easy to make the display worse. Changing adjustments indiscriminately indiscriminately can quickly ruin display quality beyond your ability to correct it. The following three guidelines will help you make the most effective adjustments with the greatest probability of improving image quality. First, mark your adjustment (Fig. 57-1). Use a narrow-tip indelible marker to make a reference mark along the body of the adjustment. It does not have to be anything fancy. By making a reference mark, you can quickly return the adjustment to the exact place it started. A reference mark can really save the day if you get lost or move the wrong adjustment. Second, concentrate concentrate on only one adjustment for any one alignment alignment procedure. For example, if you are trying to optimize horizontal linearity, you should only be concerned with the horizontal linearity linearity adjustment. adjustment. If you do not have documentation that describes describes the location of each control, check the silk-screen labels on the PC board. If you absolutely cannot locate the needed adjustment point, skip the alignment and move on to the next test. When you do move an adjustment, move it slowly and in very small increments (perhaps 1 1 ⁄ 8 to ⁄ display after each step. step. If the display fails fails to improve, return the 4 turn). Check the display
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Mark the starting point clearly VR301 vertical height
FIGURE 57-1
Mark a starting point before starting an adjustment.
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control to its original location (a snap to do if you’ve made a reference mark) and try it in the opposite direction. Finally, avoid using metal tools (such (such as screwdrivers) to make your adjustments. Some of the controls in a monitor are based on coils with permeable permeable cores. Inserting steel tools to make an adjustment will throw the setting off—the display might look fine with the tool inserted, but degrade when when the tool is removed. As a general rule, use plastic plastic tools (such as TV-alignment tools) that are available from almost any electronics store.
Tests and Procedures Testing a computer monitor is easy and it can be accomplished through the use of relatively standard test patterns. Once the companion alignment software is started, you can select the test pattern for the specific test you wish to run, Each of the following procedures covers how to interpret the pattern, and provides a step-by-step procedure for making adjustments. For the purposes of this section, you should refer to the sample main board shown in Fig. 57-2. Remember that the PC board(s) used in your particular monitor might be quite different, so examine your own PC board very closely before attempting an adjustment.
Many newer monitors forego the use of discrete adjustments in favor of electronic “onscreen” controls. Before opening a monitor for alignment, alignment, check for on-screen controls, as described in Chapter 2. VR203 horizontal phase VR205 dynamic pincushion VR201 horizontal frequency VR202 horizontal hold VR303 vertical height VR207 horizontal width
VR301 horizontal height VR801 horizontal center
VR403 blue bias VR406 blue drive VR402 green bias VR405 green drive VR401 blue bias VR404 blue drive FIGURE 57-2
VR501 high voltage VR408 white balance Flyback transformer Focus (top) Screen (bottom)
Video connector
A typical main PC board for a monitor.
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FIGURE 57-3
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High-voltage regulation test pattern.
HIGH-VOLTAGE HIGH-VOLTAGE TES T AND REGULATION The high-voltage test is one of the more important tests that you will perform on computer monitors. Excessive high-voltage high-voltage levels can allow X-radiation X-radiation to escape escape the CRT. CRT. Over long-term exposure, X-rays X-rays pose a serious biohazard. Your first check should be to use a high-voltage probe. Ground the probe appropriately and insert the metal test tip under the rubber CRT anode cap. You can then read the high-voltage level directly directly from the probe’s meter. Be certain to refer to any particular operating and safety safety instructions that accom pany your high-voltage probe. If the high-voltage level is is unusually high or unusually low, carefully adjust the level using a high-voltage control (Fig. 57-2 shows VR501 as the high-voltage control), which is usually located near the flyback transformer. Regulation is the ability of a power supply to provide a constant output as the load’s demands change. The high-voltage supply must also provide regulation within specified limits. As the display image changes, high-voltage levels should remain relatively relatively steady. If not, the display image will flinch as image brightness brightness changes. Select the Highthe High-volta voltage ge test patte pattern rn (Fig. 57-3) from the alignment-software alignment-software main menu. This is a narrow white double border with a solid white center. Watch the border as the center switches on and off at two-second intervals. A well-regulated high-voltage system system set at the correct level will keep the white border reasonably steady—there steady—there should be very little variation variation in image height or width. width. If the image flinches significantly, the high-voltage system might be damaged or failing.
SCREEN CONTROL The CRT screen grid provides a form of master control over the electron beam(s), which affects the display’s display’s overall brightness. A proper screen grid setting is important important so that the brightness and contrast controls controls work within an appropriate range. Select the Blank the Blank raster test from test from the alignment software software main menu. Adjust the monitor’s brightness brightness and contrast controls to their maximum levels—the background raster should be plainly visible. Locate the screen the screen voltage control . In Fig. 57-2, the screen screen voltage control is located located just below the focus control on the flyback transformer transformer assembly. Slowly adjust the screen screen voltage control until the background background raster is just barely visible. Set the monitor’s brightness control to its middle (detent) (detent) position. The background raster should now be invisi ble. Press any key to return to the the main menu. You might reduce reduce the monitor’s monitor’s contrast control to achieve a clear image.
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FOCUS When an electron beam is first generated in a CRT, CRT, electrons are not directed very well. A focus electrode in the CRT’s neck acts to narrow narrow the electron stream. An improperly focused image is difficult to see and can lead to excessive eye strain, resulting in headaches, fatigue, etc. The Focus test pattern allows allows you to check the image clarity and optimize optimize the focus, if necessary. Focus is a subjective measurement—it measurement—it depends depends on your perception. You would be wise to confer with another person while making focus adjustments because their perception of the display might be different than yours. Because focus is indirectly related to screen brightness and contrast, you should set the screen controls for an optimum optimum display. An image that is too bright or has has poor contrast might adversely affect affect your perception of focus. Start the companion software software if it is not running already. already. Select the Blank the Blank raster test from test from the alignment software main menu. Adjust the screen brightness to its middle (detent) position or until the dis play’s background raster disappears disappears (the screen should be perfectly dark). Press any key to return to the main menu, then select the White purity test . The display display should be filled with a solid white box. Adjust screen contrast to its maximum maximum position, or until a good white image is achieved. Once the display conditions are set properly, properly, press any key to return to the main menu. Now, select the Focus the Focus test pattern (Fig. 57-4) from the main main menu. You will see see a screen filled with the letter “m.” Review the entire screen carefully carefully to determine if the image is out of focus. focus. Again, it is wise to get a second opinion before before altering the focus. focus. If the image requires a focus adjustment, adjustment, gently and slowly alter alter the focus control. For the sample main board shown in Fig. 57-2, the focus control is located on the flyback transformer assembly. Once you are satisfied with the focus, press press any key to return to the main menu.
DYNAMIC PINCUSHION PINCUSHION A computer-generated image is produced in two dimensions—it dimensions—it is essentially flat. Unfortunately, the traditional CRT face is not flat (although some new CRT designs use an extremely flat face). face). When a flat image is projected projected onto a curved surface, surface, the image becomes distorted. Typically, the edges of the image bow outward making straight lines lines appear convex appear convex (barrel distortion). distortion). Monitor raster circuitry circuitry is designed to compensate for
FIGURE 57-4
Screen-focus test pattern.
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FIGURE 57-5
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Phase test pattern.
this distortion and allows the image to “appear” flat—even though it is being projected onto a slightly curved curved surface. This is known as the dynamic pincushion circuit (or circuit (or just the “pincushion”). However, if the pincushion circuit overcompensates overcompensates for curvature, curvature, the edges of an image will appear to bow inward, making straight lines appear concave. It is a simple matter matter to check the dynamic pincushion. Select the Convergence test (crosshatch) pattern from the main menu. menu. A white grid will will appear in the display. display. Inspect the outer border of the grid pattern. pattern. If the edges of the border appear straight straight and true, the dynamic pincushion is set properly properly and no further action is needed. If the edges appear to bow outward, the pincushion is undercompensated. undercompensated. If the edges appear to bow inward, the pincushion is overcompensated. In either case, you will need to make a minor adjustment to the dynamic pincushion control. Figure 57-2 lists VR205 as the dynamic pincushion pincushion control, but your monitor probably probably uses different nomenclature. nomenclature. Before making such an adjustment, you might wish to confer with another individual because their perception of the display might be different from yours. If you cannot locate the dynamic pincushion control, simply move on to the next test.
HORIZONTAL PHASE When brightness and contrast are set to their maximum levels, you will see a dim, dark gray rectangle formed around the screen screen image. This border is part of the raster—the raster—the overall area of the screen, which is hit by the electron beam(s). Ideally, the raster is just slightly larger than the typical image. You are able to control the position of an image within this raster area. This is known as horizontal phase. phase. The image should be horizontally centered within the raster area. The term phase term phase is used because it refers to the amount of delay between the time the horizontal horizontal scan (raster) starts and the time time where pixel data starts. By adjusting this delay, you effectively shift the image left or right in the raster area (which should remain perfectly still). Select the Phase the Phase test pattern from the alignment software main menu. A phase pattern will appear (Fig. 57-5). Set the monitor’s brightness and contrast controls to their maximum values—the raster should now be visible around the image. Locate the horizontal phase control. The sample main board shown in Fig. 57-2 indicates VR203 as the horizontal phase control. Carefully adjust the horizontal phase control until the image is ap proximately centered in the raster. This need not be a precise adjustment, but a bit of
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raster should be visible all around the image. Return the monitor brightness control to its middle (detent) position and reduce the monitor contrast control, if necessary, to achieve a crisp, clear image.
HORIZONTAL AND VERTICAL CENTERING Now that the image has been centered in the raster, it is time to center the image in the dis play. Centering ensures that the image is shown shown evenly so that you can check and adjust linearity later on without the added distortion distortion of an off-center image. Select the Conver gence test (crosshatch) pattern from the alignment software software main menu. If the image ap pears well-centered, no further action is required. The sample main PC board shown in Fig. 57-2 indicates VR801 as the horizontal centering control. Adjust the centering control control so that the image is centered horizontally horizontally in the display. Figure 57-2 also also shows VR303 as as the vertical centering control. Adjust this cencentering control so that the image is centered vertically vertically in the display. These need not be precise adjustments. Many monitors make their centering controls “user-accessible” “user-accessible” from the front or rear housings (along with brightness and contrast).
HORIZONTAL AND VERTICAL SIZE (HEIGHT AND WIDTH) Many monitors are capable of displaying more more than one video mode. Unless the monitor offers an auto-sizing feature, however, the image will shift in size (especially vertical height) for each different different video mode. Now that you have a focused, centered centered image, it should be set to the proper width width and height. Remember that image image size depends on the CRT size, so you will have to check the specifications for your particular monitor. monitor. If you do not have specifications available (or they do not specify image dimensions), you can at least approach a properly proportioned image using alignment software. Select the Convergence test (crosshatch) pattern from the main menu. menu. This pattern proproduces a grid, and each square of the grid should be roughly square. square. If the image is pro portioned correctly, correctly, no further action action is needed. Figure 57-2 uses VR301 VR301 to control vertical height. Slowly adjust the vertical height until until the grid squares are actually about square. The entire grid will will be a rectangle that that is wider than it is high. If the overall image is too small, you can adjust the horizontal width (VR207 is shown in Fig. 57-2) to make the grid wider, then adjust the vertical height again to keep the grid squares in a square shape. Of course, if the image is too large, you can reverse this procedure to shrink the image. If your monitor is a multi-mode design and is able to display images in several different graphics modes, there might be several independent vertical height adjustments—one adjustments—one for each available mode. You will have to check the test mode that you have selected selected against the vertical control to be sure that the vertical height control you are changing is appropriate for the test mode mode being used. If you are using a 640-×-480 graphics mode, for exam ple, you should be adjusting the vertical height control for the monitor’s 640- ×-480 mode. If the monitor offers an auto-sizing feature that automatically compensates image size for changes in screen mode, a vertical height adjustment might not be available on the main PC board.
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HORIZONTAL AND VERTICAL LINEARITY The concept of linearity is often difficult to grasp because there are so few real-life exam ples for us to draw from. Linearity is best related related to consistency—everything consistency—everything should be the same as everything else. For a computer monitor, there there must be both horizontal and vertical linearity linearity for an image to appear properly. An image is formed as a series series of horizontally scanned lines. Each line should be scanned at the same speed from start start to finish. If horizontal scanning speed fluctuates, vertical lines will appear closer together (or farther apart) than they actually actually are. Circles will appear compressed compressed (or elongated) in the the horizontal direction. Each horizontal line should be spaced exactly the same vertical distance distance apart. If the spacing between scanned lines should vary, horizontal lines will appear appear closer together (or farther apart) than they actually actually are. Circles will appear compressed compressed (or elongated) in the vertical direction. Any “non-linearity” will result result in distortion to the image. Before testing, you should understand that the screen mode will have an effect on the test pattern. When screen modes change, the the vertical height of the image will will also change. This is especially prevalent in multi-frequency and multi-mode monitor designs, which can display images images in more than one graphics graphics mode. Before testing for linearity, the the height and width of the image should be set properly for the selected screen mode, as described in the previous previous procedures. Otherwise, the image image might appear compressed compressed or elongated, resulting in a false diagnosis. Select the Linearity the Linearity test (lines and circles test) from the alignment software main menu. A grid will appear with an array of five circles (Fig. 57-6). 57-6). Observe the test pattern carefully. The spacing between between each horizontal horizontal line should should be equal. The spacing between between each vertical line should be equal. Assuming the vertical height and horizontal width width are set properly, each of the the five circles should appear appear round and even. Each grid square should appear square. If the image appears correct, correct, no further action is needed. needed. If the vertical lines are not spaced evenly apart, there might be a horizontal linearity problem. Find the horizontal linearity linearity adjustment adjustment (Fig. 57-2 shows VR202). VR202). Be sure to mark the starting point, then slowly adjust the control until the horizontal linearity im proves. If there is no improvement in one direction (or linearity linearity worsens), return to the starting point and try the adjustment adjustment in the opposite direction. If there is still no improvement (or linearity worsens again), return the control to its starting position and take no further action—the fault might be in the horizontal drive circuit.
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FIGURE 57-6
Linearity test pattern.
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If the horizontal lines are not spaced evenly apart, there might be a vertical linearity problem. Find the vertical linearity linearity adjustment adjustment (Fig. 57-2 shows VR201). VR201). Mark the starting point, then slowly adjust the control until until the vertical linearity improves. improves. If there is no improvement in one direction (or linearity worsens), return to the starting point and try the adjustment in the opposite direction. If there is still no improvement (or linearity worsens worsens again), return the control to its starting position and take no further action—the fault might be in the vertical drive circuit. Refer to Chapter 27 on monitor troubleshooting troubleshooting for detailed service procedures.
STATIC STATIC CONV ERGENCE Convergence is a concept that relates relates expressly to color CRTs. CRTs. A color CRT produces three electron beams—one beams—one for each of the primary colors colors (red, green, and blue). These electron beams strike color color phosphors on the CRT face. By adjusting the intensity intensity of each electron beam, any color color can be produced—including produced—including white. The three electron electron beams must converge at the shadow the shadow mask , which is mounted just behind behind the phosphor layer. The shadow mask maintains color purity by allowing the beams to impinge only where needed (any stray or misdirected misdirected electrons are physically blocked). blocked). Without the shadow mask, mask, stray electrons could excite adjacent color phosphors and result in strange or unsteady colors. If the beams are not aligned properly, properly, a beam might pass through through an adjacent mask aperture and excite an undesired color dot. Proper convergence is important important for a quality color display. It is simple matter to check convergence. Be certain to allow at least 15 minutes for the monitor to warm warm up. Select the Convergence test (dots) from the alignment software main menu. An array of white dots should should appear on the display. Observe the dots dots carefully. If you can see any “shadows” of red, green, or blue around the dots, convergence alignment might be necessary. necessary. If the dot pattern pattern looks good, then select Convergence test (crosshatch) pattern from the main main menu. A white grid should appear. Once again, observe observe the display carefully to locate any primary color “shadows” that might appear around the white lines. If the crosshatch pattern looks good, no further further action is necessary, so press any key to return to the main menu. If you determine that a convergence alignment is necessary, be sure to select the Convergence test (crosshatch) from the main menu. Locate the convergence rings rings located on the CRT’s neck, just behind behind the deflection yokes (Fig. (Fig. 57-7). Using a fine-tip black marker, mark the starting position of each convergence ring, relative to the glass CRT neck. This is a vital step because it will allow you to quickly return the rings to their original positions if you run into into trouble. Convergence alignment alignment is delicate, so it is easy easy to make the display much worse if you are not very careful. careful. Also, this alignment must must be performed with monitor power applied, so be extremely careful to protect yourself from shock hazards. The alignment process is not difficult, difficult, but requires a bit of practice and patience to become proficient. As the following procedure procedure shows, you will align the red and blue electron beams to make “magenta,” then you will align the green electron beam over the magenta pattern to make make “white.” Keep in mind that this procedure procedure uses the monitor alignment software (MONITORS) (MONITORS) included on the Companion CD. While the crosshatch pattern is displayed, press the letter “M” on the keyboard which will switch the crosshatch crosshatch pattern to a magenta magenta color. Magenta is a combination combination of blue
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Deflection yoke assembly
Purity rings Magenta convergence rings White convergence rings Convergence locking rings
FIGURE 57-7
A typical convergence-ring assembly.
and red (by choosing magenta, the green electron beam is effectively shut down, so there is less clutter in the display). display). Loosen the metal band holding the the rings in place. Do not remove the band. You might also have to loosen a locking ring before before moving the convergence rings. Move the magenta convergence rings together together or separately until the blue and red shadows overlap to form a uniform magenta magenta crosshatch pattern. Be sure to move these rings only in small, careful careful steps. By moving the rings together, you adjust adjust red and blue overlap in the vertical lines. By moving the rings separately, you adjust red and blue overlap in the horizontal lines. If you get into trouble, use the starting marks to return the rings to their original locations and start again. Once the magenta pattern is aligned, press the letter “W” on the keyboard, which will switch the crosshatch pattern to its original white color (thus activating the green electron beam). Adjust the white convergence convergence rings until any green shadows overlap overlap the crosshatch to form a uniform white grid, grid, as desired. As with the magenta rings, moving moving the white convergence rings rings together will adjust green green overlap in the vertical lines. lines. Moving the white convergence rings separately separately will adjust green overlap in the horizontal horizontal lines. If the image appears white, white, carefully secure the locking locking ring and setup band. Recheck the convergence as you tighten tighten the assembly to be sure nothing nothing has shifted. Do not overtighten the setup band—you stand a good chance of damaging the CRT.
DYNAMIC CONVERGENCE You will probably hear convergence referred to as being static being static and dynamic and dynamic.. These terms refer to the convergence in different areas of the display. Static refers to the convergence in the center area of the display. Dynamic refers to the convergence around the perimeter of the display. Although static convergence provides a good overall alignment with a minimum of fuss, dynamic convergence is a more difficult alignment because it requires inserting rubber wedges between the edge of the deflection yoke(s) and the CRT funnel—touchy procedures even for a practiced practiced hand. If a visible misconvergence misconvergence is around the display
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FIGURE 57-8
Calibrating dynamic convergence.
perimeter—even after a careful static convergence alignment, you will have to consider a dynamic convergence procedure. As you might expect, dynamic convergence is perhaps the most unforgiving alignment procedure—once you remove the wedges or alter their positions, it is extremely difficult to restore them (even with with alignment marks). Unfortunately, there there are no formal procedures for positioning the wedges, wedges, so you are often left to your own trial-and-error trial-and-error calls. In the end, you should avoid dynamic convergence adjustments, if possible. Dynamic convergence alignment alignment requires several important steps. steps. First, start the white white convergence (crosshatch) pattern and allow the monitor to warm up for at least 15 minutes. Then, mark and remove all three wedges (be sure they are free to move). When you tilt the deflection yoke up and down, you will see distortion (as illustrated in the “A” portion of Fig. 57-8). As you see, misconvergence misconvergence increases increases near the screen screen edge. Use the first two wedges to align the up/down positioning of the deflection yoke—the distortion shown in Fig. 57-8 should disappear. Next, when you tilt the deflection yoke right and left, you will see distortion (as illustrated illustrated in the “B” portion of Fig. 57-8). Use the third wedge to elimeliminate that distortion. You might have to tweak the three wedges wedges to optimize the dynamic convergence. Finally, you should secure the wedges in place with with a dab of high-temperature/high-voltage epoxy. The problem with dynamic convergence is that you often make the display worse when removing the wedges (even with installation marks) because you typically have to break the hold of any epoxy and wrench the wedges free. As a consequence, it is extremely extremely dif-
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ficult to just “tweak” an existing dynamic convergence calibration—in calibration—in virtually all cases, it is an all-or-nothing proposition. proposition. With this in mind, you should evaluate evaluate the need for dynamic convergence very carefully carefully before proceeding. If you do choose to proceed, be sure to leave yourself plenty of time.
COLOR PURITY Another concern is color purity—that is, a solid color should have the same hue across the entire display. If discoloration develops in the display, display, purity might need to be restored by degaussing (demagnetizing) (demagnetizing) the monitor. Typically, the discoloration discoloration follows a semicircular pattern around one side or corner with several bands of different color distortions (Fig. 57-9)—the 57-9)—the color banding appears almost like like a rainbow. Sometimes the discoldiscoloration involves the entire screen, but that is rare. Such discoloration might be caused by an externally induced magnetic field that has permanently magnetized magnetized some material in the monitor. monitor. The three CRT electron electron beams are guided to their appropriate appropriate phosphor dots by a magnetic deflection deflection system. The beams converge and pass through a shadow mask near the phosphor surface, ensuring that the red beam hits the red phosphor, the blue beam hits the blue phosphor, and the green beam hits the green phosphor. If some component within the CRT (frequently, (frequently, it is the shadow mask itself) has become sufficiently magnetized, the beams receive an undesired deflection and will not land on the appropriate phosphor (or will land partly on one color and partly on another). The result is an impure color color that arcs around the magnetized magnetized area. Location is an important clue clue to this problem. If the discoloration moves moves or disappears when the monitor is moved, it is not being caused by a permanent magnetization, but by some magnetic interference interference in or near the monitor. Placing a highly magnetic magnetic or electromagnetic source (such as a strong industrial magnet or power supply) on or near the monitor can cause such discoloration. discoloration. If the discoloration does not move when the monitor monitor is moved, it might be caused by permanent magnetization magnetization in the shadow mask. In that case, degaussing is necessary. Checking color purity is a straightforward straightforward procedure. Select the White purity test from test from the main menu. menu. A white box box will fill the entire screen. screen. If there are any areas of discoloration, degaussing is probably necessary. necessary. Degaussing removes permanent magnetization magnetization by introducing an alternating magnetic field that is stronger than the offending permanent
Discolored areas
FIGURE 57-9
Typical locations of color banding (color distortion).
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MONI MONITO TOR R TEST TESTIN ING G AND AND ALIG ALIGNM NMEN ENT T
magnetization. This field will energize energize the magnetic domains of the the material and induce an alternating magnetic field. Then, if the amplitude of the alternating alternating magnetic field is gradually reduced to zero, the magnetic domains in the material will be left disorganized and scrambled. This effectively demagnetizes demagnetizes the monitor. The easiest way to degauss a monitor is often to let the monitor do it. All modern color monitors have built-in degaussing degaussing coils and circuits. There will be a thick, black coil of wire wrapped in tape or other insulation insulation surrounding the CRT face plate. Usually, it is coiled around the CRT behind its mounting ears. That is the internal degaussing coil. The coil is connected to the ac supply through a thermistor thermistor current-limiting circuit. circuit. The thermistor has a low resistance when cold and a higher resistance when warm (typically a 10:1 ratio). It is in series with the degaussing degaussing coil so that when started cold, a large current current will flow through the coil, then will will decrease to a low value. The internal degaussing degaussing coil thus automatically degausses degausses the monitor every time time it is turned on. This degaussing occurs while the monitor screen is blank (the video system has not yet initialized) so that the resulting discoloration during auto-degaussing auto-degaussing is not visible. Unfortunately, design limitalimitations reduce the magnetic magnetic field strength available available from internal internal degaussing coils. That limits the amount of permanent magnetization that can be neutralized by internal degaussing. If a monitor has been strongly magnetized, internal internal degaussing might not be enough and discoloration eventually results. Manual degaussing requires a hand-held degaussing degaussing coil. You might have to search a bit to find one, but they are available. The basic principle involved in operating a manual degaussing coil is the same as the auto-degaussing assemblies already in place on color monitors—introduce a strong alternating magnetic field, then slowly reduce its amplitude to zero. Start the companion companion software (if it is not already running) and select the White purity test from test from the main menu. A white rectangle should should fill the entire image. image. Discoloration should be visible. Hold the degaussing coil near the monitor, flip the the degaussing coil switch on and slowly move the coil away from the monitor monitor as smoothly as you can. The image will discolor drastically drastically when the degaussing coil is activated. activated. When the coil is at arm’s length from the monitor flip flip the degaussing switch off. You might need to repeat this procedure several times. times. When the monitor is degaussed properly, properly, the white image should be consistent at all points on the display. Excessive degaussing can damage the monitor. Wait between 15 to 30 30 minutes before repeating a degaussing procedure.
COLOR DRIVE Once color convergence and purity are set correctly, you should turn your attention to the color drive levels (also known as white balance). balance). Select the White purity test from the alignment software main main menu. A white box will fill the the entire screen. Set the screen contrast to its highest level and reduce brightness brightness to its middle (detent) position. position. The background raster should disappear. Ideally, all three color signal signal levels should be equal and the resulting image should be a pure white—rather like a blank piece of white photocopier paper. However, judging the quality quality of a display color is largely a subjective evaluation. evaluation. You will need an oscilloscope to measure the actual voltage level of each color signal in
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order to set them equally. equally. If you do not have an oscilloscope (or do do not have access to one), do not attempt to adjust the color drive settings “by-eye.” Use your oscilloscope to measure the signal levels being generated by the red, green, and blue video drivers. These three color signals are actually actually driving the CRT. CRT. With the full white pattern being displayed, all three color signals should be equal (probably around 30 volts, although your own monitor might might use slightly different signal levels). levels). Even if you are not quite sure what the level should be, all three signals must be set to the same level to ensure a white image. If you do not know what the level should be, find the highest of the three color signals and use that as a reference. Adjust the gain levels of the other two colors until both levels match the reference. reference. Reduce contrast and inspect the image again. It should remain white white (and all three signals should be equal). equal). Disconnect your oscilloscope and press any key to return to the main menu.
CLEANING AND VACUUMING Once the monitor is checked and aligned, your final step before returning the unit to service should be to inspect the housings and PC boards for accumulations of dust and debris. Look for dust accumulating in the housing housing vents. A monitor is typically cooled by convection (hot air rises, rises, drawing in cooler air from from the lower vents). If these vents become clogged, heat will build up inside the monitor and lead to operational problems and perhaps even cause a premature breakdown. breakdown. Dust is also conductive. If enough dust builds builds up within the monitor, the dust might short circuit two or more components and cause operational problems. Vacuum away any dust or debris that might have accumulated in the outer housing. When you see dust buildup around around the monitor PC boards and and CRT, turn off and unplug the monitor, then vacuum away any buildups. buildups. Carefully re-assemble re-assemble the monitor’s housing(s) and return it to service.
Further Study That concludes Chapter Chapter 57. Be sure to review the glossary glossary and chapter questions on the the accompanying CD. If you have access to the Internet, take a look at some of these monitor maintenance resources: AnaTek: http://www.anatekcorp.com Repair World: http://www.repairworld.com
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