The ability to display images and information has evolved right along with CPUs, memory, hard drive space, and all of the other computer attributes that we associate with PC performance. Although the essential principles of a monitor have remained virtually unchanged, the small, drab monochrome displays of just a decade ago have been almost entirely replaced by flicker-free, high-resolution monitors capable of producing photo-realistic color images (Fig. 4-1). Today's monitor is more than just an output device - it has become our window into the complex virtual world created by computers. This chapter shows you what is inside the typical color monitor, and provides some guidelines for monitor disassembly and re-assembly.

As you can see from Fig. 4-2, a typical computer monitor is not terribly complicated. Compared to notebook computers and low-profile desktop systems, the monitor assembly is spacious. This is not an accident - monitors require substantial amounts of energy for operation. Much of this energy is dissipated as heat. Extra space prevents a buildup of heat from damaging the monitor's circuitry, and heat is allowed to escape through ventilation slots in the enclosure. Another reason for ample enclosure space is to ensure ample high-voltage insulation. Some monitors generate up to 30kV during normal operation (sometimes more for very large monitors), and normal plastic wire insulation is hardly sufficient to ensure safety. High-voltage insulation and plenty of unobstructed keep high-voltage from arcing to other circuits. The typical monitor can be broken down into five sections; the enclosure, the CRT, a CRT drive board (or "video drive" board), a raster drive board, and a power supply.

Monitor enclosures are built as two pieces, The front enclosure (marked 3) is used to mount the CRT and degaussing coil. This is bolted to a frame (marked 12) which forms the base of the monitor. Once other circuit boards are attached to the frame, the rear enclosure (marked 17) forms a shroud over almost all of the monitor. In most cases, the rear enclosure can be freed by removing four screws (marked 18) as shown in Fig. 4-2. A few monitor enclosures are held together by plastic latches in addition to screws. If the rear enclosure does not slide away easily, suspect the presence of snap-in latches, or extra screws installed into the frame from the bottom.

Although color monitors rely on extra video circuitry to process color signals, it is the design and construction of the CRT itself (marked CRT in Fig. 4-2) that really makes color monitors possible. The basic principles of a color CRT (Fig. 4-3) are very similar to a monochrome monitor - electrons "boil" off the cathode and are accelerated toward the phosphor-coated front face by a high positive potential. Color CRTs use three cathodes and video control grids - one for each of the three primary colors. Control (brightness), screen, and focus grids serve the same purpose as they do in monochrome CRTs. The control grid regulates the overall brightness of the electron beams, the screen grid begins accelerating the electron beams toward the front screen, and the focus grid narrows the beams. Once the electron beams are focused, vertical and horizontal deflection coils (or deflection yokes) apply magnetic force to direct the beams around the screen.

You will notice a shadow mask added to the color CRT. A shadow mask is a thin plate of metal that contains thousands of microscopic perforations - one perforation for each screen pixel. The mask is placed in close proximity to the phosphor face. There is also a substantial difference in the screen phosphors. Where a monochrome CRT uses a homogeneous layer of phosphor across the entire face, a color CRT uses phosphor triads as shown in Fig. 4-4 (the distance between the shadow mask and phosphor screen is shown greatly exaggerated). Red, green, and blue phosphor dots are arranged in sets such that the red, green, and blue electron beams will strike the corresponding phosphor. In actual operation, the color dots are so close together that each triad appears as a single point (or pixel). A degaussing coil (shown in Fig. 4-2) mounted in front of the CRT works to keep the shadow mask demagnetized.

Color CRTs must also be more precise in how the three electron beams are directed around the screen. Since there are now three phosphors instead of just one, it is critical that each electron beam strike only its corresponding phosphor color - not adjoining phosphors. This is known as color purity. A purity magnet added to the CRT yoke helps to adjust fine beam positioning. By using a shadow mask, the electron beams are only allowed to reach the phosphors where there are holes in the mask. Also realize that each of the three electron beams must converge at each hole in the shadow mask. A convergence magnet added to the CRT yoke adjusts beam convergence in the display center (known as static convergence), while a convergence coil driven by the raster circuitry optimizes beam convergence at the edges of the display (known as dynamic convergence). It is this delicate balance of purity and convergence adjustments, as well as the presence of a shadow mask, that give today's color monitors such rich, precise color.

The CRT drive board (marked 31 in Fig. 4-2) attaches directly to the CRT pins through a circular connector. Control (brightness), screen, and focus grid voltages are applied to the CRT through this board. The CRT drive board also contains the red, green, and blue video amplifiers and drivers. Since more CRT drive circuitry is needed for a color monitor than a monochrome monitor, the CRT drive board for a color monitor is usually much larger than that of a monochrome monitor. Once the monitor is unplugged and discharged, make sure that this board is attached evenly and securely to the CRT. It is the CRT drive circuit which regulates the strength of each electron beam by adjusting signal strength on the corresponding video control grid in the CRT. The CRT drive circuit must convert a small video signal (usually no more than 0.7 volts) into a signal large enough to drive the CRT (typically around 50 volts). For color monitors with three analog video lines, three separate video drive circuits are required. Problems can strike the CRT drive circuits in a number of ways, but there are clues to help guide your way. If the display should disappear, but the raster remains (raster is that dim haze you see by turning up the monitor's brightness), the video signal may have failed at the video adapter board in your PC. If there is suddenly not enough (or far too much) red, green, or blue in the displayed image, the corresponding DAC (digital-to-analog converter) on the video adapter may have failed, or the corresponding CRT drive circuit in the monitor may have broken down. Try a known-good monitor. If the correct image appears, you know the video adapter is producing the desired output, and the original monitor is probably defective. If no display appears on a known-good monitor, suspect the video adapter board in your PC. If the screen is black, suffers from fixed brightness (with or without video input), or looses focus, one or more grids in the CRT may have shorted and failed. Refer to Chapter 33 for detailed instructions on monitor troubleshooting.

The main raster board contains the vertical raster, horizontal raster, and high-voltage circuits that actually drive the CRT and direct the electron beam(s) around the screen. Depending on the design of your particular monitor, the raster board may contain part or all of the power supply circuit as well, along with some microcontroller-driven circuitry to operate on-screen monitor adjustments. Just about all monitors mount the raster board directly to the frame horizontally below the CRT neck. This assembly can be difficult to remove since it is obstructed by the CRT neck and yoke, as well as the interconnecting wiring that connects to the power supply, front panel controls, and flyback transformer.

The vertical drive circuit is used to operate the vertical deflection yoke. This is accomplished with a vertical sweep oscillator which is little more than a free-running oscillator set to run at either 60 or 70/72 Hz (depending on the design of the particular monitor). When the oscillator is triggered, it produces a sawtooth wave - the start of the sawtooth wave corresponds to the top of the screen, while the end of the sawtooth wave corresponds to the bottom of the screen. When the sawtooth cycle is complete, there is a blank period for blanking and retrace. One vertical sweep will be accomplished in less than 1/60th of a second (or 1/70th or 1/72nd of a second depending on the monitor).

Trouble with the vertical drive circuit usually strikes the vertical output driver circuit. If part of the driver should fail, either the upper or lower half of the image will disappear. If the entire driver should fail, the screen image will compress to a straight horizontal line in the center of the screen (there would be no vertical deflection - only horizontal deflection). Another problem is vertical oversweep which elongates the picture to the extent where it "wraps back" on itself in the lower portion of the screen. The area where the vertical image oversweeps will appear with a whitish haze and is typically the fault of the vertical oscillator circuit.

The horizontal drive circuit is the second part of the color monitor's raster circuit, and it is designed to operate the horizontal deflection yoke. This is accomplished with a horizontal oscillator which is little more than a free-running oscillator set to run at a frequency between 15kHz and 48kHz. A CGA monitor will typically use a horizontal sweep frequency of about 15.75kHz. The actual oscillator may be based on a transistor, but is usually designed around an integrated circuit which is more stable at the higher frequencies that are needed. When a horizontal synchronization trigger pulse is received from the video adapter board, the oscillator is forced to fire. When the oscillator is triggered, it produces a square wave. The start of the square wave corresponds to the left side of the screen. When the cycle is complete, there is a blank period for blanking and retrace. At an operating frequency of 31.5kHz, one horizontal sweep will be accomplished in about 31.7m S.

Trouble with the horizontal drive circuit usually strikes the horizontal output drive circuit since that is the circuit that sustains the greatest stress in the monitor. If the drive circuit should fail, the entire image will disappear since high-voltage generation will also be effected. Unfortunately, a fault in the horizontal oscillator will also result in an image loss since high-voltage generation depends on a satisfactory horizontal pulse. If the horizontal oscillator or amplifier fails, high-voltage fails as well, and the image becomes too faint to see - this makes troubleshooting horizontal problems a bit more difficult than troubleshooting vertical problems.

The high-voltage system is actually part of the horizontal drive circuit. A monitor's power supply generates relatively low voltages (usually not much higher than 140 volts). This means that the high positive potential needed to excite the CRT's anode is not developed in the power supply. Instead, the 15 to 30kV or more needed to power a CRT anode is generated from the horizontal output. The amplified, high-frequency pulse signal generated by the horizontal driver circuit is provided to the primary winding of a device known as the flyback transformer (or FBT). It is the FBT which produces the high-voltage. The principle is similar to the ignition system used in automobiles.

The power supply is typically a hand-sized assembly that converts AC into several DC voltage levels (usually +135, +20, +12, +6.3, and +87 volts DC) that will be needed by other monitor circuits. The AC itself may be filtered and fused by a separate small assembly near the monitor’s base. If there is no stand-alone power supply board in your particular monitor, the supply is probably incorporated into the raster board. As you saw earlier, the only voltage that is not produced in the power supply is the high-voltage source. A stand-alone power supply is typically mounted vertically to the frame. The metal frame not only provides a rigid mounting platform, but it serves as a chassis common, and helps to contain RF signals generated by the monitor.

PC monitors have traditionally been analog devices that used manually-adjusted controls to configure proper operation. However, as monitor sizes, viewing areas, and resolutions continue to increase, users demand more control over the image’s appearance. The use of microcontrollers in large modern monitors allows many display adjustments to be made through the front control panel which otherwise would require a tedious and time-consuming internal alignment. Such changes can then be easily saved in the monitor’s internal memory. Since adjustments can be set for major resolutions independently, the monitor can "remember" your optimum display configuration for your most frequently used display modes - there is almost no "tinkering" with the monitor each time you change a screen mode. On-screen controls are listed through a series of icons which indicate the general action of each control. Figure 4-5 illustrates some of the most popular icons for basic on-screen monitor adjustments.

The basic on-screen controls may seem overwhelming at first glance - that’s understandable because most PC users never get to use more than "brightness" and "contrast" knobs. Still, the basic controls are designed to help you set the overall image position and quality for your current display mode.

NOTE: Some of these adjustments can cause severe image distortion if set improperly. Before making any adjustments (beyond contrast and brightness), be sure to note the starting level of each adjustment, or find the "factory default" button which can restore default levels automatically.

Horizontal size - also called H-size. This makes the image fatter or thinner. If the image is too wide (where one or both ends are "lost" beyond the edges of the display area), you can use the H-size control to pull the image inside the display area.

Horizontal phase - also called H-phase or H-posi. This control lets you position the image left or right in the display area. For example, if the image is too far to the right, use H-phase to shift the image to the left. You may use H-phase and H-size alternately in order to size the image properly.

Vertical size - also called V-size. This makes the image taller or shorter. If the image is too tall (where one or both ends are "lost" beyond the top and bottom of the display area), you can use the V-size control to pull the image inside the display area.

Vertical phase - also called V-phase or V-posi. This control lets you position the image higher or lower in the display area. For example, if the image is too low, use V-phase to shift the image upward. You may use V-phase and V-size alternately in order to size the image properly.

PCC amp - this is also known as the pincushion adjustment. Use this control to straighten the left and right sides of the image. If the PCC control is set too low, the image will bow outward. If the PCC control is set too high, the image will draw inward. Ideally, the sides of the image should be straight.

PCC phase - this is also known as trapezoidal adjustment. Use this control to make the image perfectly rectangular. If the adjustment is set too low, the top of the image may be narrower than the bottom. If the adjustment is set too high, the top of the image may be wider than the bottom.

Pin balance - this is also known as the curvature adjustment. Use this adjustment also to straighten the image. If the control is too high, the image may curve to the left. If the image is too low, the image may curve to the right. Note that this is not a pincushion (PCC amp) adjustment because the left and right sides of the image are being effected in the same direction.

Key balance - this may also be referred to as a slant control or tilt adjustment. Use this adjustment also to straighten the image. If the control is set too high, the top of the image may be pulled right and the bottom of the image may be pulled left. If the control is set too low, the top of the image may be pulled left, and the bottom of the image may be pulled right.

Rotation - this may also be referred to as the twist adjustment. This control effects the rotation of the entire image in the display. Ideally, the image should appear straight in the display - the two bottom corners of the image should be exactly the same distance from the desk.

Horizontal static - this is an adjustment of color alignment. Use this control to adjust the alignment of the red, green, and blue electron beams.

Color purity - you may also see this called color balance. Ideally, white should be a "pure" white - containing the exact same amounts of red, green, and blue. However, age may effect CRT color guns and video driver levels, so you can tweak the RGB settings to restore color purity.

Moire level - moire is a form of distortion which occurs when certain conditions of resolution, dot pitch, screen size, and image coloring are met. The moire pattern usually appears as wavy or elliptical patterns. Use the Moire level control to adjust the amount of moire distortion that may appear in an image.

The advanced on-screen image adjustments are often quite similar to the basic adjustments, but advanced adjustments allow more precise and subtle corrections - especially in the corners of an image which are the most difficult to set correctly. Figure 4-6 highlights the advanced controls which you’ll encounter most frequently.

Vertical linearity - also called V-lin. Linearity is the geometric "correctness" of the display. For example, if there is an image of small colored boxes the same size through out, each box should "appear" to be the same size. If not, linearity might be a problem. The linearity control adjusts linearity in the vertical direction.

Vertical linearity balance - also called V-lin balance. The linearity balance control effectively centers the linearity of the display’s vertical axis. For example, it may be necessary to shift the linearity balance when changing linearity in order to avoid making uniform linearity changes across the entire display.

Center PCC - also called center pincushion. This is a precision adjustment that allows you to tweak the pincushion adjustment near the vertical center of the image, instead of across the entire left and right sides of the image. This adjustment should only be used when there is limited pincushioning around the middle of the image.

Corner PCC - also called corner pincusion. This is a precision adjustment that allows you to tweak the pincushion adjustment near the corners of image, instead of across the entire left and right sides of the image. This adjustment should only be used when there is limited pincushioning around the edges of the image.

Center balance - This feature (similar to pin balance) adjusts the curvature of the left and right sides of the display near the vertical center of the image, instead of across the entire image. Use this adjustment to correct minor curvature in the center of the image.

Corner balance - This feature (similar to pin balance) adjusts the curvature of the left and right sides of the display at the corners of the image, instead of across the entire image. Use this adjustment to correct minor curvature at the corners of the image.

Clamp pulse position - This feature is not needed with RGB inputs such as those provided by your 15-pin video cable, but may be needed when using Sync-on-Green signals through a BNC connector. Clamp pulse controls allows you to eliminate the excessive green or white background that can occur when using Sync-on-Green or external sync signals at the monitor.

Purity - This feature allows you to adjust the color purity - or color "uniformity" of the display. Do not adjust this level unless it is absolutely necessary.

The process of monitor disassembly is remarkably straightforward. In most cases, only the rear enclosure must be removed to expose the entire inner workings of the monitor. The rear enclosure itself is typically held in place with only four screws (there may be additional screws inserted at the bottom). On some occasions, you may also encounter a number of plastic latches, but this is rare. After removing the rear enclosure, you will see the bell and neck of the CRT, the CRT drive board, the raster board, and the power supply (if a separate supply is used).

NOTE: The computer monitor operates with exposed voltages that are potentially lethal! This makes monitors unusually dangerous in the hands of novice or inexperienced troubleshooters. Make sure that the monitor is unplugged and allowed several minutes to discharge before reaching into the assembly. Do not operate the monitor without its X-ray and RF shields in place (if applicable). It is also advisable to work with a second person nearby.

You should take note of any metal shrouds or coverings that are included with the monitor assembly. Metal shielding serves two very important purposes. First, the oscillators and amplifiers in a monitor produce radio-frequency (rf) signals that have the potential to interfere with radio and TV reception. The presence of metal shields or screens helps to attenuate any such interference, so always make it a point to replace shields securely before testing or operating the monitor. Second, large CRTs (larger than 17'') use very high voltages (25kV or more) at the CRT anode. With such high potentials, X-radiation becomes a serious concern. CRTs with lower anode voltages can usually contain X-rays with lead in the CRT glass. Metal shields are added to the larger CRTs as supplemental shielding in order to stop X-rays from escaping the monitor enclosure. When X-ray shielding is removed, it is vital that it be replaced before the monitor is tested and returned to service. X-ray shields will usually be clearly marked when you remove the monitor's rear cover.

Before removing any wiring or boards from the monitor, it is important to be sure that the CRT is fully discharged. Even though unplugging the monitor will prevent AC and high-voltage electrocution, there may still be enough high-voltage charge stored in the CRT to provide a fair kick to the careless. Make sure the monitor is turned off and allow several minutes for the AC supply to discharge. Use a regular-blade screwdriver with a heavy-duty alligator clip attached between the screwdriver shaft and the metal chassis. Gently insert the screwdriver blade under the high-voltage anode cap as shown in Fig. 4-7. You will probably hear a mild crackle as the CRT is grounded. Do not rotate the screwdriver or force it in the CRT - remember that the CRT is still a glass assembly, and excessive force can damage it easily. Once the crackling stops, remove the screwdriver and unplug the monitor's AC cord. The assembly should now be safe to work on.

Removing boards is often a simple matter. The CRT drive board is simply plugged into the CRT through a circular connector. Rock the video board back and forth gently to pull it away from the CRT. The raster board is typically mounted to the frame with several screws. After the screws are removed, the raster board should be free. When removing any board, be sure to make a careful note of each connector's location and orientation. A CRT is held in place with a metal bracket bolted to the front enclosure. Unfortunately, replacing the CRT usually means removing the video and raster boards along with the frame. If you must place the monitor (or front enclosure alone) face-down onto a work surface, be sure to use a layer of soft towels or foam to prevent scratches to the front enclosure or CRT.

The most important rule to remember when exchanging a board or CRT is to use an exact replacement part. Monitors are precisely timed, high-energy systems - so "close" doesn't count. An improper replacement assembly may cause the monitor to malfunction, or may it work only for a limited amount of time. When re-assembling a monitor, be extremely careful of the wiring interconnecting each board and the CRT. Be sure that all wiring and connectors are installed properly and completely. Loose connectors can cause erratic or intermittent operation. Pay close attention to wire paths - do not allow wiring to be pinched under boards or against the metal chassis. Finally, make it a point to re-install any rf or X-ray shielding that may have been removed during the repair.

Of all the PC peripherals, monitors are perhaps the most potentially dangerous in careless hands. The tips listed below will help you protect yourself from injury, and get the most working life from your monitor investment:

• For best monitor images, always keep the 15-pin video cable secured to the video adapter board. Also avoid using monitor cable extensions which can cause image ghosting.
• use simple ammonia-based cleaner to clean the monitor case, but never spray cleaner onto the monitor - only onto the cleaning cloth.
• CRT coatings (i.e. anti-glare coatings) can be extremely sensitive to chemicals, so never use any sort of cleaner other than demineralized water to clean the CRT. Remember to wet the cleaning cloth - not the CRT.
• Monitors use convection for cooling. Be sure to keep all of the vent openings on the monitor case unobstructed and free of dust and debris. Vacuum out any accumulations of dust within the monitor.
• When working on a monitor, always use a soft pillow or plush towel to cushion the CRT and avoid scratches.
• Monitors can contain potentially dangerous voltages. Be sure to keep the monitor turned off and unplugged before working inside it. Discharge the CRT’s high-voltage anode before performing any service.
• Do not use metal tools when working inside a monitor - especially when attempting to make alignments.
• When reassembling a monitor, be extremely careful to avoid damaging wiring and connectors.
• Monitors (especially large monitors) are extremely sensitive to the influences of magnetic fields. Be sure to keep any motorized or magnetic devices well clear of the monitor. Even unshielded multimedia speakers can adversely effect the monitor.
• Monitors are particularly heavy devices, so be very careful when lifting a monitor. Lift from the knees - NOT from the back. Hold the CRT face toward your chest.

This concludes Chapter 4. Be sure to review the glossary and chapter questions on the accompanying CD. If you have access to the Internet, point your web browser to some of the contacts below:

Anatek Corporation: http://www.anatekcorp.com

CTX: http://www.ctxintl.com/

NEC: http://www.nec.com

Sony: http://www.ita.sel.sony.com/support/displays/

Viewsonic: http://www.viewsonic.com/desk/desk.htm