From their humble beginnings as basic monochrome text displays, the monitor (Fig. 33-1) has grown to provide real-time photo-realistic images of unprecedented quality and color. Monitors have allowed real-time video playback, stunning graphics, and information-filled illustrations to replace the generic "command line" user interface of just a few years ago. In effect, monitors have become our "virtual window" into the modern computer. With many millions of computers now in service, the economical maintenance and repair of computer monitors represents a serious challenge to technicians and hobbyists alike. Fortunately, the basic principles and operations of a computer monitor have changed very little since the days of "terminal displays". This chapter explains the basic concepts behind today's computer monitors, and provides a cross-section of troubleshooting procedures.

While PCs are defined by a set of fairly well-understood specifications such as RAM size, hard drive space, and clock speed, monitor specifications describe a whole series of physical properties that PCs never deal with. With this in mind, perhaps the best introduction to monitor troubleshooting is to discuss each specification in detail and show you how each specification and characteristic effects a monitor's performance.

The cathode ray tube (or CRT) is essentially a large vacuum tube. One end of the CRT is formed as a long, narrow neck, while the other end is a broad, almost-flat surface. A coating of colored phosphors is applied inside the CRT along the front face. The neck end of the CRT contains an element (called the cathode) which is energized and heated to very high temperatures (much like an incandescent lamp). At high temperatures, the cathode liberates electrons. When a very high positive voltage potential is applied at the front face of the CRT, electrons liberated by the cathode (which are negatively charged) are accelerated toward the front face. When the electrons strike the phosphor on the front face, light is produced. By directing the stream of electrons across the front face, a visible image is produced. Of course, there are other elements needed to control and direct the electron stream, but this is CRT operation in a nutshell. CRT face size (or screen size) is generally measured as a diagonal dimension - that is, a 43.2cm (17") CRT is 43.2cm (17") between opposing corners. Larger CRTs are more expensive, but produce larger images which are usually easier on the eyes.

The picture element (or pixel) is the very smallest point that can be controlled on a CRT. For monochrome displays, a pixel may simply be turned on or off. For a color display, a pixel may assume any of a number of different colors. Pixels are combined in the form of an array (rows and columns). It is the size of that pixel array which defines the display's resolution. Thus, resolution is the total number of pixels in width by the total number of pixels in height. For example, a typical EGA resolution is 640 pixels wide by 350 pixels high (a total of 224000 pixels), while a typical VGA resolution is 640 pixels wide by 480 pixels high (a total of 307200 pixels). Typical Super VGA (SVGA) resolution is 800 pixels wide by 600 pixels high. Resolution is important for computer monitors since higher resolutions allow finer image detail.

While monochrome CRTs use a single, uniform phosphor coating (usually white, amber, or green), color CRTs use three color phosphors (red, green, and blue) arranged as triangles (or triads). Figure 33-2 illustrates a series of color phosphor triads. On a color monitor, each triad represents one pixel (even though there are three dots in the pixel). By using the electron streams from three electron guns - one gun for red, one for blue, and another for green - to excite each dot, a broad spectrum of colors can be produced. The three dots are placed so close together that they appear as a single point to the unaided eye.

The quality of a color image is related to just how close each of the three dots are to one another. The closer together they are, the purer the image appears. As the dots are spaced further apart, the image quality degrades because the eye can begin to discern the individual dots in each pixel. This results in lines that no longer appear straight and colors are no longer pure. Dot pitch is a measure of the distance between two adjacent phosphor dots on the display. This is also the same dimension for the distance between openings in a "shadow mask". Displays with a dot pitch of 0.31mm or less generally provide adequate image quality.

The shadow mask is a thin sheet of perforated metal that is placed in the color CRT just behind the phosphor coating. Electron beams from each of the three "electron guns" are focused to converge at each hole in the mask - not at the phosphor screen (Fig. 33-3). The microscopic holes act as apertures that let the electron beams through only to their corresponding color phosphors. In this way, any stray electrons are masked, and color is kept pure. Some CRT designs substitute a shadow mask with a slot mask (or aperture grille) which is made up of vertical wires behind the phosphor screen. The "dot pitch" for CRTs with slot masks is defined as the distance between each slot. Keep in mind that monochrome CRTs do not need a shadow mask at all since the entire phosphor surface is the same color.

Remember that three electron guns are used in a color monitor - the electrons themselves are invisible, but each gun excites a particular color phosphor. All three electron beams are tracking around the screen simultaneously, and the beams converge at holes in the shadow mask. This convergence of electron beams is closely related to color purity in the screen image. Ideally, the three beams converge perfectly at all points on the display and the resulting color is perfectly pure throughout (i.e. pure white). If one or more beams do not converge properly, the image color will not be pure. In most cases, poor convergence will result in colored shadows. For example, you may see a red, green, or blue shadow when looking at a white line. Serious convergence problems can result in a blurred or distorted image. Monitor specifications usually list typical convergence error as misconvergence at both the display center and the overall display area. Typical center misconvergence runs approximately 0.45mm, while overall display area misconvergence is about 0.65mm. Larger numbers result in poorer convergence. Fortunately, monitor convergence can be calibrated (see Chapter 63; "Monitor testing and alignment").

The front face of most CRTs is slightly convex (bulging outward). However, digital images are perfectly square (that is, two-dimensional). When a flat (2D) image is projected onto a curved (3D) surface, distortion results. Ideally, a monitor's raster circuits will compensate for this screen shape so that the image appears flat when viewed at normal distances. In actual practice, however, the image is rarely flat. The sides of the image (top-to-bottom) and (left-to-right) may be bent slightly inward or slightly outward. Figure 33-4 illustrates an exaggerated view of these effects. Pincushioning occurs when sides are bent inward making the image's boarder appear concave. Barreling occurs when the sides are bent outward making the image's boarder appear convex. In most cases, these distortions should be just barely noticeable (no more than 2.0mm or 3.0mm). Keep in mind that many technicians refer to barrel distortion as pincushioning as well, though this is not technically correct.

To understand what scanning is, you must first understand how a monitor's image is formed. A monitor's image is generated one horizontal line of pixels at a time starting from the upper left corner of the display (Fig. 33-5). As the beams travel horizontally across the line, each pixel in the line is excited based on the video data contained in the corresponding location of video RAM on the video adapter board. When a line is complete, the beam turns off (known as horizontal blanking). The beam is then directed horizontally (and slightly lower vertically) to the beginning of the next subsequent line. A new horizontal line can then be drawn. This process continues until all horizontal lines are drawn and the beam is in the lower right corner of the display. When this image "page" is complete, the beam turns off (called vertical blanking) and is redirected back to the upper left corner of the display to start all over again.

The rate at which horizontal lines are drawn is known as the horizontal scanning rate (sometimes called horizontal sync). The rate at which a complete "page" of horizontal lines is generated is known as the vertical scanning rate (or vertical sync). Both the horizontal and vertical blanking times are known as retrace times since the deactivated beams are "retracing" their path before starting a new trace. A typical horizontal retrace time is 5m S, while the typical vertical retrace time is 700m S. This continuous horizontal and vertical scanning action is generally referred to as raster.

We can easily apply numbers to scanning rates to give you an even better idea of their relationship. A typical VGA monitor with a resolution of 640x480 pixels uses a horizontal scanning rate of 31.5kHz. This means that 31500 lines can be drawn in 1 second, or a single line of 640 pixels can be drawn in 31.7m S. Since there are 480 horizontal lines to be drawn in one "page", a complete page can be drawn in (480 x 31.7m S) 15.2mS. If a single page can be drawn in 15.2mS, the screen can be refreshed 65.7 timer per second (65.7Hz) - this is roughly the vertical rate that will be set for VGA operation at 640x480 resolution. In actual practice, the vertical scanning rate will be set to a whole number such as 60Hz which leaves a lot of spare time for blanking and synchronization. It was discovered early in TV design that vertical scanning rates under 60Hz resulted in perceivable flicker that causes eye strain and fatigue. You can start to see now that horizontal scanning rates are not chosen arbitrarily. The objective is to select a horizontal frequency that will cover a page's worth of horizontal pixel lines for any given resolution at about 60 times per second (or even higher for reduced flicker). Table 33-1 compares typical monitor resolutions and scan rates.

Images are "painted" onto a display one horizontal row at a time, but the sequence in which those lines are drawn can be non-interlaced or interlaced. As you see in Fig. 33-6, a non-interlaced monitor draws all of the lines that compose an image in one pass. This is preferable since a non-interlaced image is easier on your eyes - the entire image is refreshed at the vertical scanning frequency - so a 60Hz vertical scanning rate will update the entire image 60 times in 1 second. A non-interlaced display draws an image as two passes. Once the first pass is complete, a second pass fills in the rest of the image. The effective image refresh rate is only half the stated vertical scanning rate. The typical 1024x768 SVGA monitor offers a vertical scanning rate of 87Hz, but since the monitor is "interlaced", effective refresh is only 43.5Hz - screen flicker is much more noticeable.

In the very simplest terms, the bandwidth of a monitor is the absolute maximum rate at which pixels can be sent to the monitor. Typical VGA displays offer a bandwidth of 30MHz. That is, the monitor could generate up to 30 million pixels per second on the display. Consider that each scan line of a VGA display uses 640 pixels and the horizontal scan rate of 31.45kHz allows 31450 scan lines per second to be written. At that rate, the monitor is processing (640 pixels/scan line x 31450 scan lines/second) 20128000 pixels/second - well within the monitor's 30MHz bandwidth. The very newest color monitors offer bandwidths of 135MHz. Such high-resolution 1280x1024 monitors with scanning rates of 79kHz would need to process at least (1280 pixels/scan line x 79000 scan lines/second) 101120000 pixels/second (101.12MHz), so enhanced bandwidth is truly a necessity for high resolutions.

The electron beam(s) that form an image are directed around a display using variable magnetic fields generated by separate vertical and horizontal deflection coils mounted around the CRT's neck. The analog signals that drive each deflection coil are produced by horizontal and vertical deflection circuitry. Ideally, deflection circuitry should steer the electron beam(s) precisely the same way in each pass. This would result in an absolutely rock-solid image on the display. In the real world, however, there are minute variations in the placement of images over any given period of time. Jitter is a term used to measure such variation over a 15 second period. Swim (sometimes called Wave) is a measure of position variation over a 30 second period. Drift is a measure of position variation over a 1 minute period. Note that all three terms represent essentially the same problem over different amounts of time. Swim, jitter, and drift may be expressed as fractions of a pixel or as physical measurements such as millimeters.

This specification lists signal levels and characteristics of the analog video input channel(s). In most cases, a video signal in the 0.7 Vpp (peak to peak) range is used. Circuitry inside the monitor amplifies and manipulates these relatively small signals. A related specification is input impedance which is often at 75 ohms. Older monitors using digital (on-off) video signals typically operate up to 1.5 volts.

After a line is drawn on the display, the electron beams are turned off (blanked) and repositioned to start the next horizontal line. However, no data is contained in the retrace line. In order for the new line to be "in sync" with the data for that line, a synchronization pulse is sent from the video adapter to the monitor. There is a separate pulse for horizontal synchronization and vertical synchronization. In most current monitors, synchronization signals are edge triggered TTL (transistor-transistor logic) signals. Polarity refers to the edge that triggers the synchronization. A falling trigger (marked "-" or "positive/negative") indicates that synchronization takes place at the high-to-low transition of the sync signal. A leading trigger (marked "+" or "negative/positive") indicates that synchronization takes place on the low-to-high transition of the sync signal.

In order to have a full understanding of color monitors, it is best to start with a block diagram. The block diagram for a VGA monitor is shown in Fig. 33-7. Three complete video drive circuits are needed (one for each primary color - red, green, and blue). While early color monitors used logic levels to represent video signals, current monitors use analog signals which allow the intensity of each color to be varied. The CRT is designed to provide three electron beams which are directed at corresponding color phosphors. By varying the intensity of each electron beam, virtually any color can be produced. For all practical purposes, the color monitor can be considered in three sub-sections; the video drive circuits, the vertical drive circuit, and the horizontal drive circuit (including the high-voltage system).

The schematic diagram for a typical RGB (Red, Green, and Blue) drive circuit is shown in Fig. 33-8. This schematic is actually part of a Tandy VGM-220 analog color monitor. You will see that there are three separate video drive circuits. Components with a 5xx designation (i.e. IC501) are part of the red video drive circuit. The 6xx designation (i.e. Q602) shows a part in the green video drive circuit. A 7xx marking (i.e. C704) indicates a component in the blue video drive circuit. Other components marked with 8xx designations (i.e. Q803) are included to operate the CRT control grid. Let's walk through the operation of one of these video circuits.

The red analog signal is filtered by the small array of F501. The ferrite beads on either side of the small filter capacitor serve to reduce noise that may otherwise interfere with the weak analog signal. The video signal is amplified by transistor Q501. Potentiometer VR501 adjusts the signal gain (the amount of amplification applied to the video signal). Collector signals are then passed to the differential amplifier circuit in IC501. Once again, noise is a major concern in color signals, and differential amplifiers help to improve signal strength while eliminating noise. The resulting video signal is applied to a "push-pull" amplifier circuit consisting of Q503 and Q504, then fed to a subsequent "push-pull" amplifier pair of Q505 and Q506. Potentiometer VR502 controls the amount of DC bias used to generate the final output signal. The output from this final amplifier stage is coupled directly to the corresponding CRT video control grid. The remaining two drive circuits both work the same way.

Problems with the video circuits in color monitors rarely disable the image entirely. Even if one video drive circuit should fail, there are still two others to drive the CRT. Of course, the loss of one primary color will severely distort the image colors, but the image should still be visible. You can tell when one of the video drive circuits fails; the faulty circuit will either saturate the display with that color or cut that color out completely. For example, if the red video drive circuit should fail, the resulting screen image will either be saturated with red, or red will be absent (leaving a greenish-blue or cyan image).

The vertical drive circuit is designed to operate the monitor's vertical deflection yoke (dubbed V-DY). To give you a broad perspective of vertical drive operation and its inter-relation to other important monitor circuits, Fig. 33-9 illustrates the vertical drive, horizontal drive, high-voltage, and power supply circuits all combined together in the same schematic. This schematic is essentially the main PC board for the Tandy VGM-220 monitor. Components marked with 4xx numbers (i.e. IC401) are part of the vertical drive system.

The vertical sync pulses enter the monitor at connector CH202 (the line marked "V"). A simple exclusive-OR gate (IC201) is used to condition the sync pulses and select the video mode being used. Since the polarity of horizontal and vertical sync pulses will be different for each video mode, IC201 detects those polarities and causes the digitally-controlled analog switch (IC401) to select one of three vertical size (V-SIZE) control sets which is connected to the vertical sawtooth oscillator (IC402). This mode-switching circuit allows the monitor to auto-size the display.

The vertical sync pulse fires the vertical sawtooth oscillator on pin 2 of IC402. The frequency of the vertical sweep is set to 60Hz, but can be optimized by adjusting the vertical frequency control (V-FREQ) VR404. It is highly recommended that you do NOT attempt to adjust the vertical frequency unless you have an oscilloscope available. Vertical linearity (V-LIN) is adjusted through potentiometer VR405. Vertical centering (V-CENTER) is controlled through VR406. Linearity and centering adjustments should only be made while displaying an appropriate test pattern. It is interesting to note that there are no discrete power amplifiers needed to drive the vertical deflection yoke - IC402 pin 6 drives the deflection yoke directly through an internal power amplifier.

The pincushion circuit forms a link between the vertical and horizontal deflection systems through the pincushion transformer (T304). Transistors Q401 and Q402 form a compensator circuit that slightly modulates horizontal deflection. This prevents distortion in the image when projecting a flat, two-dimensional image onto a curved surface (the CRT). Potentiometer VR407 provides the pincushion control (PCC). As with other alignments, you should not attempt to adjust the pincushion unless an appropriate test pattern is displayed.

Problems that develop in the vertical amplifier will invariably effect the appearance of the CRT image. A catastrophic fault in the vertical oscillator or amplifier will leave a narrow horizontal line in the display. The likeliest cause is the vertical drive IC (IC402) since that component handles both sawtooth generation and amplification. If only the upper or lower half of an image disappears, only one part of the vertical amplifier in IC402 may have failed. However, any fault on the PC board that interrupts the vertical sawtooth will disable vertical deflection entirely. When the vertical deflection is marginal (too expanded or too compressed), suspect a fault in IC402, but its related components may also be breaking down. An image that is over-expanded will usually appear "folded over" with a whitish haze along the bottom. It may also be interesting to note that vertical drive problems to not effect display colors.

The horizontal drive circuit is responsible for operating the horizontal deflection yoke (H-DY). It is this circuit that sweeps the electron beams left and right across the display. To understand how the horizontal drive works, you should again refer to the schematic of Fig. 33-9. All components marked 3xx numbers (i.e. IC301) relate to the horizontal drive circuit. Horizontal sync signals enter the monitor at connector CH202 (the line marked "H") and are conditioned by the executive-OR gates of IC201. Conditioned sync pulses fire the horizontal oscillator (IC301). Horizontal frequency should be locked at 31.5kHz, but potentiometer VR302 can be used to optimize the frequency. Do not attempt to adjust horizontal frequency unless you have an oscilloscope available. Horizontal phase can be adjusted with VR301. You should avoid altering any alignments until a suitable test pattern is displayed as discussed in Chapter 63.

IC301 is a highly-integrated device which designed to provide precision horizontal square wave pulses to the driver transistors Q301 and Q302. IC301 pin 3 provides the horizontal pulses to Q301. Transistor Q301 switches on and off causing current pulses in the horizontal output transformer (T303). Current pulses produced by the secondary winding of T303 fire the horizontal output transistor (Q302). Output from the HOT drives the horizontal deflection yoke (H-DY). The deflection circuit includes two adjustable coils to control horizontal linearity (H-LIN; L302) and horizontal width (H-WIDTH; L303). You will also notice that the collector signal from Q302 is directly connected to the flyback transformer (FBT). Operation of the high-voltage system is covered in the next section.

Problems in the horizontal drive circuit can take several forms. One common manifestation is the loss of horizontal sweep leaving a vertical line in the center of the display. This is generally due to a fault in the horizontal oscillator (IC301) rather than the horizontal driver transistors. The second common symptom is a loss of image (including raster), and is almost always the result of a failure in the HOT (or High-voltage Output Transistor circuit). Since the HOT also operates the flyback transformer, a loss of horizontal output will disrupt high-voltage generation - the image will disappear.

The presence of a large positive potential on the CRT's anode is needed in order to accelerate an electron beam across the distance between the cathode and CRT phosphor . Electrons must strike the phosphor hard enough to liberate visible light. Under normal circumstances, this requires a potential of 15,000 to 30,000 volts. Larger CRTs need higher voltages because there is a greater physical distance to overcome. Monitors generate high-voltage through the flyback circuit.

The heart of the high-voltage circuit is the flyback transformer (FBT) as shown in Fig. 33-9. The FBT's primary winding is directly coupled to the horizontal output transistor (Q302). Another primary winding is used to compensate the high-voltage level for changes in brightness and contrast. Flyback voltage is generated during the horizontal retrace (the time between the end of one scan line and the beginning of another) when the sudden drop in deflection signal causes a strong voltage spike on the FBT secondary windings. You will notice that the FBT in Fig. 33-9 provides one multi-tapped secondary winding. The top-most tap from the FBT secondary provides high-voltage to the CRT anode. A high-voltage rectifier diode added to the FBT assembly forms a half-wave rectifier - only positive voltages reach the CRT anode. The effective capacitance of the CRT anode will act to filter the high-voltage spikes into DC. You can read the high-voltage level with a high-voltage probe. The CRT needs additional voltages in order to function. The lower tap from the FBT secondary supplies voltage to the focus and screen grid adjustments. These adjustments, in turn, drive the CRT directly.

Trouble in the high-voltage circuit can render the monitor inoperative. Typically, a high-voltage fault manifests itself as a loss of image and raster. In many cases where the HOT and deflection signals prove to be intact, the flyback transformer has probably failed causing a loss of output in one or more of the three FBT secondary windings. The troubleshooting procedures in the next section of this chapter will cover high-voltage symptoms and solutions in more detail.

Before jumping right into troubleshooting, it would be helpful to understand how the circuits shown in Fig. 33-9 are assembled. A wiring diagram for the Tandy VGM-220 is shown in Fig. 33-10. There are two PC boards; the video drive PC board, and the main PC board. The main PC board contains the raster circuits, power supply, and high-voltage circuitry. The video drive PC board contains red, green, and blue video circuits. Video signals, focus grid voltage, screen grid voltage, and brightness and contrast controls connect to the video drive board. The video PC board plugs in to the CRT at its neck (although the diagram of Fig. 33-10 may not show this clearly). A power switch, power LED, and CRT degaussing coil plug into the main PC board. There are also connections at the main PC board for the AC line cord and video sync signals.

In spite of its age, the cathode ray tube (CRT) continues to play an important role in modern computer monitors. There are some very important reasons for this longevity. First, the CRT is relatively inexpensive to make, and it needs only simple circuitry in order to operate. Second, the CRT is extremely reliable. Typical working lives can extend to ten years or more. It is this combination of low-cost, ease of operation, and long-term reliability which has allowed the CRT to keep pace with today’s personal computers. However, CRTs are certainly not perfect devices - the delicate assemblies within the CRT used to generate and direct electron beams can eventually open, short-circuit, or wear out. And like most classical vacuum tubes, CRT failures often occur slowly over a period of weeks or months. This part of the chapter shows you the assemblies in a typical color CRT, explains the faults that often occur, and offer some alternatives for dealing with CRT problems.

Before we discuss CRT problems, you should have an understanding of the color CRT itself. Figure 33-11 shows a cross-section of a typical color CRT. To produce an image, electron beams are generated, concentrated, and directed across a phosphor-coated face. When electron beams (which are invisible) strike phosphor, light is liberated - this is the light you see from the CRT. The color of light is determined by the particular phosphor chemistry. You will note that there are three electron "guns" in the color CRT; a beam for red, a beam for green, and a beam for blue.

Electron beams start with a heater wire. When energized, the heater becomes extremely hot (this is the glow you see in the CRT neck). The heat from a heater warms its corresponding cathode, and a barium tip on the cathode begins "boiling off" electrons. Ordinarily, electrons would simply boil off into a big, clumsy cloud. But since electrons are negatively charged, they will be attracted to any large positive potential. A moderate positive potential (+500 volts or so) on the screen grid starts accelerating the electrons down the CRT’s neck, while the control grid voltage limits the electrons - effectively forcing the unruly cloud into a beam. Once electrons pass the screen grid, a high positive potential on the CRT anode (anywhere from 15 to 30kV) rockets the electrons toward the CRT face. The beam is still rather wide, so a focus grid applies another potential that works to concentrate the beam.

All this is very effective at generating narrow, high-velocity electron beams. But unless you want to watch a big, bright spot in the middle of the CRT, there has to be some method of tracing the beams around the CRT face. Beam tracing is accomplished through the use of deflection magnets placed around the CRT neck - you will see these magnets (actually electromagnets) as heavy coils of wire where the CRT funnel meets the neck. There are actually four electromagnets in this "deflection assembly"; two opposing electromagnets direct the beam in the vertical direction, while another set of opposing magnets direct the beam in the horizontal direction. Using electrical signals from the monitor’s raster circuits, an electron beam can traced across the entire CRT face.

Another element of the CRT that you should understand is called the shadow mask. A shadow mask is basically a thin metal sheet with a series of small holes punched into it. Some CRTs use a mask of rectangular openings referred to as an aperture grille or slot mask. Both types of mask perform the same purpose - to ensure that electron beams strike only the color phosphors of the intended pixel. This is a vital element of a color monitor. In a monochrome monitor, the CRT is coated with a single homogenous layer of phosphor - if stray electrons strike nearby phosphor particles, a letter or line may simply appear to be a bit out of focus. For a color CRT however, stray electrons can cause incorrect colors to appear on nearby pixels. Masks help to preserve color purity. Color purity is also aided by a purity magnet which helps correct fine beam positioning. A convergence magnet helps ensure that all three electron beams meet (or converge) at the shadow mask.

Of course, grids, heaters, and cathodes are all located inside the glass CRT vessel. Electrical connections are made through a circular arrangement of sealed pins in the neck. Table 33-2 explains the designations for each pin. Keep in mind that the high-voltage anode is attached directly to the CRT in the top right part of the glass funnel. Also remember that some CRT designs may use additional pins.

CRTs enjoy a long, 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. Let’s take a look at some of the typical problems that manifest themselves in a CRT.

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 for a 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 a new monitor outright.

Symptom 33-1. 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 may eventually fatigue the heater and cause it to open. You will see this as a complete loss of the corresponding color. Since heaters are all tied together electrically, there is no way to measure a particular heater directly, but you may 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 33-2. Heater shorts to a cathode in the CRT. This is not as strange as it might seem at first. In order to heat a cathode effectively, the heater must be in extremely close proximity to the cathode - especially to the barium element that actually liberates the electrons. Over time, the heater may 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 actual practice, some small amount of oxygen will still be present in the CRT, and oxidation may 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 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 CRT’s neck, and measuring the resistance between a heater wire and the suspect cathode. For the CRT pinout listed in Table 33-2, you could check the blue cathode by measuring resistance between the KB and H1 (or H2) pins. Ideally, there should be infinite resistance 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 may have a defect on the video drive board.

Symptom 33-3. 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 comes in contact with the control grid. When this happens, the control grid looses its effectiveness, and the corresponding color will appear saturated. In practice, 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, there should be infinite resistance 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 33-4. One or more colors appear weak. This is a common symptom in many older CRTs. Over time, 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 loose efficiency, resulting in weakened screen colors. Typically, you might expect all three cathodes to degrade evenly over time - 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 33-5. 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 may 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 for a very long period of time. If you turn the monitor off, you can see the latent image burned onto the CRT as a dark shadow. In both cases, there is no way 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 screen saver utility if an image will sit unchanging for a long time.

Symptom 33-6. The CRT suffers from bad cutoff (a.k.a. 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 video signal (i.e. 0 to 20 volts or 0 to 50 volts). As cathodes age, however, they tend to become non-linear. 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 effect beam intensity.

Symptom 33-7. Open control grid in the CRT. 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 33-8. Open screen grid in the CRT. The screen grid plays an important role in image brightness by accelerating the electron beam toward the CRT phosphors. If the screen grid opens, there will be no potential 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 33-9. Open focus grid in the CRT. A focus grid assembly serves to concentrate electron beams into narrow pinpoints by the time the beam reaches the shadow mask. There is typically a focus control 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 33-10. Control grid shorts to 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 may disappear entirely. You can measure the screen grid voltage at G2, which typically runs from 250 volts to 750 volts 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 may 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 G1 and G2 pins on the CRT neck. Ideally, there should be infinite resistance.

You can probably guess that short circuits within a CRT can be maddening - there is just no way to get to them. However, most shorts are not held in place by anything more than gravity, or a slight arc during contact. As a result, it may be possible to dislodge the short by turning the monitor over and gently rapping on the CRT neck with the plastic end of a screwdriver. Obviously, this is also a prime way to shatter the CRT, so be VERY careful if you attempt to dislodge a suspected short. If a few light taps don’t do the job, quit while the CRT is still in one piece.

Since shorts are small fragments of conductive material, they can be "burned" away using a surge of electricity - this is much safer than the "tap-and-pray" method mentioned above. Devices such as Sencore’s CR70 Universal CRT Restorer/Analyzer can help check the CRT for shorts and opens, and can also burn out a wide variety of shorts, and (in many cases) rejuvenate weak elements. As another advantage, a tester can usually check and rejuvenate a CRT without having the whole monitor available. Most CRT test equipment can perform four major operations; color balance testing, emission testing, removing shorts, and beam rejuvenation:

• Color balance testing - In order to produce pure white (and all other true colors), all three electron guns must be able to run at the same intensity. A color balance test can compare the strongest gun to the weakest gun. If there is a variation of more than 55%, the weakest gun will be displayed as "bad". But it is possible to recover a portion of the weaker beam’s operation through a "Beam Builder" or "Beam Rejuvenator" function on the tester.
• Emission testing - A cathode must be able to "emit" electrons - that is the basis of all vacuum tubes. As the cathode ages, ions generated from air in the CRT gradually block the cathode’s ability to produce electrons. This is "ion poisoning", which results in weakened electron beams. A rejuvenator function can usually overcome low-emission problems.
• Removing shorts - Generally speaking, a decent CRT tester/rejuvenator can remove shorts between the control grid and the cathode or the screen grid. In practice, you may see such a function marked "Remove G1 Short" or some similar nomenclature. However, few testers attempt to remove heater-to-cathode shorts because the energy needed to clear a short there would usually burn out the heater element entirely.
• Beam rejuvenation - The purpose of rejuvenation is basically to restore the emission of weak electron guns. This is usually accomplished by boosting the heater voltage (making the cathode extremely hot), then passing a 100-150mA current through the cathode. The effect of rejuvenation exposes fresh emitting material, which in turn adds new life to weakened guns. A current meter measures beam current - when beam current reaches its nominal range during rejuvenation, the electron gun is restored.

Any discussion of monitor troubleshooting must start with a reminder of the dangers involved. Computer monitors use very high voltages for proper operation. Potentially lethal shock hazards exist within the 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 a monitor's outer housings are removed. If you are uncomfortable with the idea of working around high voltages, defer your troubleshooting to an experienced technician.

When you finally get your monitor working again and are ready to reassemble it, be very careful to see that all wiring and connectors are routed properly. No wires should be pinched or lodged between the chassis or other metal parts (especially sharp edges). After the wiring is secure, make sure that any insulators, shielding, or protective enclosures are installed. This is even more important for larger monitors with supplemental X-ray shielding. Replace all plastic enclosures and secure them with their full complement of screws.

Regardless of the problem with your monitor or how you go about repairing it, a check of the monitor's alignment is always worthwhile before returning the unit to service. Your first procedure after a repair is complete should be to ensure that the high-voltage level does not exceed the maximum specified value. Excessive high-voltage can liberate X-radiation from the CRT. Over prolonged exposure, X-rays can present a serious biohazard. The high-voltage value is usually marked on the specification plate glued to the outer housing, or recorded on a sticker placed somewhere inside the housing. If you can not find the high-voltage level, refer to service data from the monitor's manufacturer. Once high-voltage is correct, you can proceed with other alignment tests. Refer to Chapter 63 for testing and alignment procedures. When testing (and realignment) is complete, it is wise to let the monitor for 24 hours or so (called a burn-in test) before returning it to service. Running the monitor for a prolonged period helps ensure that the original problem has indeed been resolved. This is a form of quality control. If the problem resurfaces, there may be another more serious problem elsewhere in the monitor.

Symptom 33-11. The image is saturated with red, or appears greenish-blue (cyan). If there are any user color controls available from the front or rear housings, make sure those controls have not been accidentally adjusted. If color controls are set properly (or not available externally), the red video drive circuit has probably failed. Refer to the example circuit of Fig. 33-8. Use your oscilloscope to trace the video signal from its initial input to the final output. If there is no red video signal at the amplifier input (i.e. the base of Q501), check the connection between the monitor and the video adapter board. If the connection is intact, try a known-good monitor. If the problem persists on a known-good monitor, replace the video adapter board. As you trace the video signal, you can compare the signal to characteristics at the corresponding points in the green or blue video circuits. The point at which the signal disappears is probably the point of failure, and the offending component should be replaced. If you do not have the tools or inclination to perform component-level troubleshooting, try replacing the video drive PC board entirely.

If the video signal measures properly all the way to the CRT (or a new video drive PC board does not correct the problem), suspect a fault in the CRT itself - the corresponding cathode or video control grid may have failed. If you have access to a CRT tester/rejuvenator, test the CRT. If the CRT measures bad (and can not be recovered through any available rejuvenation procedure), it should be replaced. Keep in mind that a color CRT is usually the most expensive component in the monitor. As with any CRT replacement, you should carefully consider the economics of the repair versus buying a new or rebuilt monitor.

Symptom 33-12. The image is saturated with blue, or appears yellow. If there are any user color controls available from the front or rear housings, make sure those controls have not been accidentally adjusted. If color controls are set properly (or not available externally), the blue video drive circuit has probably failed. Refer to the example circuit of Fig. 33-8. Use your oscilloscope to trace the video signal from its initial input to the final output. If there is no blue video signal at the amplifier input (i.e. the base of Q701), check the connection between the monitor and the video adapter board. If the connection is intact, try a known-good monitor. If the problem persists on a known-good monitor, replace the video adapter board. As you trace the video signal, you can compare the signal to characteristics at the corresponding points in the green or red video circuits. The point at which the signal disappears is probably the point of failure, and the offending component should be replaced. If you do not have the tools or inclination to perform component-level troubleshooting, try replacing the video drive PC board entirely.

If the video signal measures properly all the way to the CRT (or a new video drive PC board does not correct the problem), suspect a fault in the CRT itself - the corresponding cathode or video control grid may have failed. If you have access to a CRT tester/rejuvenator, test the CRT. If the CRT measures bad (and can not be recovered through any available rejuvenation procedure), it should be replaced. Keep in mind that a color CRT is usually the most expensive component in the monitor. As with any CRT replacement, you should carefully consider the economics of the repair versus buying a new or rebuilt monitor.

Symptom 33-13. The image is saturated with green, or appears bluish-red (magenta). If there are any user color controls available from the front or rear housings, make sure those controls have not been accidentally adjusted. If color controls are set properly (or not available externally), the green video drive circuit has probably failed. Refer to the example circuit of Fig. 33-8. Use your oscilloscope to trace the video signal from its initial input to the final output. If there is no green video signal at the amplifier input (i.e. the base of Q601), check the connection between the monitor and the video adapter board. If the connection is intact, try a known-good monitor. If the problem persists on a known-good monitor, replace the video adapter board. As you trace the video signal, you can compare the signal to characteristics at the corresponding points in the red or blue video circuits. The point at which the signal disappears is probably the point of failure, and the offending component should be replaced. If you do not have the tools or inclination to perform component-level troubleshooting, try replacing the video drive PC board entirely.

If the video signal measures properly all the way to the CRT (or a new video drive PC board does not correct the problem), suspect a fault in the CRT itself - the corresponding cathode or video control grid may have failed. If you have access to a CRT tester/rejuvenator, test the CRT. If the CRT measures bad (and can not be recovered through any available rejuvenation procedure), it should be replaced. Keep in mind that a color CRT is usually the most expensive component in the monitor. As with any CRT replacement, you should carefully consider the economics of the repair versus buying a new or rebuilt monitor.

Symptom 33-14. Raster is present, but there is no image. When the monitor is properly connected to a PC, a series of text information should appear as the PC initializes. We can use this as our baseline image. Isolate the monitor by trying a known-good monitor on your host PC. If the known-good monitor works, you prove that the PC and video adapter are working properly. Reconnect the suspect monitor to the PC and turn up the brightness (and contrast if necessary). You should see a faint white haze covering the display. This is the raster generated by the normal sweep of an electron beam. Remember that the PC must be on and running. Without the horizontal and vertical retrace signals provided by the video adapter, there will be no raster.

For a color image to fail completely, all three video drive circuits will have to be disabled. You should check all connectors between the video adapter board and the monitor's main PC board. A loose or severed wire can interrupt the voltage(s) powering the board. You should also check each output from your power supply. A low or missing voltage can disable your video circuits as effectively as a loose connector. If you find a faulty supply output, you can attempt to troubleshoot the supply, or you can replace the power supply outright. For monitors that incorporate the power supply onto the main PC board, the entire main PC board would have to be replaced.

If supply voltage levels and connections are intact, use an oscilloscope to trace the video signals through their respective amplifier circuits. Chances are that you will see all three video signals fail at the same location of each circuit. This is usually due to a problem in common parts of the video circuits. In the example video drive board of Fig. 33-8, such common circuitry involves the components marked with 8xx numbers (i.e. Q801). If you do not have the tools or inclination to perform such component-level troubleshooting, replace the video drive PC board.

You should also suspect a problem with the raster blanking circuits. During horizontal and vertical retrace periods, video signals are cut off. If visible raster lines appear in your image, check the blanking signals. If you are unable to check the blanking signals, try replacing the video drive PC board. If a new video drive board fails to correct the problem, replace the main PC board.

If you should find that all three video signals check correctly all the way to the CRT (or replacing the video drive circuit does not restore the image), you should suspect a major fault in the CRT itself - there is little else that can fail. If you have a CRT tester/rejuvenator available, you should test the CRT thoroughly for shorted grids or a weak cathode. If the problem can not be rectified through rejuvenation (or you do not have access to a CRT tester, try replacing the CRT. Keep in mind, however, that a CRT is usually the most expensive part of the monochrome monitor. If each step up to now has not restored your image, you should weigh the economics of replacing the CRT versus scrapping it in favor of a new or rebuilt unit.

Symptom 33-15. A single horizontal line appears in the middle of the display. The horizontal sweep is working properly, but there is no vertical deflection. A fault has almost certainly developed in the vertical drive circuit (refer to Fig. 33-9). Use your oscilloscope to check the sawtooth wave being generated by the vertical oscillator/amplifier IC (pin 6 of IC402). If the sawtooth wave is missing, the fault is almost certainly in the IC. For the circuit of Fig. 33-9, try replacing IC402. If the sawtooth wave is available on IC402 pin 6, you should suspect a defect in the horizontal deflection yoke itself, or one of its related components. If you are not able to check signals to the component level, simply replace the monitor's main PC board.

Symptom 33-16. Only the upper or lower half of an image appears. In most cases, there is a problem in the vertical amplifier. For the example circuit of Fig. 33-9, the trouble is likely located in the vertical oscillator/amplifier (IC402). Use your oscilloscope to check the sawtooth waveform leaving IC402 pin 6. If the sawtooth is distorted, replace IC402. If the sawtooth signal reads properly, check for other faulty components in the vertical deflection yoke circuit. If you do not have the tools or inclination to check and replace devices at the component level, replace the monitor's main PC board. When the image is restored, be sure to check vertical linearity as described in Chapter 63.

Symptom 33-17. A single vertical line appears along the middle of the display. The vertical sweep is working properly, but there is no horizontal deflection. However, in order to even see the display at normal brightness, there must be high-voltage present in the monitor - the horizontal drive circuit must be working (refer to Fig. 33-9). The fault probably lies in the horizontal deflection yoke. Check the yoke and all wiring connected to it. It may be necessary to replace the horizontal deflection yoke, or the entire yoke assembly.

If horizontal deflection is lost as well as substantial screen brightness, there may be a marginal fault in the horizontal drive circuit. If there is a problem with the horizontal oscillator pulses, the switching characteristics of the horizontal amplifier will change. In turn, this effects high-voltage development and horizontal deflection. Use your oscilloscope to check the square wave generated by the horizontal oscillator IC301 pin 3 as shown in Fig. 33-9. You should see a square wave. If the square wave is distorted, replace the oscillator IC (IC301). If the horizontal pulse is correct, check the horizontal switching transistors (Q301 and Q302). Replace any transistor that appears defective. If the collector signal at the HOT is low or distorted, there may be a short circuit in the flyback transformer primary winding. Try replacing the FBT. If you do not have the tools or inclination to check components to the component level (or the problem persists), replace the monitor's main PC board. When the repair is complete, check the horizontal linearity and size as described in Chapter 63.

Symptom 33-18. There is no image and no raster. When the monitor is properly connected to a PC, a series of text information should appear as the PC initializes. We can use this as our baseline image. Isolate the monitor by trying a known-good monitor on your host PC. If the known-good monitor works, you prove that the PC and video adapter are working properly. Reconnect the suspect monitor to the PC and turn up the brightness (and contrast if necessary). Start by checking for the presence of horizontal and vertical synchronization pulses. If pulses are absent, no raster will be generated. If sync pulses are present, there is likely a problem somewhere in the horizontal drive or high-voltage circuits.

Always suspect a power supply problem, so check every output from the supply (especially the 20 Vdc and 135 Vdc outputs as shown in Fig. 33-9). A low or absent supply voltage will disable the horizontal deflection and high-voltage circuits. If one or more supply outputs are low or absent, you can troubleshoot the power supply circuit, or replace the power supply outright (when the power circuit is combined on the monitor's main PC board, the entire main PC board would have to be replaced).

If the supply outputs read correctly, suspect your horizontal drive circuit. Use your oscilloscope to check the horizontal oscillator output at the base of Q301 as shown in Fig. 33-9. You should see a square wave. If the square wave is low, distorted, or absent, replace the horizontal oscillator IC (IC301). If a regular pulse is present, the horizontal oscillator is working. Since Q301 is intended to act as a switch, you should also find a pulse at the collector of Q301. If the pulse output is severely distorted or absent, Q301 is probably damaged (remove Q301 and test it). If Q301 reads as faulty, it should be replaced. If Q301 reads good, check the horizontal coupling transformer (T303) for shorted or open windings. Try replacing T303 (there is little else that can go wrong in this part of the circuit).

Check the HOT (Q302) next by removing it from the circuit and testing it. If Q302 reads faulty, it should be replaced with an exact replacement part. If Q302 reads good, the fault probably lies in the flyback transformer. Try replacing the FBT. If you do not have the tools or inclination to perform these component-level checks, simply replace the monitor's main PC board outright.

In the event that these steps fail to restore the image, the CRT has probably failed. If you have access to a CRT tester/rejuvenator, you can test the CRT. When the CRT measures as bad (and can not be restored through rejuvenation), it should be replaced. If you do not have a CRT test instrument, you can simply replace the CRT. Keep in mind, however, that a CRT is usually the most expensive part of a color monitor. If each step up to now has not restored your image, you should weigh the economics of replacing the CRT versus scrapping it in favor of a new or rebuilt unit. If you choose to replace the CRT, you should perform a full set of alignments as described in Chapter 63.

Symptom 33-19. The image is too compressed or too expanded. A whitish haze may appear along the bottom of the image. Start by checking your vertical size control to be sure that it was not adjusted accidentally. Since vertical size is a function of the vertical sawtooth oscillator, you should suspect the vertical oscillator circuit. A sawtooth signal that is too large will result in an over-expanded image, while a signal that is too small will appear to compress the image. Use your oscilloscope to check the vertical sawtooth signal. For the vertical drive circuit of Fig. 33-9, you should find a sawtooth signal on IC402 pin 6. If the signal is incorrect, try replacing IC 402. You may also wish to check the PC board for any cracks or faulty soldering connections around the vertical oscillator circuit. If the problem persists, or you do not have the tools or inclination to perform component-level troubleshooting, simply replace the monitor's main PC board outright.

Symptom 33-20. The displayed characters appear to be distorted. The term "distortion" can be interpreted in many different ways. For our purposes, we will simply say that the image (usually text) is difficult to read. Before even opening your toolbox, check the monitor's location. The presence of stray magnetic fields in close proximity to the monitor can cause bizarre forms of distortion. Try moving the monitor to another location. Remove any electromagnetic or magnetic objects (such as motors or refrigerator magnets) from the area. If the problem persists, it is likely that the monitor is at fault.

If only certain areas of the display appear effected (or effected worse than other areas), the trouble is probably due to poor linearity (either horizontal, vertical, or both). If raster speed varies across the display, the pixels in some areas of the image may appear too close together, while the pixels in other areas of the image may appear too far apart. You can check and correct horizontal and vertical linearity using a test pattern such as the one described in Chapter 63. If alignment fails to correct poor linearity, your best course is often simply to replace the monitor's main PC board. If the image is difficult to read because it is out of focus, you should check the focus alignment. If you can not achieve a sharp focus using controls either on the front panel of the monitor or on the flyback transformer assembly, there is probably a fault in the flyback transformer. Try replacing the FBT. If the problem persists, your best course is often simply to replace the monitor's main PC board.

Symptom 33-21. The display appears wavy. There are visible waves appearing along the edges of the display as the image sways back and forth. This is almost always the result of a power supply problem - one or more outputs is failing. Use your multi-meter and check each supply output. If you find a low or absent output, you can proceed to troubleshoot the supply, or you can simply replace the supply outright. If the power supply is integrated onto the main PC board, you will have to replace the entire main PC board.

Symptom 33-22. The display is too bright or too dim. Before opening the monitor, be sure to check the brightness and contrast controls. If the controls had been accidentally adjusted, set contrast to maximum, and adjust the brightness level until a clear, crisp display is produced. When front panel controls fail to provide the proper display (but focus seems steady), suspect a fault in the monitor's power supply. Refer to the example schematic of Fig. 33-9. If the 135Vdc supply is too low or too high, brightness levels controlling the CRT screen grid will shift. If you find one or more incorrect outputs from the power supply, you can troubleshoot the supply or replace the supply outright. For those monitors that incorporate the power supply on the main PC board, the entire main PC board will have to be replaced.

Symptom 33-23. You see visible raster scan lines in the display. The very first thing you should do is check the front panel brightness and contrast controls. If contrast is set too low and/or brightness is set too high, raster will be visible on top of the image. This will tend to make the image appear a bit fuzzy. If the front panel controls can not eliminate visible raster from the image, chances are that you have a problem with the power supply. Use your multi-meter and check each output from the supply. If one or more outputs appear too high (or too low), you an troubleshoot the supply or replace the supply outright. If the supply is integrated with the monitor's main PC board, the entire PC board will have to be replaced.

If the power supply is intact, you should suspect a problem with the raster blanking circuits. During horizontal and vertical retrace periods, video signals are cut off. If visible raster lines appear in your image, check the blanking signals. If you are unable to check the blanking signals, try replacing the video drive PC board. If a new video drive board fails to correct the problem, replace the main PC board.

Symptom 33-24. Colors bleed or smear. Ultimately, this symptom occurs when unwanted pixels are excited in the CRT. However, this can be caused by several different problems. Perhaps the most common problem is a fault in the video cable between the video board and the monitor. Electrical noise (sometimes called crosstalk) in the cable may allow signals representing one color to accidentally be picked up in another color signal wire. This can easily cause unwanted colors to appear in the display. Although the video cable is designed to be shielded and carefully filtered, age or poor installation can precipitate this type of problem. Try wiggling the cable. If the problem stops, appears intermittent, or shifts around, you have likely found the source of the problem - replace the cable with a proper replacement assembly.

If the video cable appears intact, suspect failing capacitors in the video amplifier circuits. You can see these capacitors in the schematic of Fig. 33-8. Capacitors such as C505 and C506 are typically low-value, high-voltage components, so they tend to degrade rather quickly. Fortunately, such capacitors are easy to spot on the video amplifier board. If the color problem appears intermittent (or occurs when the monitor warms up), try a bit of liquid refrigerant on each capacitor. If the problem disappears, the one you froze is probably defective. Otherwise, you can turn off and unplug the monitor, then check each capacitor individually. When replacing capacitors in the video amplifier circuit, be sure to replace them with the same type and voltage rating.

If capacitors are not at fault, suspect the amplifier transistors on the video amplifier board (i.e. Q504, Q505, or Q506). Turn off and unplug the monitor, then try checking each of the transistors. Chances are that your readings will be inconclusive, so try comparing readings from each transistor to find a device that gives the most unusual readings. Try replacing any defective or questionable amplifier transistors. If you do not have the time or inclination to troubleshoot the video amplifier board, try replacing the board outright.

Symptom 33-25. Colors appear to change when the monitor is warm. Either colors will appear correctly when the monitor is cold, then change as the monitor warms up, or vice versa. In both cases, there is likely to be some kind of thermal problem in the video amplifier circuits. Turn off and unplug the monitor, then start by checking the video cable - especially its connection to the raster board inside the monitor. If this connection is loose, it may be intermittent or unreliable. Tighten any loose connections and try the monitor again. Also check the cable that connects the video amplifier board to the raster board.

If the connections appear tight, your best course of action is often to remove the video amplifier board and try re-soldering each of the junctions. Chances are that age or thermal stress has fatigued one or more connections. By re-soldering the connections, you should be able to correct any potential connection problems. You might also try re-soldering the connector which passes video data from the raster board to the video amplifier board. If your problems persist, try replacing the video amplifier board.

Symptom 33-26. An image appears distorted in 350 or 400 line mode. In most cases, the "distortion" is an image that appears excessively compressed. As you probably read earlier in this book, different screen modes have a different number of horizontal lines (i.e. a 640x480 display offers 480 horizontal traces of 640 pixels each). When the screen mode changes, the number of lines changes as well (i.e. to a 320x200 mode). As you might expect, the "size" of each pixel has to be adjusted when the screen mode changes in order to keep the image roughly square - otherwise the image simply "shrinks". Monitors detect the screen mode by checking the polarity of the sync signals. You can see this function in the schematic of Fig. 33-9.

Typically, each screen mode size can be optimized by an adjustment on the raster board. However, if a mode adjustment is thrown off (or the sync sensing circuit fails), an image can easily appear with an incorrect size. If you notice this kind of distortion without warning, suspect a problem with the sync sensing circuit. If the sync sensing circuit is incorporated into a single IC (such as IC201), replace the IC outright. If you notice a size problem after aligning the monitor, you may have accidentally upset a size adjustment. Re-adjust the size controls to restore proper image dimensions.

Symptom 33-27. The fine detail of high-resolution graphic images appears a bit fuzzy. At best, this kind of symptom may not appear noticeable without careful inspection, but it may signal a serious problem in the video amplifier circuit. High resolutions demand high bandwidth - a video amplifier must respond quickly to the rapid variations between pixels. If a weakness in the video amplifier(s) occurs, it can limit bandwidth and degrade video performance at high resolutions. The problem will likely disappear at lower resolutions.

The particular problem with this symptom is that it is almost impossible to isolate a defective component - the video amplifier board is working. As a result, your best course of action is to check all connectors for secure installation first. Nicked or frayed video cables can also contribute to the problem. If the problem remains, replace the video amplifier board.

Symptom 33-28. The display changes color, flickers, or cuts out when the video cable is moved. Check the video cable’s connection to the video adapter at the PC - a loose connection will almost certainly result in such intermittent problems. If the connection is secure, there is an intermittent connection in the video cable. Before replacing the cable, check its connections within the monitor itself. When connections are intact, replace the intermittent video cable outright. Do not bother cutting or splicing the cable - any breaks in the signal shielding will cause crosstalk which will result in color bleeding.

Symptom 33-29. The image expands in the horizontal direction when the monitor gets warm. One or more components in the horizontal retrace circuit are weak - and changing value a bit once the monitor gets warm. Turn off and unplug the monitor. You should inspect any capacitors located around the horizontal output transistor (HOT). The problem is that thermal problems such as this can be extremely difficult to isolate because you can’t measure capacitor values while the monitor is running, and after the monitor is turned off, the parts will cool too quickly to catch a thermal problem. It is often most effective to simply replace several of the key capacitors around the HOT outright. If you don’t want to bother with individual components, replace the raster board.

Symptom 33-30. The image shrinks in the horizontal direction when the monitor gets warm. This is another thermal-related problem which indicates either a weakness in one or more components, or a mild soldering-related problem. Turn off and unplug the monitor. Start by checking for a poor solder connection - especially around the horizontal deflection yoke wiring, the horizontal output transistor (HOT), and the flyback transformer. If nothing appears obvious, you might consider resoldering all of the components in the HOT area of the raster board. If problems continue, suspect a failure in the HOT itself. Semiconductors rarely become marginal - they either work or they don’t. Still, semiconductor junctions can become unstable when temperatures change, and result in circuit characteristic changes. You could also try replacing the HOT outright.

It is also possible that one or more mid-range power supply outputs (i.e. 12 or 20 volts) are sagging when the monitor warms up. Use a voltmeter and measure the outputs from your power supply. If the 12 or 20 volt outputs appear to drop a bit once the monitor has been running for a bit, you should troubleshoot the power supply.

Symptom 33-31. High-voltage fails after the monitor is warm. There are a large number of possible causes behind this problem, but no matter what permutation you find, you will likely be dealing with soldering problems, or thermal-related failures. Turn off and unplug the monitor. Inspect the HOT’s heat sink assembly - there may be a bad solder connection on the heat sink ground. There may also be an open solder connection on one or more of the flyback transformer pins. If you cannot locate a faulty soldering connection, you may simply choose to re-solder all of the connections in the flyback area.

If the problem persists, you should suspect that either your HOT or flyback transformer is failing under load (after the monitor warms up). One possible means of isolating the problem is to measure pulses from the HOT output with your oscilloscope. If the pulses stop at the same time your high-voltage fails, you can suspect a problem with your HOT or other horizontal components. Try replacing the HOT. If high-voltage fails but the HOT pulses remain, your flyback transformer has likely failed. Replace the flyback transformer. If you do not have an oscilloscope, try replacing the HOT first since that is the least-expensive part, then replace the flyback transformer if necessary.

In the unlikely event that both a new HOT and flyback transformer do not correct the problem, you should carefully inspect the capacitors in the HOT circuit. One or more might be failing. Unfortunately, it is very difficult to identify a marginal capacitor (especially one that is suffering from a thermal failure). You may try replacing the major capacitors in the HOT circuit, or replace the raster board entirely.

Symptom 33-32. The image blooms intermittently. The amount of high-voltage driving the CRT is varying intermittently. Since high-voltage is related to the HOT circuit and flyback transformer, you should concentrate your search in those two areas of the raster board. Examine the soldering of your HOT and FBT connections - especially the ground connections if you can identify them. You may try resoldering all of the connections in those areas (remember to turn off and unplug the monitor before soldering). There may also be a ground problem on the video amplifier board which allows all three color signals to vary in amplitude. When this happens, the overall brightness of the image changes, and the image may grow or shrink a bit in response. Try resoldering connections on the video amplifier board.

If the problem remains (even after soldering). Your FBT may be failing - probably due to an age-related internal short. High-end test equipment such as Sencore’s monitor test station provides the instrumentation to test a flyback transformer. If you do not have access to such dedicated test equipment, however, try replacing the FBT assembly. If you do not have the time or inclination to deal with component replacement, go ahead and replace the raster board outright. In the unlikely event that your problem persists, suspect a fault in the CRT itself. If you have access to a CRT tester/rejuvenator, you can check the CRT’s operation. Some weaknesses in the CRT may be corrected (at least temporarily) by rejuvenation. If the fault cannot be corrected, you may have to replace the CRT.

Symptom 33-33. The image appears out of focus. Before suspecting a component failure, try adjusting the focus control. In most cases, the focus control is located adjacent to the flyback transformer. Keep in mind that the focus control should be adjusted with brightness and contract set to optimum values - excessively bright images may loose focus naturally. If the focus control is unable to restore a proper image, check the CRT focus voltage. In Fig. 33-9, you can find the focus voltage off a flyback transformer tap. If the focus voltage is low (often combined with a dim image), you may have a failing FBT. It is possible to test the FBT if you have the specialized test instrumentation - otherwise, you should just replace the FBT outright. If you lack the time or inclination to replace the FBT, you can simply replace the raster board.

If a new FBT does not resolve your focus problem, suspect a fault in the CRT - probably in the focus grid. You can use a CRT tester/rejuvenator to examine the CRT, and it may be possible to restore normal operation (at least temporarily). If you do not have such equipment, you will simply have to try a new CRT.

Symptom 33-34. The image appears to flip or scroll horizontally. There is a synchronization problem in your horizontal raster circuit. Begin by checking the video cable to be sure that it is installed and connected securely. Cables that behave intermittently (or that appear frayed or nicked) should be replaced. If the cable is intact, suspect a problem in your horizontal circuit. If there is a horizontal sync (or "horizontal hold") adjustment on the raster board, adjust it in small increments until the image snaps back into sync. If there is no such adjustment on your particular monitor, try resoldering all of the connections in the horizontal processing circuit. If the problem persists, replace the horizontal oscillator IC, or replace the entire raster board.

Symptom 33-35. The image appears to flip or scroll vertically. There is a synchronization problem in your vertical raster circuit. Begin by checking the video cable to be sure that it is installed and connected securely. Cables that behave intermittently (or that appear frayed or nicked) should be replaced. If the cable is intact, suspect a problem in your vertical circuit. If there is a vertical sync (or "vertical hold") adjustment on the raster board, adjust it in small increments until the image snaps back into sync. If there is no such adjustment on your particular monitor, try resoldering all of the connections in the vertical processing circuit. If the problem persists, replace the vertical oscillator IC, or replace the entire raster board.

Symptom 33-36. The image appears to shake or oscillate in size. This may occur in bursts, but it typically occurs constantly. In most cases, this is due to a fault in the power supply - usually the 135 volt (B+) output. Try measuring your power supply outputs with an oscilloscope and see if an output is varying along with the screen size changes. If you locate such an output, the filtering portion of that output may be malfunctioning. Track the output back into the supply and replace any defective components. If you are unable to isolate a faulty component, replace the power supply. When the power supply is integrated onto the raster board, you may have to replace the raster board entirely.

If the outputs from your power supply appear stable, you should suspect a weak capacitor in your horizontal circuit. Try resoldering the FBT, HOT, and other horizontal circuit components to eliminate the possibility of a soldering problem. If the problem remains, you will have to systematically replace the capacitors in the horizontal circuit. If you do not have the time or inclination to replace individual components, replace the raster board outright.

Here’s an unusual problem. The shaking you see may be related to a problem in the degaussing coil located around the CRT funnel. Ordinarily, the degaussing coil should unleash the most of its energy in the initial moments after monitor power is turned on. Thermistors (or posistors) in the power supply quickly diminish coil voltage - effectively cutting off the degaussing coil’s operation. A fault in the degaussing coil circuit (in the power supply) may continue to allow enough power to the coil to effect the image’s stability. Try disconnecting the degaussing coil. If the problem remains, the degaussing coil is likely operating properly. If the problem disappears, you have a fault in the degaussing coil circuit.

That concludes Chapter 33. Be sure to review the glossary and chapter questions on the accompanying CD. If you have access to the Internet, take a look at some of the monitor resources listed below:

Acer: http://www.aci.acer.com.tw

CTX: http://www.ctxintl.com

Hitachi: http://www.hitachi.com

Magnavox: http://www.magnavox.com

Nanao: http://www.traveller.com/nanao/

NEC: http://webserver.nectech.com/textgraph/tocmon.htm

Sony: http://www.sel.sony.com/SEL/ccpg/index.html

Viewsonic: http://www.viewsonic.com