Monitor Tshooting

27 MONITOR TROUBLESHOOTING

CONT CO NTEN ENTS TS AT A GL GLAN ANCE CE Monitor Specifications and Characteristics CRT Pixels and resolution Triads and dot pitch Shadow and slot masks Convergence Pincushion and barrel distortion Horizontal scanning, vertical scanning, raster, and retrace Interlacing Bandwidth Swim, jitter, and drift Video signal Synchronization and polarity

Horizontal drive circuit The flyback circuit Construction

Troubleshooting Tr oubleshooting a CRT Inside the CRT Identifying Ident ifying CRT probl problems ems Correcting shorts CRT teste testers/reju rs/rejuvenat venators ors

Troubleshooting a Color Monitor Wrapping it up Post-repair testing and alignment Symptoms

Further Study The Color Circuits Video drive circuits Vertical drive circuit

906

MONITOR MONI TOR SPECI SPECIFICAT FICATIONS IONS AND CHARA CHARACTERI CTERISTICS STICS

907

From their humble beginnings as basic monochrome text displays, the monitor (Fig. 27-1) has grown to provide real-time photo-realistic photo-realistic images of unprecedented quality and color. Monitors have allowed real-time video playback, stunning graphics, and informationfilled illustrations illustrations to replace the generic “command line” user interface of just a few years ago. In effect, monitors have have become our “virtual window” window” into the modern computer. 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.” displays.” This chapter explains the basic concepts behind today’s computer monitors, and provides a cross-section of troubleshooting procedures.

Monitor Specifications Monitor and Characteristics Although 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

FIGURE FIG URE 2727-1 1

A CTX EX910 color monitor.

CTX International, Inc.

2

S Y S T E M D A T A A N D T R O U B L E S H O O T I N G

908 908

MONI MONITO TOR R TROU TROUBL BLES ESHO HOOT OTIN ING G

best introduction to monitor troubleshooting is to cover each specification in detail and show you how each specification and characteristic affects a monitor’s performance. performance.

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

PIXELS AND RESOLUTION The picture element (or pixel) is the very smallest point that can be controlled on a CRT. For monochrome displays, a pixel can simply simply be turned on or off. For a color display, a pixel could assume any of a number of different colors. Pixels are combined in the form of an array (rows and columns). The size of the pixel array defines the display’s resoluresolution. Thus, resolution is the the total number of pixels in width width by the total number of pixels pixels in height. For example, a typical EGA EGA resolution is 640 pixels wide by 350 pixels pixels high (a total of 224,000 pixels), and a typical VGA resolution is 640 pixels wide by 480 pixels high (a total of 307,200 pixels). Typical Super VGA (SVGA) (SVGA) resolution is 800 pixels wide by 600 pixels high. Resolution is important important for computer monitors because because higher resolutions allow finer image detail.

TRIADS AND DOT PITCH Although monochrome CRTs use a single, uniform phosphor coating (usually white, am ber, or green), color CRTs use three color phosphors (red, green, and blue) arranged as triangles (or triads). triads). Figure 27-2 illustrates illustrates a series of color color phosphor triads. triads. On a color monitor, each triad represents one pixel (even (even though three dots are 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 spaced further apart, the image quality degrades because the eye can begin to discern the individual dots in each pixel. This results in lines lines that no longer appear straight and colors are no longer

MONITOR MONITOR SPECIFICAT SPECIFICATIONS IONS AND CHARACTERI CHARACTERISTICS STICS

909

3 dots compose a “pixel” 1 color “dot”

2 Dot pitch

FIGUR FIGURE E 27-2 27-2

Arranging color phosphors in a triad.

Phosphor layer

Shadow mask

Pixel

A convergence point

Electron beams

*Sizes and distances are NOT shown to scale.

FIGU FIGURE RE 27-3 27-3

The importance of convergence in a color monitor.

pure. Dot pitch is a measure of the distance between two adjacent phosphor dots on the dis play. This is also the same dimension for for the distance between openings in a “shadow mask.” mask.” Displays with a dot pitch of 0.31 mm (or less) generally provide adequate image quality.

SHADOW AND SLOT MASKS 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 three “electron guns” are focused to converge at each hole in the mask—not at the phosphor screen (Fig. (Fig. 27-3). The


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microscopic holes act as apertures that let the electron beams through only to their corresponding color phosphors. In this way, any stray electrons electrons are masked and color is kept pure. Some CRT designs substitute substitute a shadow mask with a slot mask mask (or aperture grille), which is made up of vertical wires wires behind the phosphor screen. The “dot pitch” for CRTs CRTs with slot masks is defined as the distance between each slot. slot. Remember that monochrome monochrome CRTs do not need a shadow mask at all because the entire phosphor surface is the same color.

CONVERGENCE Remember that three electron guns are used in a color monitor—the electrons themselves are invisible, but each gun excites a particular 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 related to color purity in the screen image. Ideally, the three beams converge converge perfectly at all points on the display and the resulting color is perfectly pure throughout 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 might see a red, green, or blue shadow when looking at a white line. Serious convergence problems problems can result in a blurred or distorted image. Monitor specifications usually list typical convergence error as misconver gence at both the display center and the overall display area. Typical center misconvergence runs approximately 0.45 mm, and overall overall display area misconvergence misconvergence is about 0.65 mm. Larger numbers result in in poorer convergence. Fortunately, monitor monitor convergence can be calibrated (see Chapter 57: “Monitor testing and alignment”).

PINCUSHION AND BARREL DISTORTION The front face of most CRTs is slightly slightly convex (bulging outward). However, digital images are perfectly square (that is, is, two dimensional). When a flat (2D) image is projected projected onto a curved (3D) surface, distortion results. Ideally, a monitor’s raster raster circuits will compensate for this screen shape so that the image appears flat when viewed at normal distances. In actuality, however, however, the image is is rarely flat. The sides of the image (top-to bottom) and (left-to-right) might might be bent slightly inward or slightly outward. Figure 27-4 illustrates an exaggerated view of these effects. Pincushioning occurs Pincushioning occurs when sides are bent inward, making the image’s border appear concave. Barreling occurs Barreling occurs when the sides are bent outward making the image’s image’s border appear convex. In most cases, these distortions distortions should be just barely noticeable (no more more than 2.0 or 3.0 mm). Many technicians refer to barrel distortion as pincushioning as well, although this is not technically correct.

HORIZONTAL SCANNING, VERTICAL SCANNING, RASTER, AND RETRACE To understand what scanning is, you must first understand how a monitor’s image is formed. A monitor’s image is generated generated one horizontal line of pixels at a time, starting starting from the upper left corner of the display display (Fig. 27-5). As the beams travel horizontally horizontally across the line, each pixel in the line is excited, based on the video data contained in the corresponding location of video video RAM on the video adapter board. board. When a line is com-


911

Barrel distortion (undercompensated pincushion)

Normal pincushion

2

Overcompensated pincushion

FIGURE FIGURE 27-4 27-4

The effects of pincushion and barrel distortion. Horizontal retrace (blanking)

Start

Vertical retrace (blanking)

End


Forming a screen image on a CRT.

plete, the beam turns off (known as horizontal blanking ). ). The beam is then directed horihorizontally (and slightly lower lower vertically) to the beginning of the next subsequent line. A new horizontal line can then be drawn. drawn. This process continues until until all horizontal lines 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 (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 called horizontal sync). sync). The rate at which which a complete “page” “page” of horizontal lines is generated is known as the vertical scanning rate (vertical sync). sync). Both the horizontal and vertical blanking lines are known as retrace lines because the deactivated beams are “retracing” their their path before starting a new trace. A typical horizontal retrace retrace time is 5 s, and the typical vertical retrace time time is 700 s. This continuous horizontal and


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MONI MO NITO TOR R TR TROU OUBL BLES ESHO HOOT OTIN ING G

TABLE TABL E 27-1 SCAN RAT RATES ES VS. VS. MONIT MONITOR OR RESOL RESOLUTIO UTION N

MONITOR

RESOLUTION

HORIZONTAL SCAN

VERTICAL SCAN

MDA

720 × 348

18.43kHz

50Hz

CGA

320 × 200

15.85kHz

60.5Hz

EGA

640

×

350

21.8kHz

60Hz

VGA

640

×

350

31.5kHz

70.1Hz Alternate config.

VGA

640

×

480

31.47kHz

60Hz

VGA

640

×

480

37.9kHz

72Hz

SVGA

800

×

600

38.0kHz

60Hz

"

800 × 600

35.16kHz

56Hz

"

800 × 600

37.60kHz

72Hz

"

1024 × 768

35.52kHz

87Hz

"

1024 × 768

48.8kHz

60Hz

Sony config.

"

1024 × 864

54kHz

60Hz

DEC config.

"

1006 × 1048

62.8kHz

59.8Hz Samsung config.

"

1280 × 1024

70.7kHz

66.5Hz DEC config.

"

1600 × 1280

89.2kHz

66.9Hz Sun config.

VESA config.

Interlaced (8514A)

vertical scanning action is generally referred to as raster. raster. Numbers can easily be ap plied to scanning rates to give you an even better idea of their relationship. relationship. A typical VGA monitor with a resolution of 640 × 480 pixels uses a horizontal scanning rate of 31.5kHz. This means that 31,500 lines lines can be drawn in one second or a single single line of 640 pixels can be drawn in 31.7 31.7 s. Because 480 horizontal lines lines are drawn in one “page,” a complete page can be drawn in (480 × 31.7 s) 15.2 ms. ms. If a single page can be drawn in 15.2 ms, the screen can be refreshed 65.7 times per second (65.7Hz)—this is roughly the vertical rate that will be set for VGA operation at 640- ×-480 resolution. In actuality, the vertical scanning rate will be set to a whole number, such as 60Hz, which leaves a lot of spare time time for blanking and synchronization. synchronization. It was discovered early in TV design that vertical scanning rates less than 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 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 27-1 compares typical monitor resolutions and scan rates.

INTERLACING 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 non-interlaced or or interlaced interlaced . As you see in Fig. 27-6, a non-interlaced monitor draws all of the lines that compose an image in one pass. This is preferable because 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 one second. second. A non-interlaced display display draws an image as two


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passes. Once the first pass is is complete, a second pass fills fills in the rest of the image. image. The effective image-refresh image-refresh rate is only half the stated vertical vertical scanning rate. The typical 1024×-768 SVGA monitor offers a vertical scanning rate of 87Hz, but because the monitor is “interlaced,” effective refresh is only 43.5Hz—screen flicker is much more noticeable.

BANDWIDTH In the very simplest terms, the bandwidth of a monitor is the absolute maximum rate at which pixels can be be sent to the monitor. monitor. Typical VGA displays displays offer a bandwidth bandwidth of 30MHz. That is, the monitor could generate up to 30 million pixels pixels per second on the dis play. Consider that each scan line of a VGA display display uses 640 pixels and the horizontal scan rate of 31.45kHz allows 31,450 scan lines per second to be written. written. At that rate, the monitor is processing (640 pixels/scan line × 31,450 scan lines/second) 20,128,000 pixels/second—well els/second—well within the monitor’s 30MHz bandwidth. bandwidth. The very newest color monitors offer bandwidths of 135MHz. Such high-resolution high-resolution 12801280-×-1024 monitors with scanning rates of 79kHz would need to process at least (1280 pixels/scan line × 79,000 scan lines/second) 101,120,000 pixels/second (101.12MHz), so enhanced bandwidth is truly a necessity for high resolutions.

SWIM, JITTER, AND DRIFT 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 deflection coil are produced by horizontal and vertical deflection circuitry. circuitry. Ideally, deflection circuitry circuitry should steer the electron beam(s) precisely precisely the same way in each each pass. This would result in an absolutely rock-solid image on the display. display. In the real world, however, there 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 called wave)) is a measure of position variation over a 30-second period. Drift is Drift is a measure of position variation over a one-minute period. Notice that all three terms represent essentially the same problem over different amounts of time. time. Swim, jitter, jitter, and drift can be expressed as fractions of a pixel or as physical measurements, such as millimeters. Noninterlaced

1 2 3 4 5 6 7

Interlaced

1 5 2 6 3 7 4

FIGUR FIGURE E 27-6 27-6 1st pass 2nd pass

Interlaced vs. non-interlaced scanning.

2


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

SYNCHRONIZATION AND POLARITY 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. For the new line to be “in sync” with the data for that line, a synchronization pulse is sent from the video adapter to to the monitor. There is a separate pulse pulse for horizontal synchrosynchronization and vertical synchronization. synchronization. In most current monitors, monitors, synchronization signals signals are edge-triggered TTL (transistor-transistor logic) signals. Polarity refers to the edge that triggers the synchronization. synchronization. A falling trigger (marked (marked “-” or “positive/negative”) indiindicates that synchronization occurs at the high-to-low high-to-low transition of the sync signal. A leading trigger (marked “+” or “negative/positive”) indicates that synchronization occurs on the low-to-high transition of the sync signal.

The Color Circuits 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. 27-7. Three complete video drive circuits are needed (one for each primary color—red, green, and blue). Although early color monitors used logic levels to represent video signals, current monitors use analog signals that allow the intensity of each color to be varied. The CRT is designed to provide three electron beams that are directed directed at corresponding color phosphors. phosphors. By varying the intensity of each electron beam, virtually virtually any color can be produced. For all practical purposes, 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).

VIDEO DRIVE CIRCUITS The schematic diagram for a typical RGB typical RGB (Red, Green, and Blue) Blue) drive circuit is shown in Fig. 27-8. This schematic is actually actually part of a Tandy VGM-220 VGM-220 analog color monitor. You will see that there are three separate video drive circuits. Components with a 5xx designation (e.g., IC501) IC501) are part of the red video drive circuit. circuit. The 6xx designation (e.g., (e.g., Q602) shows a part in the green video drive drive circuit. A 7xx marking (e.g., C704) C704) indicates a component in the blue video drive circuit. circuit. Other components marked marked with 8xx designations (e.g., 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 might otherwise interfere with the weak analog signal. signal. The video signal is amplified by transistor Q501. Q501. Potentiometer

THE COLOR COLOR CIRCUI CIRCUITS TS

Red video in

Green video in

Blue video in

Video amp.

Video driver

Red

Video amp.

Video driver

Green

Video amp.

Video driver

Blue

CRT

Vert. drive cir. Vsync

Vertical oscillator

915

Anode Vertical driver High-voltage

2

Hor. drive cir. Hsync

ac in

Horizontal oscillator

Power supply


Horizontal driver

FBT

dc out

Block diagram of a color (VGA) monitor.

VR501 adjusts the signal gain (the amount of amplification applied to the video signal). Collector signals are then passed to the differential amplifier amplifier circuit in IC501. Once again, noise is a major concern in color signals, and differential amplifiers amplifiers help to improve signal strength while eliminating eliminating noise. The resulting video signal signal is applied to a “push-pull” “push-pull” amplifier circuit consisting of Q503 and Q504, then fed to a subsequent “push-pull” am plifier pair of Q505 and Q506. Potentiometer VR502 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 video control grid. The remaining two drive circuits 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 should fail, two others are still left left 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. visible. You can tell when one of the video drive drive circuits fails: the faulty faulty circuit will either saturate saturate the display with that color or cut that color out completely. 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).

VERTICAL DRIVE CIRCUIT 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 inter-relation to other important monitor circuits, Fig. 27-9 illustrates the vertical drive, horizontal drive, highvoltage, and power-supply power-supply circuits—all combined combined together in the same schematic. schematic. This


9 1 6 M O N I T O R T R O U B L E S H O O T I N G


Schematic of a VGM-220 video circuit. Tandy Corporation

T H


T H E C O L O R C I R C U I T S

Schematic of a VGM-220 main (raster) circuit. Tandy Corporation

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G N I T O O H S E L B U O R T D N A A T A D M E T S Y S

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schematic is essentially the main PC board board for the Tandy VGM-220 monitor. Components marked with 4xx numbers (e.g., IC401) are part of the vertical drive system. The vertical sync pulses enter the monitor at connector CH202 (the line marked V marked V ). ). A simple exclusive-OR gate (IC201) is used to condition the sync pulses and select the video mode being used. Because the polarity of horizontal horizontal and vertical sync pulses will 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 oscillator (IC402). This mode-switching 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 it 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. VR406. Linearity and centering centering adjustments should should only be made

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schematic is essentially the main PC board board for the Tandy VGM-220 monitor. Components marked with 4xx numbers (e.g., IC401) are part of the vertical drive system. The vertical sync pulses enter the monitor at connector CH202 (the line marked V marked V ). ). A simple exclusive-OR gate (IC201) is used to condition the sync pulses and select the video mode being used. Because the polarity of horizontal horizontal and vertical sync pulses will 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 oscillator (IC402). This mode-switching 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 it 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. VR406. Linearity and centering centering adjustments should should only be made while displaying an appropriate test pattern. pattern. It is interesting that no discrete power amplifiers are 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). 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 amplifier will leave a narrow horizontal line in the display. The likeliest cause is the vertical drive IC (IC402) because that component handles handles both sawtooth generation generation and amplification. If only the upper or lower half of an image disappears, only one part of the vertical amplifier in IC402 might have failed. However, any fault on the PC board that interrupts interrupts the vertical sawtooth will disable vertical deflection entirely. entirely. When the vertical deflection is marginal (too expanded or too compressed), suspect a fault in IC402, but its related components might also be breaking down. An image that is over-expanded will will usually appear “folded over” with a whitish haze along the bottom. It might also be interesting to note that vertical drive problems do not affect display colors.

HORIZONTAL DRIVE CIRCUIT The horizontal drive circuit is responsible for operating the horizontal deflection yoke (H-DY). This circuit sweeps the electron beams left and right right across the display. To understand how the horizontal drive works, you should again refer to the schematic of Fig. 27-9. All components marked 3xx numbers (e.g., IC301) IC301) relate to the horizontal drive circuit. Horizontal sync signals enter the monitor 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). (IC301). Horizontal frequency should should be locked at 31.5kHz, but potentiometer VR302 can be used to optimize optimize the frequency. Do not attempt to adjust horizontal frequency, unless unless you have an oscilloscope available. available. Horizontal phase can be be

THE COLOR COLOR CIRCUI CIRCUITS TS

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adjusted with VR301. VR301. You should avoid altering any alignments alignments until a suitable test test pattern is displayed, as covered in Chapter 57. IC301 is a highly integrated device that is designed to provide precision horizontal square-wave pulses to the driver 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 transformer (T303). Current pulses produced by the secondary winding winding of T303 fire the horizontal output transistor transistor (Q302). Output from the HOT drives the horizontal deflection yoke (H-DY). (H-DY). The deflection circuit includes includes two adjustable coils to control horizontal linearity linearity (H-LIN: L302) and horizontal width (H-WIDTH: (H-WIDTH: L303). You will also notice that the collector signal from Q302 is directly connected to the flyback transformer (FBT). (FBT). Operation of the high-voltage system is covered in the next section. Problems in the horizontal horizontal drive circuit can take several several forms. One common manifestation is the loss of horizontal sweep, leaving a vertical line in the center of the display. This is generally caused by a fault in the horizontal oscillator (IC301), rather than the horizontal driver transistors. transistors. The second common symptom is a loss of of image (including raster), and is almost always the result of a failure in the HOT the HOT (High-voltage Output Tran sistor circuit) . Because the HOT also also operates the flyback transformer, transformer, a loss of horizonhorizontal output will disrupt high-voltage generation—the image will disappear.

THE FLYBACK CIRCUIT The presence of a large positive potential on the CRT’s anode is needed in order to accelerate an electron beam across the distance distance between the cathode and CRT phosphor. Electrons must strike strike the phosphor hard enough to liberate visible light. Under normal circumstances, this this requires a potential of 15,000 to 30,000 V. Larger CRTs need higher higher voltages because a greater physical physical distance must be overcome. Monitors generate highvoltage through the flyback circuit. The heart of the high-voltage circuit is the flyback the flyback transformer (FBT) , as shown in Fig. 27-9. The FBT’s primary primary winding is directly directly coupled to the horizontal output output transistor (Q302). Another primary primary winding is used to compensate compensate the high-voltage high-voltage level for changes in brightness and contrast. contrast. Flyback voltage is generated during 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 FBT in Fig. 27-9 provides one multi-tapped multi-tapped secondary winding. The top-most tap from the FBT secondary 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 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. probe. The CRT needs additional voltages voltages in order to function. 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 monitor inoperative. Typically, a highvoltage fault manifests manifests itself as a loss of image and raster. raster. In many cases where the HOT and deflection signals are intact, the flyback transformer has probably failed, causing a loss of output in one or more of the three three FBT secondary windings. windings. The troubleshooting proprocedures in the next section of this chapter will cover high-voltage symptoms and solutions in more detail.

2


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CONSTRUCTION Before jumping right into troubleshooting, it would be helpful to understand how the circuits shown in Fig. Fig. 27-9 are assembled. A wiring diagram for the Tandy VGM-220 VGM-220 is shown in Fig. 27-10. The two PC boards boards are the video drive PC board board and the main PC board. The main PC board contains the raster circuits, power supply, and high-voltage high-voltage cir-


A wiring diagram for the VGM-220.

Tandy Corporation

TROUBLESHO TROUBLESHOOTIN OTING G A CRT

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cuitry. The video drive PC PC board contains red, 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. 27-10 might not show this this clearly). A power switch, power LED, LED, and CRT degaussing coil plug into the main PC board. board. There are also connections at the main PC board for the ac line cord and video sync signals.

Troubleshooting a CRT In spite of its age, the Cathode-Ray Tube (CRT) continues to play an important role in modern computer monitors. monitors. There are some very important important reasons for this longevity. First, the CRT is relatively inexpensive to make, and it requires only simple circuitry. Second, the CRT is extremely reliable. reliable. Typical working lives lives can extend to 10 years or longer. This combination of low-cost, ease of operation, and long-term reliability reliability has allowed the CRT to keep pace with today’s today’s personal computers. However, CRTs CRTs are certainly not perfect devices—the delicate assemblies within the CRT used to generate and direct electron beams beams can eventually open, short-circuit, short-circuit, or wear out. Like most classic vacuum tubes, CRT failures 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.

INSIDE THE CRT Before reading about CRT problems, you should have an understanding of the color CRT itself. Figure 27-11 shows shows a cross-section cross-section of a typical color color CRT. To produce an image, 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. CRT. The color of light is determined determined by the particular phosphor phosphor chemistry. Notice that there are three electron “guns” in the color CRT: CRT: a beam for red, a beam for green, and a beam for blue. Electron beams start with with a heater wire. When energized, the heater becomes becomes extremely hot (this is the glow you see in the CRT neck). neck). The heat from a heater warms its its corresponding cathode, and a barium tip on the cathode begins begins “boiling off” electrons. Ordinarily, electrons would simply simply boil off into a big, clumsy cloud. But because electrons are negatively charged, they will be attracted attracted to any large positive potential. A moderate positive potential (+500 V 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 screen grid, a high positive potential potential on the CRT anode (anywhere from 15 to 30 kV) rockets the electrons toward the CRT face. The beam is still rather wide, so a focus grid applies another potential that concentrates the beam. All this is very effective at generating narrow, narrow, high-velocity electron beams. 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 tracing is accomplished through the use

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Phosphors Vertical deflection assy.

Focus grid

Screen grid Horizontal deflection assy.

Control grid Red Green

CRT Vacuum with an inert gas at low pressure

Blue

C

S F

Convergence magnet

Shadow mask

Purity magnet


Cross-section of a color CRT.

of deflection magnets placed around the CRT neck—these magnets (actually electromagnets) are heavy coils of wire where where the CRT funnel meets meets the neck. Four electromagnets electromagnets are in this “deflection assembly”: two opposing electromagnets direct the beam in the vertical direction, and another set of opposing magnets direct the beam in the horizontal direction. Using electrical signals from the monitor’s monitor’s raster circuits, an electron beam can trace across the entire CRT face. Another element of the CRT that you should understand is called the shadow 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 slot mask . Both types of mask perform the same purpose—to ensure that electron beams strike only the color phosphors of the intended pixel. pixel. This is a vital element of a color monitor. monitor. In a monochrome monitor, the CRT is coated with a single homogeneous layer of phos phor—if stray electrons strike nearby phosphor particles, a letter or line might simply ap pear to be a bit out of focus. For a color CRT, however, however, stray electrons can cause incorrect incorrect colors to appear on nearby nearby pixels. Masks help to preserve preserve color purity. Color purity is also also aided by a purity a purity magnet , magnet , which helps correct correct fine beam positioning. positioning. A convergence magnet helps net 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. vessel. Electrical connections are made through a circular arrangement of sealed pins in the neck. Table 27-2 explains the designations for each pin. Remember 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 might use additional pins.


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IDENTIFYING CRT PROBLEMS CRTs enjoy a long, reliable working life because there are really no moving parts—merely a set of stationary metal elements. elements. However, the arrangement arrangement of grids and cathodes are located in very close proximity to one another. another. Physical shocks can dislodge elements elements and cause sudden short circuits. Eventually, regular use will 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. 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 monitor’s overall cost. In many cases, the cost for a re placement CRT approaches approaches the original cost of the entire monitor. monitor. As a consequence, you should carefully evaluate the economics of replacing the CRT versus buying a new monitor outright. Symptom 27-1. Heater opens in the CRT Each time the heater runs, it expands.

When the CRT turns off, the the heater cools and contracts again. This regular thermal ex pansion and contraction might eventually fatigue fatigue the heater and cause it to open. You will see this as a complete loss of the corresponding color. Because heaters are all tied together electrically, there is no way to measure a particular heater directly, but you might see only two glowing heaters heaters in the CRT neck, instead instead of three. An open heater cannot be recovered, and the only available alternative is to replace the CRT itself. Symptom 27-2. Heater shorts to a cathode in the CRT This is not as strange

as it might seem at first. To heat a cathode effectively, the heater must be extremely extremely close to the cathode—especially to the barium element that actually liberates the electrons. Over time, the heater might 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 gasses inside the CRT should prevent prevent this. But in actuality, some small small amount of oxygen will still still be present in the CRT, and oxidation oxidation might 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 saturated with the color of the defective defective electron gun. T AB AB LE LE 2 77- 2 T YP YP IC IC AL AL C RT RT P IN IN DESIGNATIONS

DESIGNATION

DESCRIPTION

G1

Control grid voltage

G2

Screen grid voltage

G3 or F

Focus grid voltage

KG

Green video signal

KR

Red video signal

KB

Blue video signal

H1

Heater voltage in

H2

Heater voltage out

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For example, if the blue heater shorts to the cathode, the image will appear to be saturated with blue. You will also likely likely see visible retrace lines in 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 27-2, you could check the blue cathode by measuring resistance resistance between the KB and H1 (or H2) pins. Ideally, an infinite resistance should should be between the heaters and cathodes. If there is measurable resistance (or a direct short-circuit), short-circuit), you have found the problem. If the resistance measures infinity, as expected, you might have a defect on the video drive board. Symptom 27-3. Cathode shorts to the control grid in the CRT A cathode can also short-circuit to the the control grid. Often, corrosion flakes flakes off the cathode and contacts the control grid. When this happens, the control control grid loses its effectiveness, effectiveness, and the corresponding color will appear saturated. This symptom will appear very much like a heater short. Fortunately, you should be able to verify this problem problem with your meter by measuring resistance between the control grid grid and the suspect cathode. Ideally, an infinite resistance should be between between the control grid and all cathodes. cathodes. If you read a measurable measurable resistance (or a direct short-circuit), chances are good that you’re facing a cathode-to-control grid short. Symptom 27-4. 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 afflicted cathode will will lose efficiency, resulting in weakened 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 increasing signal gain should not have a substantial effect on the color brightness, and you should consider replacing the CRT. Symptom 27-5. 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 will last for the lifetime of the monitor, monitor, but age and normal use will eventually reduce the sensitivity of the phosphors—for old CRTs, you might see this as dull, low-contrast colors. colors. Perhaps a more dramatic problem problem occurs with “phos phor 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 rejuvenate phosphors, so the CRT will have to be replaced. replaced. You can advise customers to prolong 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 27-6. The CRT suffers 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


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video signal (e.g., 0 to 20 V or 0 to 50 V). V). As cathodes age, however, however, they tend to become non-linear. When this happens, images tend tend to be too “black and white,” rather than disdis play a smooth transition of colors. Technicians often refer to this as a “gassy” CRT, which is actually a CRT gamma gamma problem. In addition to cathode wear, control control grid failure can adversely affect beam intensity. Symptom 27-7. The control grid in the the CRT is open The control grid is used to

limit the beam intensity intensity produced by a cathode by applying a potential on the grid. Occasionally, you will find find that a control grid might open. open. In that case, there is no longer a potential available to control the beam intensity, 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, multimeter, the control gr grid id is probably open, and the CRT will have to be replaced. Symptom 27-8. The CRT screen grid is open The screen grid plays an important

role in image brightness by accelerating the electron beam toward the CRT phosphors. phosphors. If the screen grid opens, no potential potential will be available to begin begin accelerating the beam. This will result in a very dark image—even with the screen voltage voltage at maximum. You might think this is a control-to-screen grid short, but if you can’t find the short with your multimeter, the screen grid is probably open, and the CRT will have to be replaced. Symptom 27-9. The CRT focus grid is open A focus grid assembly concentrates

electron beams into narrow pinpoints by the time the beam reaches the shadow mask. Typically, a focus control is located around the flyback transformer. If the focus grid fails, the image will appear highly distorted, distorted, and the focus adjustment will will have no effect. When a focus grid fails, the entire CRT will have to be replaced. Symptom 27-10. The control grid shorts to the screen grid in the CRT The

same flakes of oxidation that can short a cathode to the control grid can also short the control grid to the screen grid. The screen grid starts accelerating accelerating the electrons toward the CRT face. If the screen grid is shorted, shorted, it will reduce reduce the energy imparted to the the electrons—in effect, a shorted screen grid will significantly reduce the overall image brightness (even with the brightness at maximum). In extreme cases, the image might disappear entirely. You can measure the screen grid voltage voltage at G2, which typically runs from 250 to 750 V in normal operation. If the voltage is low (even with the screen grid control at maximum), power down the monitor, remove the video drive board from the monitor’s neck, restart the monitor, and and measure the screen voltage again. If the screen voltage returns to normal, you can be confident that the screen grid is shorted. If screen voltage remains low, you might have a fault in the screen voltage circuit. You can also verify a short between the control and screen grids by powering down the monitor and measuring resistance between the G1 and G2 pins on the CRT CRT neck. Ideally, it should be an infinite infinite resistance.

CORRECTING SHORTS 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 shorts are not held in place by anything more more than

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gravity, or a slight arc during contact. As a result, it might 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, 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.

CRT TESTERS/REJUVENATORS Because 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 previously. Such devices as Sencore’s CR70 CR70 Universal CRT Restorer/Analyzer Restorer/Analyzer can help check the CRT for shorts and opens, burn out a wide variety of shorts, and (in many cases) rejuvenate weak elements. elements. As another advantage, a tester tester can usually check and rejuvenate a CRT without having having the whole monitor available. available. Most CRT test equipment equipment can perform four major operations: color balance testing, emission testing, removing shorts, and beam rejuvenation: Color-balance testing 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 the variation is greater 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. s 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 air in the CRT gradually block the cathode’s ability to produce electrons. This is “ion poisoning,” which results in in weakened electron beams. A rejuvenator function can usually overcome low-emission low-emission problems. s Removing shorts Generally speaking, a decent CRT tester/rejuvenator can remove shorts between the control control grid and the cathode or the screen screen grid. In actuality, you might 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. s 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 (mak(making the cathode extremely hot), then passing a 100- to 150-mA current through the cathode. The effect of rejuvenation exposes fresh fresh emitting material, which, which, in turn, adds new life to weakened guns. A current meter measures beam beam current; when beam current reaches its nominal range during rejuvenation, the electron gun is restored. s

Troubleshooting Troubleshooting a Color Monitor Any chapter about monitor troubleshooting must start with a reminder of the dangers involved. Computer monitors monitors use very high voltages for proper operation. Potentially lethal lethal shock hazards exist within the monitor assembly—both from ordinary ac line voltage, as well as from the CRT anode anode voltage developed by the flyback transformer. transformer. You must use extreme caution whenever a monitor’s monitor’s outer housings are removed. removed. If you are uncomfort-

TROUBL TROUBLESH ESHOOT OOTING ING A COLOR COLOR MONITO MONITOR R

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able with the idea of working around high voltages, defer your troubleshooting to an ex perienced technician.

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

POST-REPAI POST-REPAIR R TESTING AND ALIGNMEN T 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 usually marked on the specification plate plate glued to the outer housing, or recorded recorded on a sticker placed somewhere somewhere inside the housing. housing. If you cannot find the high-voltage level, refer to service data from the monitor’s manufacturer. Once high-voltage is correct, correct, you can proceed with other alignment tests. tests. Refer to Chapter 57 for testing and alignment procedures. procedures. When testing (and realignment) realignment) is complete, it is wise to let the monitor run for 24 hours or so (called a burn-in test ) before returning it to service. Running the monitor for a prolonged prolonged period helps ensure that the original original problem has indeed been resolved. resolved. This is a form of quali quality ty control. If the problem resurfaces, resurfaces, a more serious problem might be elsewhere in the monitor.

SYMPTOMS Symptom 27-11. The image is saturated with red or appears greenishblue (cyan) If any user color controls are available from the front or rear housings, be

sure that those controls have not been accidentally accidentally adjusted. If color controls are set properly (or not available externally), externally), the red video drive circuit has probably failed. Refer to the example circuit of Fig. 27-8. Use your oscilloscope to trace the video signal from its initial input to the final output. output. If no red video signal is at the amplifier input (i.e., (i.e., the base of Q501), check the connection between between the monitor and the video adapter board. If the connection is intact, try try a known-good monitor. If the problem persists on a known-good known-good monitor, replace the video 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 com ponent-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 might might have failed. If you have access to a CRT CRT tester/rejuvenator, tester/rejuvenator,

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test the CRT. If the CRT measures bad (and (and cannot be recovered through any available available re juvenation procedure), it should should be replaced. A color CRT is usually the most expensive expensive component in the monitor. As with any CRT replacement, you should carefully carefully consider the economics of the repair versus buying a new or rebuilt monitor. Symptom 27-12. The image is saturated with blue or appears yellow If

any user color controls are available from the front or rear housings, be sure that those controls have not been accidentally adjusted. adjusted. If color controls are set properly properly (or are not available externally), externally), the blue video drive circuit has probably probably failed. Refer to the exam ple circuit of Fig. 27-8. Use your oscilloscope to trace the video video signal from its initial inin put to the final output. If no blue video signal is at the amplifier input (i.e., (i.e., the base of Q701), check the connection between the monitor and the video adapter board. If the connection is intact, try try a known-good monitor. If the problem persists persists on a known-good monitor, replace the video adapter 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 com ponent-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 that a fault is in the CRT itself—the corresponding cathode or video control grid might have failed. If you have access to a CRT tester/rejuvenator, test the CRT. If the CRT measures bad (and cannot be recovered through any available rejuvenation procedure), it should be replaced. 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 27-13. The image is saturated saturated with green, or appears bluish-red (magenta) If any user color controls are available from the front or rear housings, be

sure that those controls have not been accidentally accidentally adjusted. If color controls are set properly (or not available externally), externally), the green video drive circuit circuit has probably failed. Refer to the example circuit of Fig. 27-8. Use your oscilloscope to trace the video signal signal from its initial input to the final output. If no green video signal is 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 known-good monitor. If the problem persists on a knowngood monitor, replace replace the video adapter board. board. As you trace the video 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 probably the point of failure, and the offending component should be replaced. 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 might have failed. If you have access to a CRT tester/rejuvenator, test the CRT. If the CRT measures bad (and cannot be recovered recovered through any available rejuvenation procedure), procedure), it should be replaced. A color CRT is usually the most expensive component in the monitor. As with any CRT replacement, replacement, you should carefully consider the economics of the repair versus buying a new or rebuilt monitor.


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Symptom 27-14. Raster is present, but there there is no image When the monitor

is properly connected to a PC, a series of text information should appear as the PC initializes. You can use this as your baseline baseline image. Isolate the monitor by trying a known-good monitor on your host PC. If the known-good monitor works, works, you prove that the PC and video adapter are working properly. properly. Reconnect the suspect monitor to to the PC and turn up the brightness (and contrast if necessary). You should see a faint white haze covering the display. This raster is generated generated by the normal sweep sweep of an electron beam. beam. Remember that the PC must be on and running. Without the horizontal and vertical 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 connectors between the video adapter board and the monimonitor’s main PC board. A loose or severed wire can interrupt the voltage(s) voltage(s) powering the board. You should also also check each output from your your power supply. A low or missing voltage can disable your video circuits 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 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 respective amplifier circuits. circuits. Chances are that you will see all all three video signals fail at the same location location of each circuit. This is usually because of a problem in common parts of the video circuits. In the example video drive board of Fig. 27-8, such common circuitry involves the components marked marked with 8xx numbers (e.g., Q801). If you do not have the tools or inclination to perform such component-level troubleshooting, re place the video drive PC board. You should also suspect a problem with the raster-blanking circuits. circuits. During horizontaland vertical-retrace periods, 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 re placing 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—little itself—little else can fail. If you have a CRT tester/rejuvenator available, you should test the the CRT thoroughly for shorted shorted grids or a weak cathode. If the problem cannot be rectified through rejuvenation (or you do not have access to a CRT tester), try replacing replacing the CRT. A CRT is usually the most expensive part 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 re built unit. Symptom 27-15. A single horizontal line appears in the middle of the display The horizontal sweep is is working properly, but there there is no vertical deflection. deflection. A

fault has almost certainly certainly developed in the vertical drive drive circuit (refer to Fig. 27-9). 27-9). Use your oscilloscope to check the sawtooth wave being generated by the vertical oscillator/amplifier oscillator/amplifier IC (pin 6 of IC402). If the sawtooth wave is missing, missing, the fault is almost certainly in in the IC. For the circuit of Fig. 27-9, try try replacing IC402. If the sawtooth wave is available on IC402 pin 6, suspect that a defect is in the horizontal deflection yoke

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itself or in one of its related components. If you are not able to check signals to the com ponent level, simply replace the monitor’s main PC board. Symptom 27-16. Only the upper or lower half of an image appears In most

cases, a problem is in the vertical amplifier. amplifier. For the example circuit of Fig. Fig. 27-9, the trou ble is likely in the vertical oscillator/amplifier oscillator/amplifier (IC402). Use your oscilloscope to check the sawtooth waveform waveform leaving IC402 IC402 pin 6. If the sawtooth sawtooth is distorted, distorted, replace IC402. If the sawtooth signal reads properly, check for other faulty components in the vertical deflection yoke circuit. circuit. If you do not have the tools or inclination inclination to check and replace dedevices at the component level, replace the monitor’s monitor’s main PC board. When the image is restored, be sure to check vertical linearity (Chapter 57). Symptom 27-17. A single vertical line appears along the the middle of the display The vertical sweep is working properly, properly, but there is no horizontal deflection. How-

ever, to even see the display at normal brightness, high-voltage must be present in the monitor—the horizontal drive circuit circuit must be working (refer to Fig. 27-9). The fault probably lies in the horizontal deflection deflection yoke. Check the yoke and all wiring connected to it. It might be necessary to replace the horizontal deflection yoke or the entire yoke assembly. If horizontal deflection is lost, as well as substantial screen brightness, a marginal fault might be in the horizontal drive drive circuit. If the problem is with with the horizontal oscillator oscillator pulses, the switching characteristics characteristics of the horizontal amplifier amplifier will change. In turn, this affects high-voltage high-voltage development and horizontal horizontal deflection. Use your oscilloscope oscilloscope to check the square wave generated by the horizontal oscillator IC301 IC301 pin 3 (Fig. 27-9). You should see a square square wave. If the square wave wave is distorted, replace the oscillator oscillator IC (IC301). If the horizontal pulse pulse is correct, check check the horizontal switching switching transistors (Q301 and Q302). Replace any transistor transistor that appears defective. defective. If the collector signal at the HOT is low or distorted, a short circuit might be in the flyback transformer primary winding. Try replacing the the FBT. If you do not have the tools or inclination inclination to check com ponents to the component level (or the problem persists), replace the monitor’s main PC board. When the repair is complete, check the horizontal linearity linearity and size (Chapter 57). Symptom 27-18. There is no image and no raster When the monitor is prop-

erly connected to a PC, a series of text information should appear as the PC initializes. You 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 synchroniza tion pulses. If pulses are absent, no raster will be generated. If sync pulses are present, the problem is probably 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- and 135-Vdc outputs, outputs, as shown in Fig. 27-9). A low or absent supply voltage will disable the horizontal deflection deflection and high-voltage circuits. circuits. If one or more supply outputs are low or absent, you can troubleshoot the power-supply circuit or replace the power supply outright (if 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, correctly, suspect your horizontal horizontal drive circuit. Use your oscilloscope to check the horizontal oscillator oscillator output at the base of Q301 (Fig. 27-9). You


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should see a square wave. If the square wave is low, distorted, distorted, or absent, replace replace the horizontal oscillator IC (IC301). If a regular pulse is present, the horizontal oscillator is working. Because Q301 is intended intended to act as a switch, you you should also find a pulse at the the collector of Q301. If the pulse output is severely distorted or absent, absent, Q301 is probably damaged (remove (remove Q301 and test test it). If Q301 reads as faulty, it should be replaced. replaced. If Q301 reads good, check the horizontal coupling transformer (T303) for shorted or open windings. Try replacing T303 (little else 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 replaced with an exact replacement part. If Q302 reads good, the fault probably lies in in the flyback transformer. 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. If these steps fail to restore the image, the the CRT has probably failed. If you have access to a CRT tester/rejuvenator, tester/rejuvenator, you can test the CRT. If the CRT measures as bad (and can not be restored through rejuvenation), it should be replaced. replaced. If you do not have a CRT test instrument, you can simply replace the CRT. 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 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 (Chapter 57). Symptom 27-19. The image is too compressed or too expanded A whitish

haze might appear along the bottom of the the image. Start by checking your vertical size control to be sure that it was not adjusted accidentally. Because vertical size is a function of the vertical sawtooth oscillator, you should suspect tha t a problem is in the vertical oscillator circuit. A sawtooth signal that is too large will result in an over-expanded image, but 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. Fig. 27-9, you should find a sawtooth signal on IC402 pin 6. If the signal is incorrect, try replacing IC402. You might 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 27-20. The displayed characters appear to be distorted The term

distortion can be interpreted in many different ways. ways. For the purposes of this book, the defdefinition is simply that the image (usually text) is difficult difficult to read. Before even opening your toolbox, check the monitor’s location. The presence of stray magnetic fields 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 objects (such as motors or refrigerator magnets) from the area. If the problem persists, it is likely likely that the monitor is at fault. If only certain areas of the display appear affected (or affected worse than other areas), the trouble is probably caused caused by poor linearity (either (either horizontal, vertical, vertical, or both). If raster speed varies across the display, the pixels in some areas of the image might appear too close together, although the pixels in other areas of the image might appear too far apart. You can check and correct horizontal and vertical vertical linearity using a test pattern, pattern, such as the one described in Chapter 57. If alignment fails to correct 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 focus alignment. If you cannot

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achieve a sharp focus using controls either on the front panel of the monitor or on the fly back transformer assembly, assembly, a fault is probably in the flyback transformer. Try replacing the FBT. If the problem persists, your best course is often simply to replace the monitor’s monitor’s main PC board. Symptom 27-21. The display appears wavy Waves appear along the edges of the

display as the image sways back and forth. This is almost always the the result of a powersupply problem—one or more outputs is failing. failing. Use your multimeter and check each sup ply output. If you find a low or absent output, you can proceed to troubleshoot troubleshoot the supply or you can simply replace the supply supply outright. If the power supply is integrated integrated onto the main PC board, you will have to replace the entire main PC board. Symptom 27-22. The display is too too bright or too dim Before opening the mon-

itor, be sure to check the brightness brightness and contrast controls. If the controls had been acciaccidentally adjusted, set contrast to maximum, and adjust the brightness level until a clear, crisp display is produced. If the front-panel controls fail to provide the proper proper display (but focus seems steady), suspect that that a fault is in the monitor’s power supply. Refer to the example schematic of Fig. 27-9. If the 135-Vdc supply is too low or too high, brightness levels controlling the CRT screen screen grid will shift. If you find one or more incorrect outputs from the power supply, you can troubleshoot the 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 27-23. Visible raster scan lines are in the the display The very first

places that you should check are the front-panel brightness 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 tend to make the image appear appear a bit fuzzy. If the front-panel controls cannot eliminate visible raster from the image, chances are that you have a problem with the power supply. Use your multimeter and check each each output from the supply. If one or or more outputs appear too high (or too low), you can troubleshoot the supply or replace the supply outright. If the supply is integrated integrated with the monitor’s main main PC board, the entire PC board will have to be replaced. If the power supply is intact, you should suspect that a problem is in the raster blanking circuits. During horizontal and vertical retrace retrace periods, video signals signals are cut off. If visi ble raster lines appear appear in your image, check the blanking signals. signals. If you are unable to check the blanking signals, signals, try replacing the video video drive PC board. If a new video drive board fails to correct the problem, replace the main PC board. Symptom 27-24. 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 problem is a fault in the video cable between the video board and the monitor. monitor. Electrical noise noise (sometimes called called crosstalk crosstalk ) in the cable might allow signals representing one color to accidentally be picked up in another colorsignal wire. This can easily easily cause unwanted 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 type of problem. Try wiggling the cable. cable. If the problem stops, appears appears in-


933

termittent, 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. Fig. 27-8. Such capacitors as as C505 and C506 are typically low-value, high-voltage components, so they tend to degrade rather quickly. Fortunately, such capacitors are easy easy to spot on the video 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. capacitor. If the problem disappears, the one you froze is probably probably defective. Otherwise, you can turn off and unplug the monitor, then check each capacitor individually. When replacing capacitors in the video amplifier amplifier circuit, be sure to replace them with those of the same type and voltage rating. If capacitors are not at fault, suspect a problem in the amplifier transistors transistors on the video amplifier board (i.e., (i.e., Q504, Q505, or Q506). Turn off and unplug the monitor, then check each of the transistors. transistors. Chances are that your readings readings will be inconclusive, inconclusive, so try com paring readings from each transistor to find a device that gives the most unusual readings. Replace any defective or questionable questionable amplifier transistors. transistors. If you do not have the time or inclination to troubleshoot the video amplifier board, try replacing the board outright. Symptom 27-25. Colors appear to change change when the monitor is warm warm Either

colors will appear correctly when the monitor monit or 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 ca ble—especially its connection to the raster board inside the monitor. If this connection is loose, it might be intermittent or unreliable. Tighten any loose connections and try the monmonitor 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 re-soldering each of the junctions. Chances are that age or thermal stress has fatigued one or more connections. By re-soldering the connections, connections, you should be able to correct any potential potential connection problems. You might also try re-soldering re-soldering the connector which passes video data data from the raster board to the video amplifier amplifier board. If your problems persist, try replacing the video amplifier board. Symptom 27-26. An image appears distorted distorted in 350- or 400-line mode In

most cases, the “distortion” “distortion” is an image image that appears excessively excessively compressed. As you probably read earlier in this book, different screen modes have a different number of horizontal lines (e.g., a 640-×-480 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 320-×-200 mode). As you might expect, the “size” “size” of each pixel has to be adjusted when the screen mode changes to keep the image roughly square—otherwise the image simply “shrinks.” Monitors detect the screen mode by checking the polarity polarity of the sync signals. You can see this function in the schematic of Fig. 27-9. 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. size. If you notice this kind of distortion without without warning, suspect a problem with with the sync-sensing circuit. If the sync-sensing circuit circuit is incor porated into a single single IC (such as IC201), IC201), replace the IC outright. If you notice a size

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problem after aligning the monitor, you might have accidentally upset a size adjustment. Re-adjust the size controls to restore proper image dimensions. Symptom 27-27. The fine detail of high-resolution graphic images appears a bit fuzzy At best, this kind of symptom might not appear noticeable without

careful inspection, but it might 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. pixels. If a weakness in the video amplifier(s) occurs, occurs, it can limit bandwidth and degrade video performance performance at high resolutions. The problem will likely 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 first check al alll connectors for secure installation. Nicked or frayed video cables can also contribute to the problem. If the problem remains, replace the video amplifier board. Symptom 27-28. 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 will almost certainly result in such intermittent intermittent problems. If the connection is secure, an intermittent intermittent connection is in the video cable. Before replacing the cable, check its connections within the monitor itself. If connections are intact, replace the intermittent video cable outright. outright. Do not bother cutting or splicing the cable—any cable—any breaks in the signal shielding will cause crosstalk, which will result in color bleeding. Symptom 27-29. The image expands in the the horizontal direction when the monitor gets warm One or more components in the horizontal retrace circuit are

weak—and changing value value a bit once the monitor gets warm. warm. Turn off and unplug the monitor. You should inspect any capacitors located located around the Horizontal the Horizontal Output Tran sistor (HOT) . The problem is that thermal problems, problems, such as this, can be extremely diffidifficult to isolate because you can’t measure capacitor values while the monitor is running; 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 replace the raster board. Symptom 27-30. The image shrinks in the horizontal direction when the monitor gets warm This is another thermal-related problem that indicates either a

weakness in one or more components or a mild soldering-related soldering-related problem. Turn off and unplug the monitor. Start by checking for a poor solder connection—especially connection—especially around the horizontal deflection yoke wiring, the Horizontal the Horizontal Output Transistor (HOT) , and the fly back transformer. If nothing appears obvious, consider consider resoldering all of the components components in the HOT area of the raster board. board. If problems continue, suspect suspect that a failure is in the HOT itself. Semiconductors rarely rarely become marginal—they marginal—they either work or they don’t. don’t. Still, semiconductor junctions can become unstable when temperatures change, and result in circuit characteristic changes. 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 V) are sagging when the monitor warms up. Use a voltmeter and measure the outputs from your power supply. If the 12- or 20-V outputs appear to drop drop a little once the monitor has been running for a bit, you should troubleshoot the power supply.


935

Symptom 27-31. High-voltage fails after the monitor is warm A large num-

ber of possible causes are behind this problem, but no matter what permutation you find, you will likely be dealing with soldering soldering problems or thermal-related thermal-related failures. Turn off and unplug the monitor. Inspect the HOT’s heatsink assembly—a assembly—a bad solder connection might be on the heatsink ground. ground. An open solder connection might might be on one or more of the flyback transforme transformerr pins. If you cannot locate a faulty soldering connection, connection, you might simply choose to re-solder all of the connections in the flyback area. If the problem persists, suspect that either your HOT or flyback transformer is failing under load (after the monitor warms 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 that the problem is 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 transtransformer. If you do not have an oscilloscope, try replacing the HOT HOT first because 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, carefully carefully inspect the capacitors in the HOT HOT circuit. One or more might be failing. Unfortunately, it is very difficult to identify a marginal capacitor (especially (especially one that is suffering from a thermal thermal failure). You might try replacing the major major capacitors in the HOT circuit or replace the raster board entirely. Symptom 27-32. The image blooms intermittently The amount of high voltage

driving the CRT is varying intermittently. Because high voltage is related to the HOT circuit and flyback transformer, 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 might try resoldering all of the connections in those areas (remember to turn off and unplug the monitor before soldering). A ground problem might also be on the video amplifier board, which allows all three color signals to vary in amplitude. If this happens, the overall brightness of the image changes, and the image might grow or shrink a bit in response. Try resoldering connections on the video amplifier board. If the problem remains (even after soldering), your FBT might be failing—probably because of an age-related internal short. High-end test equipment, such as Sencore’s monimonitor test station provides provides the instrumentation instrumentation to test a flyback transformer. 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, replace the raster board outright. In the unlikely event that your problem persists, persists, suspect a fault in the CRT itself. If you have access to a CRT tester/rejuvenator, tester/rejuvenator, you can check the CRT’s operation. Some weaknesses in the CRT might be corrected (at least temporarily) by rejuvenation. If the fault cannot be corrected, corrected, you might have to replace the CRT. CRT. Symptom 27-33. The image appears out of focus Before suspecting a compo-

nent failure, try adjusting the focus control. In most cases, the focus control is located ad jacent to the flyback transformer. transformer. Remember that the focus focus control should be adjusted adjusted with brightness and contrast set to optimum values—excessively bright images might lose focus naturally. If the focus control is unable to restore a proper image, check the CRT CRT focus voltage. In Fig. 27-9, you can find the focus focus voltage off a flyback flyback transformer transformer tap. If the focus voltage is low (often combined with a dim image), you might have a failing FBT.

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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 that a fault is in the CRT— probably in the focus grid. You can use a CRT tester/rejuvenator to examine the CRT, and it might be possible to restore normal operation operation (at least temporarily). temporarily). If you do not have such equipment, you will simply have to try a new CRT. Symptom 27-34. The image appears to flip or scroll horizontally A syn-

chronization problem is in your horizontal raster raster circuit. Begin by checking the video ca ble to be sure that it is installed and connected securely. Cables that behave intermittently intermittently (or that appear frayed or nicked) should be replaced. If the cable is intact, suspect a problem in your horizontal circuit. If a horizontal-sync (or “horizontal “horizontal hold”) adjustment is on the raster board, adjust it in small increments increments until the image snaps back into sync. If no such adjustment is on your particular monitor, try resoldering all of the connections in the horizontal-processing horizontal-processing circuit. If the problem persists, replace the horizontal horizontal oscillator IC or replace the entire raster board. Symptom 27-35. The image appears to flip or scroll vertically A synchro-

nization problem is in your vertical vertical raster circuit. Begin by checking the video cable to be sure that it is installed and connected connected securely. Cables that behave intermittently intermittently (or that ap pear frayed or nicked) should be replaced. If the cable is intact, suspect that the problem is in your vertical circuit. If a vertical-sync (or “vertical “vertical hold”) adjustment is on the raster board, board, adjust it in small increments until the image snaps back into sync. If no such adjustment is on your particular monitor, try resoldering all of the connections in the vertical-processing circuit. If the problem persists, replace the vertical oscillator oscillator IC or replace the entire raster board. Symptom 27-36.

The image appears to shake or oscillate in size This

might occur in bursts, but it typically typically occurs constantly. In most cases, this is caused by a fault in the power supply—usually the 135-V (B+) output. Try measuring your power-sup ply 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 filtering portion of that that output might be malfunctioning. Track the output back into the supply and replace any defective components. components. If you are unable to isolate a faulty component, replace the power supply. supply. If the power sup ply is integrated onto the raster board, you might have to replace the raster board entirely. If the outputs from your power supply appear to be stable, you should suspect that a weak capacitor is in your horizontal circuit. Try resoldering the FBT, HOT, HOT, and other horizontal circuit components to eliminate the possibility 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 individual components, re place the raster board outright. Here’s an unusual problem. The shaking you see might be related related to a problem in the degaussing coil located around the CRT funnel. funnel. Ordinarily, the degaussing coil coil should unleash the most of its energy in the initial moments moments after monitor power is turned on. Thermistors (or posistors) in the power supply quickly diminish coil voltage—effectively cutting off the degaussing degaussing coil’s operation. A fault in the degaussing-coil degaussing-coil circuit (in the the

FURTHER FURTHER STUDY

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power supply) might continue to allow enough power to the coil to affect the image’s sta bility. Try disconnecting the degaussing coil. If the problem remains, remains, the degaussing coil coil is probably operating properly. If the problem disappears, a fault is in the degaussing-coil degaussing-coil circuit.

Further Study That concludes Chapter Chapter 27. Be sure to review the glossary glossary and chapter questions on the the accompanying CD. If you have access to the Internet, take a look at some of these monitor resources: 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

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