Contents:
Notes on the Troubleshooting and Repair of Computer and Video Monitors
Copyright (c) 1994, 1995, 1996, 1997, 1998
All Rights Reserved
Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:
In the early days of small computers, a 110 baud teletype with a personal paper tape reader was the 'preferred' input-output device (meaning that this was a great improvement over punched cards and having to deal with the bozos in the computer room. Small here, also meant something that would comfortably fit into a couple of 6 foot electronics racks!) The earliest personal computers didn't come with a display - you connected them to the family TV. You and your kids shared the single TV and the Flintstones often won out. The Commodore 64 would never have been as successful as it was if an expensive monitor were required rather than an option. However, as computer performance improved, it quickly became clear that a dedicated display was essential. Even for simple text, a TV can only display 40 characters across the screen with any degree of clarity. When the IBM PC was introduced, it came with a nice 80x25 green monochrome text display. It was bright, crisp, and stable. Mono graphics (MGA or MDA) was added at 720x350, CGA at a range of resolutions from 160x200 to 640x200 at 2 to 16 colors, and EGA extended this up to a spectacular resolution of 640x350. This was really fine until the introduction of Windows (well, at least once Windows stayed up long enough for you to care). All of these displays used digital video - TTL signals which coded for a specific discrete number of possible colors and intensities. Both the video adapter and the monitor were limited to 2, 4, or 16 colors depending on the graphics standard. The video signals were logic bits - 0s and 1s. With the introduction of the VGA standard, personal computer graphics became 'real'. VGA and its successors - PGA, XGA, and all of the SVGA (non) standards use analog video - each of the R, G, and B signals is a continuous voltage which can represent a continuous range of intensities for each color. In principle, an analog monitor is capable of an unlimited number of possible colors and intensities. (In practice, unavoidable noise and limitations of the CRT restricts the actual number to order of 64-256 distinguishable intensities for each channel.) Note that analog video was only new to the PC world. TVs and other video equipment, workstations, and image analysis systems had utilized analog signals for many years prior to the PC's 'discovery' of this approach. In all fairness, both the display adapter and monitor are more expensive so it is not surprising that early PCs did not use analog video. Most of the information in the document applies to color computer video monitors and TV studio monitors as well as the display portions of television sets. Black and white, gray scale, and monochrome monitors use a subset of the circuitry (and generally at lower power levels) in color monitors so much of it applies to these as well. For most descriptions of symptoms, testing, diagnosis, and repair, an auto-scan PC SVGA monitor is assumed. For a fixed frequency workstation monitor, studio video monitor, or closed circuit TV monitor, only a subset of the possible faults and procedures will apply. Note: we use the term 'auto-scan' to describe a monitor which accepts a wide (and possibly continuous) range of scan rates. Usually, this refers mostly to the horizontal frequency as the vertical refresh rate is quite flexible on many monitors of all types. Fixed scan or fixed frequency monitors are designed to work with a single scan rate (though a 5% or so variation may actually be accepted). Multi-scan monitors sync at two or more distinct scan rates. While not very common anymore, multi-scan monitors may still be found in some specific applications.
See the manuals on "Troubleshooting and Repair of Small Switchmode Power Supplies" and "Troubleshooting and Repair of Television Sets" for additional useful pointers. Since a monitor must perform a subset of the functions of a TV, many of the problems and solutions are similar. For power related problems the info on SMPSs may be useful as well. If you are considering purchasing a monitor or have one that you would like to evaluate, see the companion document: "Performance testing of Computer and Video Monitors".
Note: throughout this document, we use the term 'raster' to refer to the entire extent of the scanned portion of the screen and the terms 'picture', 'image'. or 'display', to refer to the actual presentation content. Monitors designed for PCs, workstations, and studio video have many characteristics in common. Modern computer monitors share many similarities with TVs but the auto-scan and high scan rate deflection circuitry and more sophisticated power supplies complicates their servicing. Currently, most computer monitors are still based on the Cathode Ray Tube (CRT) as the display device. However, handheld equipment, laptop computers, and the screens inside video projectors now use flat panel technology, mostly Liquid Crystal Displays - LCDs. These are a lot less bulky than CRTs, use less power, and have better geometry - but suffer from certain flaws. First, the picture quality in terms of gray scale and color is generally inferior to a decent analog monitor. The number of distinct shades of gray or distinct colors is a lot more limited. They are generally not as responsive as CRTs when it comes to real-time video which is becoming increasingly important with multimedia computers. Brightness is generally not as good as a decent CRT display. And last but not least, the cost is still much much higher due both to the increased complexity of flat panel technology and lower production volumes (though this is certainly increasing dramatically). It is really hard to beat the simplicity of the shadow mask CRT. For example, a decent quality active matrix color LCD panel may add $1000 to the cost of a notebook computer compared to $200 for a VGA monitor. More of these panels go into the dumpster than make it to product due to manufacturing imperfections. However, a variety of technologies are currently competing for use in the flat panel displays of the future. Among these are advanced LCD, plasma discharge, and field emission displays. Only time will tell which, if any survives to become **the** picture-on-the-wall or notepad display - at reasonable cost. Projection - large screen - TVs and monitors, on the other hand, may be able to take advantage of a novel development in integrated micromachining - the Texas Instruments Inc. Digital Micromirror Device (DMD). This is basically an integrated circuit with a tiltable micromirror for each pixel fabricated on top of a static memory - RAM - cell. This technology would permit nearly any size projection display to be produced and would therefore be applicable to high resolution computer monitors as well as HDTV. Since it is a reflective device, the light source can be as bright as needed. This is still not a commercial product but stay on line.
The following describe the capabilities which characterize a display:
1. Resolution - the number of resolvable pixels on each line and the
number of scanning lines. Bandwidth of the video source, cable, and
monitor video amplifiers as well as CRT focus spot size are all critical.
However, maximum resolution on a color CRT is limited by the dot/slot/line
pitch of the CRT shadow/slot mask or aperture grille.
2. Refresh rate - the number of complete images 'painted' on the screen
each second. Non-interlaced or progressive scanning posts the entire
frame during each sweep from top to bottom. Interlaced scanning posts
1/2 of the frame called a field - first the even field and then the
odd field. This interleaving reduces the apparent flicker for a given
display bandwidth when displaying smooth imagery such as for TV. It is
usually not acceptable for computer graphics, however, as thin horizontal
lines tend to flicker at 1/2 the vertical scan rate. Refresh rate is the
predominant factor that affects the flicker of the display though the
persistence of the CRT phosphors are also a consideration. Long persistence
phosphors decrease flicker at the expense of smearing when the picture
changes or moves. Vertical scan rate is equal to the refresh rate for
non-interlaced monitors but is the twice the refresh rate for interlaced
monitors (1 frame equals 2 fields). Non-interlaced vertical refresh rates
of 70-75 Hz are considered desirable for computer displays. Television
uses 25 or 30 Hz (frame rate) interlaced scanning in most countries.
3. Horizontal scan rate - the frequency at which the electron beam(s) move
across the screen. The horizontal scan rate is often the limiting factor
in supporting high refresh rate high resolution displays. It is what may
cause failure if scan rate speed limits are exceeded due to the component
stress levels in high performance deflection systems.
4. Color or monochrome - a color monitor has a CRT with three electron
guns each associated with a primary color - red, green, or blue.
Nearly all visible colors can be created from a mix of primaries
with suitable spectral characteristics using this additive color
system.
A monochrome monitor has a CRT with a single electron gun. However,
the actual color of the display may be white, amber, green, or whatever
single color is desired as determined by the phosphor of the CRT selected.
5. Digital or analog signal - a digital input can only assume a discrete
number of states depending on how many bits are provided. A single bit
input can only produce two levels - usually black or white (or amber,
green, etc.). Four bit EGA can display up to 16 colors (with a color
monitor) or 16 shades of gray (with a monochrome monitor).
Analog inputs allow for a theoretically unlimited number of possible gray
levels or colors. However, the actual storage and digital-to-analog
convertors in any display adapter or frame store and/or unavoidable
noise and other characteristics of the CRT - and ultimately, limitations
in the psychovisual eye-brain system will limit this to a practical
maximum of 64-256 discernible levels for a gray scale display or for
each color channel.
However, very high performance digital video sources may have RAMDACs (D/A
convertors with video lookup tables) of up to 10 or more bits of intensity
resolution. While it is not possible to perceive this many distinct gray
levels or colors (per color channel), this does permit more accurate tone
scale ('gamma') correction to be applied (via a lookup table in the RAMDAC)
to compensate for the unavoidable non-linearity of the CRT phosphor
response curve or to match specific photometric requirements.
Monitors can be classified into three general categories: 1. Studio video monitors - Fixed scanning rate for the TV standards in the country in which they are used. High quality, often high cost, utilitarian case (read: ugly), underscan option. Small closed circuit TV monitors fall into the class. Input is usually composite (i.e., NTSC or PAL) although RGB types are available. 2. Fixed frequency RGB - High resolution, fixed scan rate. High quality, high cost, very stable display. Inputs are analog RGB using either separate BNC connectors or a 13W3 (Sun) connector. These often have multiple sync options. The BNC variety permit multiple monitors to be driven off of the same source by daisychaining. Generally used underscanned for computer workstation (e.g., X-windows) applications so that entire frame buffer is visible. There are also fixed frequency monochrome monitors which may be digital or analog input using a BNC, 13W3, or special connector. 3. Multi-scan or auto-scan - Support multiple resolutions and scan rates or multiple ranges of resolutions and scan rates. The quality and cost of these monitors ranges all over the map. While cost is not a strict measure of picture quality and reliability, there is a strong correlation. Input is most often analog RGB but some older monitors of this type (e.g., Mitsubishi AUM1381) support a variety of digital (TTL) modes as well. A full complement of user controls permits adjustment of brightness, contrast, position, size, etc. to taste. Circuitry in the monitor identifies the video scan rate automatically and sets up the appropriate circuitry. With more sophisticated (and expensive) designs, the monitor automatically sets the appropriate parameters for user preferences from memory as well. The DB15 high density VGA connector is most common though BNCs may be used or may be present as an auxiliary (and better quality) input.
Thank IBM. Since the PC has evolved over a period of 15 years, display adapters have changed and improved a number of times. With an open system, vendors with more vision (and willing to take more risks) than IBM were continuously coming up with improved higher resolution display adapters. With workstations and the Apple MacIntosh, the primary vendor can control most aspects of the hardware and software of the computer system. Not so with PCs. New improved hardware adapters were being introduced regularly which were not following any standards for the high resolution modes (but attempted to be backward compatible with the original VGA as well as EGA and CGA (at least in terms of software)). Vast numbers of programs were written that were designed to directly control the CGA, EGA, and VGA hardware. Adapter cards could be designed to emulate these older modes on a fixed frequency high resolution monitor (and these exist to permit high quality fixed scan rate workstation monitors to be used on PCs) However, these would be (and are) much more expensive than basic display adapters that simply switch scan rates based on mode. Thus, auto-scan monitors evolved to accommodate the multiple resolutions that different programs required. Note: we will use the generic term 'auto-scan' to refer to a monitor which automatically senses the input video scan rate and selects the appropriate horizontal and vertical deflection circuitry and power supply voltages to display this video. Multi-scan monitors, while simpler than true auto-scan monitors, will still have much of the same scan rate detection and selection circuitry. Manufacturers use various buzz words to describe their versions of these monitors including 'multisync', 'autosync','panasync', 'omnisync', as well as 'autoscan' and 'multiscan'. Ultimately, the fixed scan rate monitor may reappear for PCs. Consider one simple fact: it is becoming cheaper to design and manufacture complex digital processing hardware than to produce the reliable high quality analog and power electronics needed for an auto-scan monitor. This is being done in the specialty market now. Eventually, the development of accelerated chipsets for graphics mode emulation may be forced by the increasing popularity of flat panel displays - which are basically similar to fixed scan rate monitors in terms of their interfacing requirements.
There are two aspects of monitor design that can be described in terms of analog or digital characteristics: 1. The video inputs. Early PC monitors, video display terminal monitors, and mono workstation monitors use digital input signals which are usually TTL but some very high resolution monitors may use ECL instead. 2. The monitor control and user interface. Originally, monitors all used knobs - sometimes quite a number of them - to control all functions like brightness, contrast, position, size, linearity, pincushion, convergence, etc. However, as the costs of digital circuitry came down - and the need to remember settings for multiple scan rates and resolutions arose, digital - microprocessor control - became an attractive alternative in terms of design, manufacturing costs, and user convenience. Now, most better quality monitors use digital controls - buttons and menus - for almost all adjustments except possibly brightness and contrast where knobs are still more convenient. Since monitors with digital signal inputs are almost extinct today except for specialized applications, it is usually safe to assume that 'digital' monitor refers to the user interface and microprocessor control.
Whether a monitor runs interlaced or non-interlaced is almost always strictly a function of the video source timing. The vertical sync pulse is offset an amount equal to 1/2 the line time on alternate fields (vertical scans - two fields make up a frame when interlaced scanning is used). Generally, a monitor that runs at a given resolution non-interlaced can run at a resolution with roughly twice the number of pixels interlaced at the same horizontal scan rate. For example, a monitor that will run 1024x768 non-interlaced at 70 Hz will run 1280x1024 interlaced at a 40 Hz frame rate. Whether the image is usable at the higher resolution of course also depends on many other factors including the dot pitch of the CRT and video bandwidth of the video card and monitor video amplifiers, as well as cable quality and termination. The flicker of fine horizontal lines may also be objectionable.
The ultimate perceived quality of your display is influenced by many aspects of the total video source/computer-cable-monitor system. Among them are: 1. Resolution of the video source. For a computer display, this is determined by the number of pixels on each visible scan line and the number of visible scan lines on the entire picture. 2. The pitch of the shadow mask or aperture grille of the CRT. The smallest color element on the face of the CRT is determined by the spacing of the groups of R, G, and B colors phosphors. The actual conversion from dot or line pitch to resolution differs slightly among dot or slot mask and aperture grille CRTs but in general, the finer, the better - and more expensive. Typical television CRTs are rather coarse - .75 mm might be a reasonable specification for a 20 inch set. High resolution computer monitors may have dot pitches as small as .22 mm for a similar size screen. A rough indication of the maximum possible resolution of the CRT can be found by determining how many complete phosphor dot groups can fit across the visible part of the screen. Running at too high a resolution for a given CRT may result in Moire - an interference pattern that will manifest itself as contour lines in smooth bright areas of the picture. However, many factors influence to what extent this may be a problem. See the section: "Contour lines on high resolution monitors - Moire". 3. Bandwidth of the video source or display card - use of high performance video amplifiers or digital to analog convertors. 4. Signal quality of the video source or display card - properly designed circuitry with adequate power supply filtering and high quality components. 5. High quality cables with correct termination and of minimal acceptable length without extensions or switch boxes unless designed specifically for high bandwidth video. 6. Sharpness of focus - even if the CRT dot pitch is very fine, a fuzzy scanning beam will result in a poor quality picture. 7. Stability of the monitor electronics - well regulated power supplies and low noise shielded electronics contribute to a rock solid image.
WARNING: No monitor is perfect. Running comprehensive tests on your
monitor or one you are considering may make you aware of deficiencies you
never realized were even possible. You may never be happy with any monitor
for the rest of your life!
Note: the intent of these tests is **not** to evaluate or calibrate a monitor
for photometric accuracy. Rather they are for functional testing of the
monitor's performance.
Obviously, the ideal situation is to be able to perform these sorts of
tests before purchase. With a small customer oriented store, this may
be possible. However, the best that can be done when ordering by mail
is to examine a similar model in a store for gross characteristics and
then do a thorough test when your monitor arrives. The following should
be evaluated:
* Screen size and general appearance.
* Brightness and screen uniformity, purity and color saturation.
* Stability.
* Convergence.
* Edge geometry.
* Linearity.
* Tilt.
* Size and position control range.
* Ghosting or trailing streaks.
* Sharpness.
* Moire.
* Scan rate switching.
* Acoustic noise.
The companion document: "Performance Testing of Computer and Video Monitors"
provides detailed procedures for the evaluation of each of these criteria.
CAUTION: since there is no risk free way of evaluating the actual scan
rate limits of a monitor, this is not an objective of these tests. It
is assumed that the specifications of both the video source/card and the
monitor are known and that supported scan rates are not exceeded. Some
monitors will operate perfectly happily at well beyond the specified range
or will shut down without damage. Others will simply blow up instantly and
require expensive repairs.
Unlike PC system boards where any disasters are likely to only affect your pocketbook, monitors can be very dangerous. Read, understand, and follow the set of safety guidelines provided later in this document whenever working on TVs, monitors, or other similar high voltage equipment. If you do go inside, beware: line voltage (on large caps) and high voltage (on CRT) for long after the plug is pulled. There is the added danger of CRT implosion for carelessly dropped tools and often sharp sheetmetal shields which can injure if you should have a reflex reaction upon touching something you should not touch. In inside of a TV or monitor is no place for the careless or naive. Having said that, a basic knowledge of how a monitor works and what can go wrong can be of great value even if you do not attempt the repair yourself. It will enable you to intelligently deal with the service technician. You will be more likely to be able to recognize if you are being taken for a ride by a dishonest or just plain incompetent repair center. For example, a faulty picture tube CANNOT be the cause of a color monitor only displaying in black-and-white (this is probably a software or compatibility problem). The majority of consumers - and computer professionals - may not know even this simple fact. This document will provide you with the knowledge to deal with a large percentage of the problems you are likely to encounter with your monitors. It will enable you to diagnose problems and in many cases, correct them as well. With minor exceptions, specific manufacturers and models will not be covered as there are so many variations that such a treatment would require a huge and very detailed text. Rather, the most common problems will be addressed and enough basic principles of operation will be provided to enable you to narrow the problem down and likely determine a course of action for repair. In many cases, you will be able to do what is required for a fraction of the cost that would be charged by a repair center. Should you still not be able to find a solution, you will have learned a great deal and be able to ask appropriate questions and supply relevant information if you decide to post to sci.electronics.repair. It will also be easier to do further research using a repair text such as the ones listed at the end of this document. In any case, you will have the satisfaction of knowing you did as much as you could before taking it in for professional repair. With your new-found knowledge, you will have the upper hand and will not easily be snowed by a dishonest or incompetent technician.
The following probably account for 95% or more of the common monitor ailments: * Intermittent changes in color, brightness, size, or position - bad connections inside the monitor or at the cable connection to the computer or or video source. * Ghosts, shadows, or streaks adjacent to vertical edges in the picture - problems with input signal termination including use of cable extensions, excessively long cables, cheap or improperly made video cables, improper daisychaining of monitors, or problems in the video source or monitor circuitry. * Magnetization of CRT causing color blotches or other color or distortion problems - locate and eliminate sources of magnetic fields if relevant and degauss the CRT. * Electromagnetic Interference (EMI) - nearby equipment (including and especially other monitors), power lines, or electrical wiring behind walls, may produce electromagnetic fields strong enough to cause noticeable wiggling, rippling, or other effects. Relocate the monitor or offending equipment. Shielding is difficult and expensive. * Wiring transmitted interference - noisy AC power possibly due to other equipment using electric motors (e.g., vacuum cleaners), lamp dimmers or motor speed controls (shop tools), fluorescent lamps, and other high power devices, may result in a variety of effects. The source is likely local - in your house - but could be several miles away. Symptoms might include bars of noise moving up or down the screen or diagonally. The effects may be barely visible as a couple of jiggling scan lines or be broad bars of salt and pepper noise, snow, or distorted video. Plugging the monitor into another outlet or the use of a line filter may help. If possible, replace or repair the offending device. * Monitor not locking on one or more video scan ranges - settings of video adapter are incorrect. Use software setup program to set these. This could also be a fault in the video source or monitor dealing with the sync signals. * Adjustments needed for background brightness or focus - aging CRT reduces brightness. Other components may affect focus. Easy internal (or sometimes external) adjustments. * Dead monitor due to power supply problems - very often the causes are simple such as bad connections, blown fuse or other component.
If you need to send or take the monitor to a service center, the repair could easily exceed half the cost of a new monitor. Service centers may charge up to $50 or more for providing an initial estimate of repair costs but this will usually be credited toward the total cost of the repair (of course, they may just jack this up to compensate for their bench time). Some places offer attractive flat rates for repairs involving anything but the CRT, yoke, and flyback. Such offers are attractive if the repair center is reputable. However, if by mail, you will be stuck with a tough decision if they find that one of these expensive components is actually bad. Monitors become obsolete at a somewhat slower rate than most other electronic equipment. Therefore, unless you need the higher resolution and scan rates that newer monitors provide, repairing an older one may make sense as long as the CRT is in good condition (adequate brightness, no burn marks, good focus). However, it may just be a good excuse to upgrade. If you can do the repairs yourself, the equation changes dramatically as your parts costs will be 1/2 to 1/4 of what a professional will charge and of course your time is free. The educational aspects may also be appealing. You will learn a lot in the process. Thus, it may make sense to repair that old clunker for your 2nd PC (or your 3rd or your 4th or....).
A computer or video monitor includes the following functional blocks:
1. Low voltage power supply (some may also be part of (2)). Most of the lower
voltages used in the TV may be derived from the horizontal deflection
circuits, a separate switching power supply, or a combination of the two.
Rectifier/filter capacitor/regulator from AC line provides the B+ to the
switching power supply or horizontal deflection system. Auto-scan
monitors may have multiple outputs from the low voltage power supply
which are selectively switched or enabled depending on the scan rate.
Degauss operates off of the line whenever power is turned on (after
having been off for a few minutes) to demagnetize the CRT. Better
monitors will have a degauss button which activates this circuitry
as well since even rotating the monitor on its tilt-swivel base can
require degauss.
2. Horizontal deflection. These circuits provide the waveforms needed to
sweep the electron beam in the CRT across and back at anywhere from
15 KHz to over 100 KHz depending on scan rate and resolution. The
horizontal sync pulse from the sync separator or the horizontal sync
input locks the horizontal deflection to the video signal. Auto-scan
monitors have sophisticated circuitry to permit scanning range of
horizontal deflection to be automatically varied over a wide range.
3. Vertical deflection. These circuits provide the waveforms needed to
sweep the electron beam in the CRT from top to bottom and back at
anywhere from 50 - 120 or more times per second. The vertical sync
pulse from the sync separator or vertical sync input locks the vertical
deflection to the video signal. Auto-scan monitors have additional
circuitry to lock to a wide range of vertical scan rates.
4. CRT high voltage (also part of (2)). A modern color CRT requires
up to 30 KV for a crisp bright picture. Rather than having a totally
separate power supply, most monitors derive the high voltage (as well
as many other voltages) from the horizontal deflection using a special
transformer called a 'flyback' or 'Line OutPut Transformer (LOPT) for
those of you on the other side of the lake. Some high performance
monitors use a separate high voltage board or module which is a self
contained high frequency inverter.
5. Video amplifiers. These buffer the low level inputs from the computer
or video source. On monitors with TTL inputs (MGA, CGA, EGA), a resistor
network also combines the intensity and color signals in a kind of poor
man's D/A. Analog video amplifiers will usually also include DC restore
(black level retention, back porch clamping) circuitry stabilize the
black level on AC coupled video systems.
6. Video drivers (RGB). These are almost always located on a little
circuit board plugged directly onto the neck of the CRT. They boost
the output of the video amplifiers to the hundred volts or so needed
to drive the cathodes (usually) of the CRT.
7. Sync separator. Where input is composite rather than separate H and
V syncs, this circuit extracts the individual sync signals. Output is
horizontal and vertical sync pulses to control the deflection circuits.
This is not needed on a monitor that only uses separate sync inputs.
8. System control. Most higher quality monitors use a microcontroller
to perform all user interface and control functions from the front panel
(and sometimes even from a remote control). So called 'digital monitors'
meaning digital controls not digital inputs, use buttons for everything
except possibly user brightness and contrast. Settings for horizontal
and vertical size and position, pincushion, and color balance for each
scan rate may be stored in non-volatile memory. The microprocessor
also analyzes the input video timing and selects the appropriate scan
range and components for the detected resolution. While these circuits
rarely fail, if they do, debugging can be quite a treat.
Most problems occur in the horizontal deflection and power supply sections.
These run at relatively high power levels and some components run hot.
This results in both wear and tear on the components as well as increased
likelihood of bad connections developing from repeated thermal cycles.
The high voltage section is prone to breakdown and arcing as a result
of hairline cracks, humidity, dirt, etc.
The video circuitry is generally quite reliable. However, it seems that
even after 15+ years, manufacturers still cannot reliably turn out circuit
boards that are free of bad solder connections or that do not develop them
with time and use.
The books listed in the section: "Suggested references" include additional information on the theory and implementation of the technology of monitors and TV sets.Go to [Next] segment
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