Notes on the Troubleshooting and Repair of Computer and Video Monitors


[Document Version: 2.73] [Last Updated: 05/25/1998]

Chapter 1) About the Author & Copyright

Notes on the Troubleshooting and Repair of Computer and Video Monitors

Author: Samuel M. Goldwasser
Corrections/suggestions: | Email

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:

  1. This notice is included in its entirety at the beginning.
  2. There is no charge except to cover the costs of copying.

Chapter 2) Introduction

  2.1) Monitors, monitors, and more monitors

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.

  2.2) Related Information

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".

  2.3) Monitor fundamentals

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.

  2.4) Monitor characteristics

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

   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.

  2.5) Types of monitors

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.

  2.6) Why auto-scan?

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

  2.7) Analog vs. digital monitors

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.

  2.8) Interlacing

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

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

  2.9) Monitor performance

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.

  2.10) Performance testing of monitors

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.

  2.11) Monitor repair

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.

  2.12) Most Common Problems

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

* 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.

  2.13) Repair or replace

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....).

Chapter 3) Monitors 101

  3.1) Subsystems of a monitor

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.

  3.2) For more information on monitor technology

The books listed in the section: "Suggested references" include additional
information on the theory and implementation of the technology of monitors
and TV sets.

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