Preface

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Introduction

Monitors, monitors, and more monitors

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 80×25 green monochrome
text display. It was bright, crisp, and stable. Mono graphics (MGA or MDA)
was added at 720×350, CGA at a range of resolutions from 160×200 to 640×200
at 2 to 16 colors, and EGA extended this up to a spectacular resolution of
640×350. 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, 16, or a whopping 64 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 this 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.

Related Information

See the documentss:
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.

Monitor fundamentals

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 inexpensive 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. As the price of LCD (and other technology) flat
screen technology decreases, such monitors will become dominant for desktop
computers as well and CRT based monitors will eventually go the way of
dinosaurs, core memory, and long playing records that dominated their
respective industries for decades but eventually yielded to fundamentally new
technology.

However, there are still problems with (low cost, at least) LCD monitors.
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. This is partly due to
the response of the LCD material itself but also a result of the scan
conversion that’s needed for non-native resolution formats. Brightness
is generally not as good as a decent CRT display. And last but not least,
the cost is still somewhat 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.

Nonetheless, 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 displays, on the other hand, can 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. DMD technology would
permit nearly any size projection display to be produced and would
therefore be applicable to HDTV as well as PCs. Since it is a reflective
device, the light source can be as bright as needed. This technology is
already appearing in commercial high performance computer projectors and
is competing for use in totally digital movie theaters to replace the film
projector, but to my knowledge is not in any consumer TV sets - yet.

As noted, the plasma panel flat screen display has been around for several
years in high-end TVs, typically in the 42 inch diagonal range. But
they are very expensive ($5,000 to $15,000 as of Winter, 2003), and their
life expectancy may be limited due to the gradual degradation of the active
pixel cells - which occurs faster than for a CRT. The physical resolution
is also probably still too low to really justify the large screen size for
computer displays. However, there is little doubt that this or a similar
technology will eventually replace the direct view CRT and 3-tube projection
TVs in the mid to large screen sizes in the not too distant future. But to
what extent it is used for computer monitors is still unclear.

The remainder of this document concentrates on CRT based computer and video
monitors since these still dominate the market and realistically, they are
the only type where there is a good chance of repair without access to
specialized test equipment and parts. I wouldn’t recommend any sort of
attempt at repair of flat screen TVs or monitors - no matter what the size -
beyond checking for bad connections, dead power supplies, or other obvious
problems. The chance of success is vanishingly small and it’s very likely
that even with great care, damage could occur to the panels or circuitry.

Monitor characteristics

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.
   

Types of monitors

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.
   

Why auto-scan?

Note: The generic term ‘auto-scan’ is used 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.

Analog versus digital monitors

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. And, except perhaps
for the very cheapest monitors, all now have digital controls.

Interlacing

• Generally, a monitor that runs at a given resolution non-interlaced can run interlaced at a resolution with the same number of pixels per line but twice
  the number of lines vertically at roughly the same horizontal and vertical
  scan rates and video bandwidth (but half the frame rate).
  

• Alternatively, it may be possible to increase the resolution in both directions while keeping the horizontal scan rate the same thus permitting a
  monitor to display the next larger size format. However, in this case, the
  video bandwidth will increase.
  

Here are a couple of examples:

• A monitor that will run 640×240 at 60 frames per second non-interlaced will run 640×480 at 30 frames per second interlaced. This would permit a monitor
  with a horizontal scan rate of 15.7 kHz (NTSC TV compatible) to display VGA
  resolution images - though they will likely flicker since the 30 Hz is way
  too low for most graphics.
  

• A resolution of 1024×768 at 50 frames per second interlaced requires roughly the same horizontal scan rate (about 42 kHz) as 800×600 at 66 frames
  per second non-interlaced. The flicker may be acceptable in this case being
  at 50 Hz for the worst case of single horizontal lines as the high 100 Hz
  vertical scan rate will reduce flicker otherwise.
  

Whether the image is usable at the higher resolution of course depends on many
other factors (in addition to flicker) 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.

Monitor performance

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.
   

The following are only partly dependent on the monitor’s design:

8. Anti-glare treatment of screen and ambient lighting conditions - No matter how good are the monitor’s electronics, the display can still be washed out
   and difficult or tiring to view if there is annoying glare or reflections.
   The lighting and location are probably more important than how the screen
   itself is designed to minimize glare.
   

9. Electromagnetic interference - Proximity to sources of magnetic fields and power line noise can degrade the performance of any monitor, no matter how
   well shielded it might be.
   

Performance testing of monitors

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,
will shut down without damage, or will display an error message. Others will
simply blow up instantly and require expensive repairs.

Monitor repair

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.

Most Common Problems

• 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. These are often easy
  internal (or sometimes external) adjustments but some manufacturers have
  gone to digital setup requiring expensive an adapter (serial cable) to a PC
  and their own (expensive and/or unavailable) software.
  

• Dead monitor due to power supply problems - very often the causes are simple such as bad connections, blown fuse or other component.
  

Repair or replace

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….).

Monitors 101

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 monitor may be derived from the horizontal deflection
   circuits, a separate switchmode power supply (SMPS), or a combination of
   the two. Rectifier/filter capacitor/regulator from AC line provides the
   B+ to the SMPS 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, or an power
   supply with programmable output voltage for the deflection system.
   A common configuration is a pair of SMPSs where one provides all the fixed
   voltages and the other is programmable based on 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 ‘flyback’ power supply (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 processor. This accepts separate, composite, or ’sync-on-green’ signals to control the timing of the horizontal and vertical deflection
   systems. Where input is composite rather than separate H and V syncs (as
   is used with VGA/SVGA), this circuit extracts the individual sync signals.
   For workstation monitors which often have the sync combined with the green
   video signals, it needs to separate this as well. The output of the sync
   processor is horizontal and vertical sync pulses to control the deflection
   circuits.
   

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. It may communicate with
   the video card over the serial VESA bus to inform if of its capabilities.
   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.

For more information on monitor technology

Philips/Magnavox used to have a very nice on-line introduction to a variety
of consumer electronics technologies. Although their site has disappeared -
and even people who work for them have no clue - I have now recovered
several of the articles including those on TVs, VCRs, camcorders, satellite
reception, and connections. See the Introductory Consumer Electronics
Technology Series.

On-line tech-tips databases

A tech-tips database is a collection of problems and solutions accumulated by
the organization providing the information or other sources based on actual
repair experiences and case histories. Since the identical failures often
occur at some point in a large percentage of a given model or product line,
checking out a tech-tips database may quickly identify your problem and
solution.

In that case, you can greatly simplify your troubleshooting or at least
confirm a diagnosis before ordering parts. My only reservation with respect
to tech-tips databases in general - this has nothing to do with any one in
particular - is that symptoms can sometimes be deceiving and a solution that
works in one instance may not apply to your specific problem. Therefore,
an understanding of the hows and whys of the equipment along with some good
old fashioned testing is highly desirable to minimize the risk of replacing
parts that turn out not to be bad.

The other disadvantage - at least from one point of view - is that you do not
learn much by just following a procedure developed by others. There is no
explanation of how the original diagnosis was determined or what may have
caused the failure in the first place. Nor is there likely to be any list
of other components that may have been affected by overstress and may fail
in the future. Replacing Q701 and C725 may get your equipment going again
but this will not help you to repair a different model in the future.

Please see the document: On-Line Tech-Tips
Databases for the most up to date compilation of these resources for TVs,
VCRs, computer monitors, and other consumer electronic equipment.

CRT Basics

Note: Most of the information on TV and monitor CRT construction, operation,
interference and other problems. has been moved to the document:
TV and Monitor CRT (Picture Tube) Information.
The following is just a brief introduction with instructions on degaussing.

Color CRTs - shadow masks and aperture grills

The shadow mask consists of a thin steel or InVar (a ferrous alloy)
with a fine array of holes - one for each trio of phosphor
dots - positioned about 1/2 inch behind the surface of the phosphor
screen. With some CRTs, the phosphors are arranged in triangular
formations called triads with each of the color dots at the apex
of the triangle. With many TVs and some monitors, they are
arranged as vertical slots with the phosphors for the 3 colors
next to one another.

An aperture grille, used exclusively in Sony Trinitrons (and now
their clones as well), replaces the shadow mask with an array of finely
tensioned vertical wires. Along with other characteristics of the
aperture grille approach, this permits a somewhat higher possible
brightness to be achieved and is more immune to other problems like
line induced moire and purity changes due to local heating causing
distortion of the shadow mask.

However, there are some disadvantages of the aperture grille design:

• Weight - a heavy support structure must be provided for the tensioned wires (like a piano frame).
  

• Price (proportional to weight).

• Always a cylindrical screen (this may be considered an advantage depending on your preference.
  

• Visible stabilizing wires which may be objectionable or unacceptable for certain applications. (Definitely on 15″ and larger sizes, possibly
  on smaller ones as well.)
  

Apparently, there is no known way around the need to keep the fine
wires from vibrating or changing position due to mechanical shock
in high resolution tubes and thus all Trinitron monitors require
1, 2, or 3 stabilizing wires (depending on tube size) across the
screen which can be see as very fine lines on bright images. Some
people find these wires to be objectionable and for some critical
applications, they may be unacceptable (e.g., medical diagnosis).

Degaussing (demagnetizing) a CRT

• if the TV or monitor is moved or even just rotated.

• if there has been a lightning strike nearby. A friend of mine had a lightning strike near his house which produced all of the
  effects of the EMP from a nuclear bomb.
  

• If a permanent magnet was brought near the screen (e.g., kid’s magnet or megawatt stereo speakers).
  

• If some piece of electrical or electronic equipment with unshielded magnetic fields is in the vicinity of the TV or monitor.
  

Degaussing should be the first thing attempted whenever color
purity problems are detected. As noted below, first try the
internal degauss circuits of the TV or monitor by power cycling a few
times (on for a minute, off for at least 20 minutes, on for a minute,
etc.) If this does not help or does not completely cure the problem,
then you can try manually degaussing.

Note: Some monitors have a degauss button, and monitors and TVs that are
microprocessor controlled may degauss automatically upon power-on (but may
require pulling the plug to do a hard reset) regardless of the amount of off
time. However, repeated use of these ‘features’ in rapid succession may
result in overheating of the degauss coil or other components. The 20 minutes
off/1 minute on precedure is guaranteed to be safe. (Some others may degauss
upon power-on as long as the previous degauss was not done within some
predetermined amount of time - they keep track with an internal timer.)

Commercial CRT Degaussers are available from parts distributors
like MCM Electronics and consist of a hundred or so turns of magnet wire
in a 6-12 inch coil. They include a line cord and momentary switch. You
flip on the switch, and bring the coil to within several inches of the
screen face. Then you slowly draw the center of the coil toward one edge
of the screen and trace the perimeter of the screen face. Then return to
the original position of the coil being flat against the center of the
screen. Next, slowly decrease the field to zero by backing straight up
across the room as you hold the coil. When you are farther than 5 feet
away you can release the line switch.

The key word here is ** slow **. Go too fast and you will freeze the
instantaneous intensity of the 50/60 Hz AC magnetic field variation
into the ferrous components of the CRT and may make the problem worse.

WARNING: Don’t attempt to degauss inside or in the back of the set (near the
CRT neck. This can demagnetize the relatively weak purity and convergence
magnets which may turn a simple repair into a feature length extravaganza!

It looks really cool to do this while the CRT is powered. The kids will
love the color effects (but then lock your degaussing coil safely away so they
don’t try it on every TV and monitor in the house!).

Bulk tape erasers, tape head degaussers, open frame transformers, and the
“butt-end” of a weller soldering gun can be used as CRT demagnetizers but
it just takes a little longer. (Be careful not to scratch the screen
face with anything sharp. For the Weller, the tip needs to be in place
to get enough magnetic field.) It is imperative to have the CRT running when
using these whimpier approaches, so that you can see where there are
still impurities. Never release the power switch until you’re 4 or 5
feet away from the screen or you’ll have to start over.

I’ve never known of anything being damaged by excess manual degaussing
as long as you don’t attempt to degauss inside or the back of the monitor -
it is possible to demagnetize geometry correction, purity, and static
converence magnets in the process! However, I would recommend keeping really
powerful bulk tape erasers-turned-degaussers a couple of inches from the CRT.

If an AC degaussing coil or substitute is unavailable, I have even done
degaussed with a permanent magnet but this is not recommended since it is more
likely to make the problem worse than better. However, if the display
is unusable as is, then using a small magnet can do no harm. (Don’t use
a 20 pound speaker or magnetron magnet as you may rip the shadow mask right
out of the CRT - well at least distort it beyond repair. What I have in
mind is something about as powerful as a refrigerator magnet.)

Keep degaussing fields away from magnetic media. It is a good idea to
avoid degaussing in a room with floppies or back-up tapes. When removing
media from a room remember to check desk drawers and manuals for stray
floppies, too.

It is unlikely that you could actually affect magnetic media but better
safe than sorry. Of the devices mentioned above, only a bulk eraser or
strong permanent magnet are likely to have any effect - and then only when
at extremely close range (direct contact with media container).

All color CRTs include a built-in degaussing coil wrapped around the
perimeter of the CRT face. These are activated each time the CRT is
powered up cold by a 3 terminal thermistor device or other control
circuitry. This is why it is often suggested that color purity problems
may go away “in a few days”. It isn’t a matter of time; it’s the number
of cold power ups that causes it. It takes about 15 minutes of the power
being off for each cool down cycle. These built-in coils with thermal
control are never as effective as external coils.

Note that while the monochrome CRTs used in B/W and projection TVs and mono
monitors don’t have anything inside to get magnetized, the chassis or other
cabinet parts of the equipment may still need degaussing. While this isn’t
likely from normal use or even after being moved or reoriented, a powerful
magnet (like that from a large speaker) could leave iron, steel, or other
ferrous parts with enough residual magnetism to cause a noticeable problem.

See the document: TV and Monitor CRT (Picture Tube)
Information for some additional discussion of degaussing tools,
techniques, treatments for severe magnetization from lightning strikes,
and cautions.

How often to degauss

If one or two activations of the degauss button do not clear up the color
problems, manual degaussing using an external coil may be needed
or the monitor may need internal purity/color adjustments. Or, you may have
just installed your megawatt stereo speakers next to the monitor!

You should only need to degauss if you see color purity problems
on your CRT. Otherwise it is unnecessary. The reasons it only works the
first time is that the degauss timing is controlled by a thermistor
which heats up and cuts off the current. If you push the button
twice in a row, that thermistor is still hot and so little happens.

One word of clarification: In order for the degauss operation to be
effective, the AC current in the coil must approach zero before the
circuit cuts out. The circuit to accomplish this often involves a
thermistor to gradually decrease the current (over a matter of several
seconds), and in better monitors, a relay to totally cut off the current
after a certain delay. If the current was turned off suddenly, you would
likely be left with a more magnetized CRT. There are time delay elements
involved which prevent multiple degauss operations in succession. Whether
this is by design or accident, it does prevent the degauss coil - which is
usually grossly undersized for continuous operation - to cool.

Why are there fine lines across my Trinitron monitor or TV?

All Trinitron (or clone) CRTs - tubes that use an aperture grille - require
1, 2, or 3 very fine wires across the screen to stabilize the array of
vertical wires in the aperture grille. Without these, the display would be
very sensitive to any shock or vibration and result in visible shimmering or
rippling. (In fact, even with these stabilizing wires, you can usually see
this shimmering if you whack a Trinitron monitor.) The lines you see are the
shadows cast by these fine wires.

The number of wires depends on the size of the screen. Below 15″ there
is usually a single wire; between 15″ and 21″ there are usually 2 wires;
above 21″ there may be 3 wires. (Some very small Trinitron CRTs may not
need these but they will be present on most of the sizes of interest here.)

Only you can decide if this deficiency is serious enough to avoid the
use of a Trinitron based monitor. Some people never get used to the fine
lines but many really like the generally high quality of Trinitron based
displays and eventually totally ignore them.

Monitor Placement and Preventive Maintenance

General monitor placement considerations

• Subdued lighting is preferred for best viewing conditions. Avoid direct overhead light falling on the screen or coming from behind the monitor
  if possible.
  

• Locate the monitor away from extremes of hot and cold. Avoid damp or dusty locations if possible. (Right you say, keep dreaming!) This will help
  keep your PC happy as well.
  

• Allow adequate ventilation - monitors use a fair amount of power - from 60 watts for a 12 inch monochrome monitor to over 200 W for a 21 inch
  high resolution color monitor. Heat is one major enemy of electronics.
  

• Do not put anything on top of the monitor that might block the ventilation grill in the rear or top of the cover. This is the major avenue for
  the convection needed to cool internal components.
  

• Do not place two monitors close to one another. The magnetic fields may cause either or both to suffer from wiggling or shimmering images.
  Likewise, do not place a monitor next to a TV if possible.
  

• Locate loudspeakers and other sources of magnetic fields at least a couple of feet from the monitor. This will minimize the possibility of color purity
  or geometry problems. The exception is with respect to good quality shielded
  multimedia speakers which are designed to avoid magnetic interference
  problems.

  Other devices which may cause interference include anything with power
  transformers including audio equipment, AC or DC wall adapters, and laptop
  power supplies; fluorescent lamps with magnetic ballasts; and motorized
  or heavy duty appliances.
  

• Situate monitors away from power lines - even electric wiring behind or on the other side of walls - and heavy equipment which may cause
  noticeable interference like rippling, wiggling, or swimming of the
  picture. Shielding is difficult and expensive.
  

• Make sure all video connections are secure (tighten the thumbscrews) to minimize the possibility of intermittent or noisy colors. Keep the
  cables as short as possible. Do not add extension cables if at all
  possible as these almost always result in a reduction in image crispness
  and introduce ghosting, smearing, and other termination problems.
  If you must add an extension, use proper high quality cable only long
  enough to make connections conveniently. Follow the termination
  recommendations elsewhere in this document.
  

• Finally, store magnetic media well away from all electronic equipment including and especially monitors and loudspeakers. Heat and magnetic
  fields will rapidly turn your diskettes and tapes into so much trash. The
  operation of the monitor depends on magnetic fields for beam deflection.
  Enough said.
  

Non-standard monitor mounting considerations

(From: Bob Myers (myers@fc.hp.com).)

Your mileage may vary, but (and please take the following for what it is, a
very general answer)…

There are basically two potential problems here; one is cooling, and the other
is the fact that the monitor has no doubt been set up by the factory assuming
standard magnetic conditions, which probably DIDN’T involve the monitor
tilting at much of an angle. If you’re happy with the image quality when it’s
installed in the cabinet, that leaves just the first concern. THAT one can be
addressed by simply making sure the cabinet provides adequate ventilation (and
preferably adding a fan for a bit of forced-air cooling), and making sure that
the whole installation isn’t going to be exposed to high ambient temperatures.
(Most monitors are speced to a 40 deg. C ambient in their normal orientation;
adding forced-air cooling will usually let you keep that rating in positions
somewhat beyond the normal.) Under no circumstances should you block the
cabinet’s vents, and - depending on the installation - it may be preferable to
remove the rear case parts of the monitor (but NOT the metal covers beneath
the plastic skin) in order to improve air circulation.

Your best bet is to simply contact the service/support people of the monitor
manufacturer, and get their input on the installation. Failing to get the
manufacturer’s blessing on something like this most often voids the warranty,
and can probably lead to some liability problems. (Note - I’m not a lawyer,
and I’m not about to start playing one on the net.)

Preventive maintenance - care and cleaning

There is some dispute as to what cleaners are safe for CRTs with antireflective
coatings (not the etched or frosted variety). Water, mild detergent, and
isopropyl alcohol should be safe. Definitely avoid the use of anything with
abrasives for any type of monitor screen. And some warn against products with
ammonia (which may include Windex, Top-Job, and other popular cleaners, as
this may damage/remove some types of antireflective coatings. To be doubly
sure, test a small spot in corner of the screen.

In really dusty situations, periodically vacuuming inside the case and the use
of contact cleaner for the controls might be a good idea but realistically,
you will not do this so don’t worry about it.

(From: Bob Myers (myers@fc.hp.com).)

Windex is perfectly fine for the OCLI HEA coating or equivalents; OCLI’s
coating is pretty tough and chemical-resistant stuff. There may be
alternative (er..cheaper) coatings in use which could be damaged by various
commercial cleaners, (For what it’s worth, OCLI also sells their own brand of
glass cleaner under the name “TFC”, for “Thin Film Cleaner”.)

I have cleaned monitors of various brands with both Windex and the OCLI-brand
cleaner, with no ill results. But then, I’m usually pretty sure what sort of
coating I’m dealing with…

Monitor coatings are always changing; besides the basic “OCLI type”
quarter-wave coatings and their conductive versions developed to address
E-field issues, just about every tube manufacturer has their own brew or three
of antiglare/antistatic coatings. There are also still SOME tubes that aren’t
really coated at all, but instead are using mechanically or chemically etched
faceplates as a cheap “anti-glare” (actually, glare-diffusing) treatment.

In general, look in the user guide/owner’s manual and see what your monitor’s
manufacturer recommends in the way of cleaning supplies.

(From: Tom Watson (tsw@johana.com).)

If you are maintaining a site, consider periodic cleaning of the monitors.
Depending on the location, they can accumulate quite a bit of dust. In normal
operation there is a electrostatic charge on the face of the crt (larger
screens have bigger charges) which act as ‘dust magnets’. If the operator
smokes (thankfully decreasing), it is even worse. At one site I helped out
with, most of the operators smoked, and the screens slowly got covered with a
film of both dust and smoke particles. A little bit of glass cleaner applied
with reasonable caution and the decree of “adjustments” to make the screen
better (these were character monochrome terminals), and lo and behold, “what
an improvement!”. Yes, even in my dusty house, the TVs get a coating of
film/goo which needs to be cleaned, and the picture quality (BayWatch viewers
beware) improves quite a bit. Try this on your home TV to see what comes off,
then show everyone else. You will be surprised what a little bit of cleaning
does.

(From: Bob Myers (myers@fc.hp.com).)

1. Don’t block the vents; make sure the monitor has adequate ventilation, and don’t operate it more than necessary at high ambient temperatures.
   

2. If the monitor is used in particularly dusty environments, it’s probably a good idea to have a qualified service tech open it up every so often
   (perhaps once a year, or more often depending on just how dirty it gets)
   and clean out the dust.
   

3. The usual sorts of common-sense things - don’t subject the monitor to mechanical shock and vibration, clean up spills, etc., promptly, and
   so forth. And if you’re having repeated power-supply problems with your
   equipment, it may be time to get suspicious of the quality of your AC
   power (are you getting noise on the line, sags, surges, spikes, brownouts,
   that sort of thing?).
   

And most importantly:

1. Turn the monitor OFF when it’s not going to be used for an extended period (such as overnight, or if you’ll be away from your desk for the
   afternoon, etc.). Heat is the enemy of all electronic components, and
   screen-savers do NOTHING in this regard. Many screen-savers don’t even
   do a particularly good job of going easy on the CRT. With modern
   power-management software, there’s really no reason to be leaving a
   monitor up and running all the time.
   

These won’t guarantee long life, of course - nothing can do that, as there
will always be the possibility of the random component failure. But these
are the best that the user can do to make sure the monitor goes as long as
it can.

Monitor tuneup?

Most manufacturers will quote an MTBF (Mean Time Before Failure) of
somewhere in the 30,000 to 60,000 hour range, EXCLUSIVE OF the CRT. The
typical CRT, without an extended-life cathode, is usually good for
10,000 to 15,000 hours before it reaches half of its initial brightness.
Note that, if you leave your monitor on all the time, a year is just about
8,000 hours.

The only “tuneup” that a monitor should need, exclusive of adjustments
needed following replacement of a failed component, would be video amplifier
and/or CRT biasing adjustments to compensate for the aging of the tube.
These are usually done only if you’re using the thing in an application where
exact color/brightness matching is important. Regular degaussing of the
unit may be needed, of course, but I’m not considering that a “tuneup” or
adjustment.

Monitor Troubleshooting

SAFETY

There are two areas which have particularly nasty electrical dangers: the
non-isolated line power supply and the CRT high voltage.

Major parts of nearly all modern TVs and many computer monitors are directly
connected to the AC line - there is no power transformer to provide the
essential barrier for safety and to minimize the risk of equipment damage.
In the majority of designs, the live parts of the TV or monitor are limited
to the AC input and line filter, degauss circuit, bridge rectifier and main
filter capacitor(s), low voltage (B+) regulator (if any), horizontal output
transistor and primary side of the flyback (LOPT) transformer, and parts
of the startup circuit and standby power supply. The flyback generates most
of the other voltages used in the unit and provides an isolation barrier so
that the signal circuits are not line connected and safer.

Since a bridge rectifier is generally used in the power supply, both
directions of the polarized plug result in dangerous conditions and an
isolation transformer really should be used - to protect you, your test
equipment, and the TV, from serious damage. Some TVs do not have any
isolation barrier whatsoever - the entire chassis is live. These are
particularly nasty.

The high voltage to the CRT, while 200 times greater than the line input,
is not nearly as dangerous for several reasons. First, it is present in a
very limited area of the TV or monitor - from the output of the flyback
to the CRT anode via the fat HV wire and suction cup connector. If you
don’t need to remove the mainboard or replace the flyback or CRT, then
leave it alone and it should not bite. Furthermore, while the shock from
the HV can be quite painful due to the capacitance of the CRT envelope, it
is not nearly as likely to be lethal since the current available from the
line connected power supply is much greater.

Of particular note in: Major Parts of Typical SVGA
Monitor with Cover Removed are the CRT HV cable and connector, flyback
or LOPT, and the horizontal output transistor and its heat sink. With many
TVs and some monitors, this may be line-connected and electrically hot.
However, this monitor uses a separate switchmode power supply and in any case,
there is likely an insulator between the transistor and heat sink.

Safety Guidelines:

These guidelines are to protect you from potentially deadly electrical shock
hazards as well as the equipment from accidental damage.

Note that the danger to you is not only in your body providing a conducting
path, particularly through your heart. Any involuntary muscle contractions
caused by a shock, while perhaps harmless in themselves, may cause collateral
damage - there are many sharp edges inside this type of equipment as well as
other electrically live parts you may contact accidentally.

The purpose of this set of guidelines is not to frighten you but rather to
make you aware of the appropriate precautions. Repair of TVs, monitors,
microwave ovens, and other consumer and industrial equipment can be both
rewarding and economical. Just be sure that it is also safe!

• Don’t work alone - in the event of an emergency another person’s presence may be essential.
  

• Always keep one hand in your pocket when anywhere around a powered line-connected or high voltage system.
  

• Wear rubber bottom shoes or sneakers.

• Don’t wear any jewelry or other articles that could accidentally contact circuitry and conduct current, or get caught in moving parts.
  

• Set up your work area away from possible grounds that you may accidentally contact.
  

• Know your equipment: TVs and monitors may use parts of the metal chassis as ground return yet the chassis may be electrically live with respect to the
  earth ground of the AC line. Microwave ovens use the chassis as ground
  return for the high voltage. In addition, do not assume that the chassis
  is a suitable ground for your test equipment!

• If circuit boards need to be removed from their mountings, put insulating material between the boards and anything they may short to. Hold them in
  place with string or electrical tape. Prop them up with insulation sticks -
  plastic or wood.
  

• If you need to probe, solder, or otherwise touch circuits with power off, discharge (across) large power supply filter capacitors with a 2 W or greater
  resistor of 100 to 500 ohms/V approximate value (e.g., for a 200 V capacitor,
  use a 20K to 100K ohm resistor). Monitor while discharging and verify that
  there is no residual charge with a suitable voltmeter. In a TV or monitor,
  if you are removing the high voltage connection to the CRT (to replace the
  flyback transformer for example) first discharge the CRT contact (under the
  suction cup at the end of the fat HV wire). Use a 1M to 10M ohm 5 W or
  greater wattage (for its voltage holdoff capability, not power dissipation)
  resistor on the end of an insulating stick or the probe of a high voltage
  meter. Discharge to the metal frame which is connected to the outside of
  the CRT.
  

• For TVs and monitors in particular, there is the additional danger of CRT implosion - take care not to bang the CRT envelope with your tools.
  An implosion will scatter shards of glass at high velocity in every
  direction. There are several tons of force attempting to crush the typical
  CRT. While implosion is not really likely even with modest abuse, why take
  chances? However, the CRT neck is relatively thin and fragile and breaking
  it would be very embarrassing and costly. Always wear eye protection when
  working around the back side of a CRT.
  

• Connect/disconnect any test leads with the equipment unpowered and unplugged. Use clip leads or solder temporary wires to reach cramped
  locations or difficult to access locations.
  

• If you must probe live, put electrical tape over all but the last 1/16″ of the test probes to avoid the possibility of an accidental short which
  could cause damage to various components. Clip the reference end of the
  meter or scope to the appropriate ground return so that you need to only
  probe with one hand.
  

• Perform as many tests as possible with power off and the equipment unplugged. For example, the semiconductors in the power supply section of a TV or
  monitor can be tested for short circuits with an ohmmeter.
  

• Use an isolation transformer if there is any chance of contacting line connected circuits. A Variac™ is not an isolation transformer!
  The use of a GFCI (Ground Fault Circuit Interrupter) protected outlet is a
  good idea but will not protect you from shock from many points in a line
  connected TV or monitor, or the high voltage side of a microwave oven, for
  example. (Note however, that, a GFCI may nuisanse trip at power-on or at
  other random times due to leakage paths (like your scope probe ground) or
  the highly capacitive or inductive input characteristics of line powered
  equipment.) A fuse or circuit breaker is too slow and insensitive to provide
  any protection for you or in many cases, your equipment. However, these
  devices may save your scope probe ground wire should you accidentally connect
  it to a live chassis.
  

• Don’t attempt repair work when you are tired. Not only will you be more careless, but your primary diagnostic tool - deductive reasoning - will
  not be operating at full capacity.
  

• Finally, never assume anything without checking it out for yourself! Don’t take shortcuts!
  

Warning about disconnecting CRT neck board

This is probably not a problem on small CRTs but for large ones with high
high voltages and high deflection angles where the glass of the neck is
very thin to allow for maximum deflection sensitivity, the potential does
exist for arcing through the glass to the yoke to occur, destroying the CRT.

There is really no way to know which models will self destruct but it
should be possible to avoid such a disaster by providing a temporary return
path to the DAG ground of the CRT (NOT SIGNAL GROUND!!) via the focus or G2
pins preferably through a high value high voltage rated resistor just in
case one of these is shorted.

This probably applies mostly to large direct-view TVs since they use high
deflection angle CRTs but it won’t hurt to take appropriate precautions with
video and computer monitors as well.

Troubleshooting tips

Many problems have simple solutions. Don’t immediately assume that
your problem is some combination of esoteric complex convoluted
failures. For a monitor, it may just be a bad connection or blown fuse.
Remember that the problems with the most catastrophic impact on operation
like a dead monitor usually have the simplest solutions. The kind of
problems we would like to avoid at all costs are the ones that are
intermittent or difficult to reproduce: the occasional jitter or a monitor
that blows its horizontal output transistor every six months.

If you get stuck, sleep on it. Sometimes, just letting the problem
bounce around in your head will lead to a different more successful
approach or solution. Don’t work when you are really tired - it is both
dangerous (especially with respect to monitors) and mostly non-productive
(or possibly destructive).

Whenever working on complex equipment, make copious notes and diagrams.
You will be eternally grateful when the time comes to reassemble the unit.
Most connectors are keyed against incorrect insertion or interchange
of cables, but not always. Apparently identical screws may be of differing
lengths or