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Repair and maintenance of Computer Monitors / CRTs, 200 Page Manual

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Sunday, 13 August 2006

I used this manual when
I owned my computer/laptop
repair business

It is a very comprehensive
piece of work taht goes into
great detail on the
repair and upkeep of monitors






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Notes on the Troubleshooting and Repair of Computer and Video Monitors

Contents:

[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

	   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.

 

  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

	requirements.

 

  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

	used).

	

	Generally, a monitor that runs at a given resolution non-interlaced can run

	at a resolution with roughly twice the number of pixels interlaced at the

	same horizontal scan rate.  For example, a monitor that will run 1024x768

	non-interlaced at 70 Hz will run 1280x1024 interlaced at a 40 Hz frame rate.

	Whether the image is usable at the higher resolution of course also depends

	on many other factors including the dot pitch of the CRT and video bandwidth

	of the video card and monitor video amplifiers, as well as cable quality

	and termination.  The flicker of fine horizontal lines may also be

	objectionable.

 

  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

	  circuitry.

	

	* Magnetization of CRT causing color blotches or other color or distortion

	  problems - locate and eliminate sources of magnetic fields if relevant

	  and degauss the CRT.

	

	* Electromagnetic Interference (EMI) - nearby equipment (including and

	  especially other monitors), power lines, or electrical wiring behind walls,

	  may produce electromagnetic fields strong enough to cause noticeable

	  wiggling, rippling, or other effects.  Relocate the monitor or offending

	  equipment.  Shielding is difficult and expensive.

	

	* Wiring transmitted interference - noisy AC power possibly due to other

	  equipment using electric motors (e.g., vacuum cleaners), lamp dimmers or

	  motor speed controls (shop tools), fluorescent lamps, and other high power

	  devices, may result in a variety of effects.  The source is likely local - in

	  your house - but could be several miles away.  Symptoms might include bars of

	  noise moving up or down the screen or diagonally.  The effects may be barely

	  visible as a couple of jiggling scan lines or be broad bars of salt and

	  pepper noise, snow, or distorted video.  Plugging the monitor into another

	  outlet or the use of a line filter may help.  If possible, replace or repair

	  the offending device.

	

	* Monitor not locking on one or more video scan ranges - settings of

	  video adapter are incorrect.  Use software setup program to set these.

	  This could also be a fault in the video source or monitor dealing with

	  the sync signals.

	

	* Adjustments needed for background brightness or focus - aging CRT reduces

	  brightness.  Other components may affect focus.  Easy internal (or sometimes

	  external) adjustments.

	

	* Dead monitor due to power supply problems - very often the causes are

	  simple such as bad connections, blown fuse or other component.

 

  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.

 

  3.3) On-line tech-tips databases

  

	A number of organizations have compiled databases covering thousands of common

	problems with VCRs, TVs, computer monitors, and other electronics equipment.

	Most charge for their information but a few, accessible via the Internet, are

	either free or have a very minimal monthly or per-case fee.  In other cases, a

	limited but still useful subset of the for-fee database is freely available.

	

	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.

	

	Having said that, here are three tech-tips sites for computer monitors, TVs,

	and VCRs:

	

	* http://www.anatekcorp.com/techforum.htm            (Free).

	* http://www.repairworld.com/                        ($8/month).

	* http://elmswood.guernsey.net/                      (Free, somewhat limited).

	

	The following is just for monitors.  Some portions are free but others require

	a $5 charge.  However, this may include a personal reply from a technician

	experienced with your monitor so it could be well worth it.

	

	* http://www.netis.com/members/bcollins/monitor.htm

	

	Some free monitor repair tips:

	

	* http://www.kmrtech.com/

	

	Tech-tips of the month and 'ask a wizard' options:

	

	* http://members.tripod.com/~ADCC/          (Home page)

	* http://members.tripod.com/~ADCC/tips.htm  (Tech-tips of the month)

	

	The Resolve Monitor Tech-Tips database is a diskette that is priced out of

	the reach of most hobbyists.  However, a reduced shareware version may be

	downloaded from a number of web sites.  Go to http://www.filez.com/ and look

	for res16sw.zip.

 

Chapter 4) 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.

 

  4.1) Color CRTs - shadow masks and aperture grills

  

	All color CRTs utilize a shadow mask or aperture grill a fraction of an inch

	(1/2" typical) behind the phosphor screen to direct the electron beams 

	for the red, green, and blue video signals to the proper phosphor dots.

	Since the electron beams for the R, G, and B phosphors originate from

	slightly different positions (individual electron guns for each)

	and thus arrive at slightly different angles, only the proper phosphors

	are excited when the purity is properly adjusted and the necessary

	magnetic field free region is maintained inside the CRT.  Note that

	purity determines that the correct video signal excites the

	proper color while convergence determines the geometric

	alignment of the 3 colors.  Both are affected by magnetic fields.

	Bad purity results in mottled or incorrect colors.  Bad convergence

	results in color fringing at edges of characters or graphics.

	

	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.

	

	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).

 

  4.2) Degaussing (demagnetizing) a CRT

  

	Degaussing may be required if there are color purity problems with the

	display.  On rare occasions, there may be geometric distortion caused

	by magnetic fields as well without color problems.  The CRT can get

	magnetized:

	

	* 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 30 minutes, on for a minute, etc.)

	If this does not help or does not completely cure the problem,

	then you can try manually degaussing.

	

	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.

	

	It looks really cool to do this while the CRT is powered.  The kids will

	love the color effects.

	

	Bulk tape erasers, tape head degaussers, open frame transformers, and the

	"ass-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.) 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

	though 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 thermister 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.

	

	See the document: " TV and Monitor CRT (Picture Tube) Information" for

	some additional discussion of degaussing tools, techniques, and cautions.

 

  4.3) How often to degauss

  

	Some monitor manufacturers specifically warn about excessive use of degauss,

	most likely as a result of overstressing components in the degauss circuitry

	which are designed (cheaply) for only infrequent use.  In particular,

	there is often a thermister that dissipates significant power for the second

	or two that the degauss is active.  Also, the large coil around the CRT

	is not r