Notes on the Troubleshooting and Repair of Computer and Video Monitors

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 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, 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

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.

The really bad news from the perspective of repair is that they generally cannot be repaired outside of a manufacturer authorized service center and the way they do the repair most likely will be to swap the entire LCD/driver panel, if not the entire monitor. Only repair of the most simple problems like obvious bad connections, a bad cable, a bad backlight lamp, or a failure of the power supply or backlight inverter, can realistically be accomplished without fancy specialized test equipment and facilities. Access to the backlight lamps might substantial disassembly.

Buying a broken LCD monitor to repair may have better odds than the State Lottery, but probably not by much. Where one or more columns or rows or an entire half screen are not displaying properly, I wouldn't consider it unless nearly totally free, hoping for a miracle, and even then it might not be worth it. Loose connectors and solder joints are possible, though not nearly as common as with CRT monitors.

Also a note to those with less than perfect vision: If you tend to view your monitor from less than 10 to 15 inches, you may be disappointed, or at least have a hard time getting used to LCD monitors. The appearance of a CRT display is nearly independent of viewing angle. But for an LCD display, this is not the case. Only the central part of your field of vision will have the proper brightness, contrast, and color rendition. If the curser isn't within this central area, it will be harder to locate than on a CRT. In short, don't just depend on the hype. An LCD with a slightly lower contrast ratio and lower price may have a substantially wider viewing angle and better match to your needs than a top-of-the-line model. Test drive multiple LCD monitors before committing to one!

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 640x240 at 60 frames per second non-interlaced will run 640x480 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 1024x768 at 50 frames per second interlaced requires roughly the same horizontal scan rate (about 42 kHz) as 800x600 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. These as well as most or all of the other articles, as well a glossary and much more, can be also be accessed via the Internet Archive Wayback Machine. Copy and paste the following URL into the search box:

• http://www.magnavox.com/electreference/electreference.html

The earliest (Nov 09, 1996) archive seems to be the most complete.

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.

Additional monitor technology and repair information

CRT Basics

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.

Another alternative which has been known to work is to place another similar size monitor face-to-face with the suspect monitor (take care not to bump or scratch the screens!) and activate degauss function on the working monitor. While not ideal, this may be enough to also degauss the broken one.

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 a 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.

Note that a drop of oil or other contamination might appear like a defect (hole) in the AR coating. Before getting upset, try cleaning the screen.

(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(tm) 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

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 have slightly different thread types. Little parts may fit in more than one place or orientation. Etc. Etc.

Pill bottles, film canisters, and plastic ice cube trays come in handy for sorting and storing screws and other small parts after disassembly. This is particularly true if you have repairs on multiple pieces of equipment under way simultaneously.

Select a work area which is wide open, well lighted, and where dropped parts can be located - not on a deep pile shag rug. The best location will also be relatively dust free and allow you to suspend your troubleshooting to eat or sleep or think without having to pile everything into a cardboard box for storage.

Another consideration is ESD - Electro-Static Discharge. Some components (like ICs) in a TV are vulnerable to ESD. There is no need to go overboard but taking reasonable precautions such as getting into the habit of touching a **safe** ground point first.

WARNING: even with an isolation transformer, a live chassis should **not** be considered a safe ground point. When the monitor is unplugged, the shields or other signal ground points should be safe and effective.

A basic set of precision hand tools will be all you need to disassemble a monitor and perform most adjustments. These do not need to be really expensive but poor quality tools are worse than useless and can cause damage. Needed tools include a selection of Philips and straight blade screwdrivers, socket drivers, needlenose pliers, wire cutters, tweezers, and dental picks. For adjustments, a miniature (1/16" blade) screwdriver with a non-metallic tip is desirable both to prevent the presence of metal from altering the electrical properties of the circuit and to minimize the possibility of shorting something from accidental contact with the circuitry. A set of plastic alignment tools will be useful for making adjustments to coils (though you can forgo these until the (rare) need arises.

A low power (e.g., 25 W) fine tip soldering iron and fine rosin core solder will be needed if you should need to disconnect any soldered wires (on purpose or by accident) or replace soldered components. A higher power iron or small soldering gun will be needed for dealing with larger components. Never use acid core solder or the type used for sweating copper pipes!

CAUTION: You can easily turn a simple repair (e.g., bad solder connections) into an expensive mess if you use inappropriate soldering equipment and/or lack the soldering skills to go along with it. If in doubt, find someone else to do the soldering or at least practice, practice, practice, soldering and desoldering on a junk circuit board first! See the document: Troubleshooting and Repair of Consumer Electronic Equipment for additional info on soldering and rework techniques.

For thermal or warmup problems, a can of 'cold spray' or 'circuit chiller' (they are the same) and a heat gun or blow dryer come in handy to identify components whose characteristics may be drifting with temperature. Using the extension tube of the spray can or making a cardboard nozzle for the heat gun can provide very precise control of which components you are affecting.

For info on useful chemicals, adhesives, and lubricants, see "Repair Briefs, an Introduction" as well as other documents available at this site.

Test equipment

However, some test equipment will be needed:

• Multimeter (DMM or VOM) - This is essential for checking of power supply voltages and voltages on the pins of ICs or other components - service literature like the SAMs Photofacts described elsewhere in this document include voltage measurements at nearly every circuit tie point for properly functioning equipment. The multimeter will also be used to check components like transistors, resistors, and capacitors for correct value and for shorts or opens. You do not need a fancy instrument. A basic DMM - as long as it is reliable - will suffice for most troubleshooting. If you want one that will last for many years, go with a Fluke. However, even the mid range DMMs from Radio Shack have proven to be reliable and of acceptable accuracy. For some kinds of measurements - to deduce trends for example - an analog VOM is preferred (though some DMMs have a bar graph scale which almost as good).

• Oscilloscope - While many problems can be dealt with using just a multimeter, a 'scope will be essential as you get more into advanced troubleshooting. Basic requirements are: dual trace, 10-20 MHz minimum vertical bandwidth, delayed sweep desirable but not essential. A good set of proper 10X/1X probes. Higher vertical bandwidth is desirable but most consumer electronics work can be done with a 10 MHz scope. A storage scope or digital scope might be desirable for certain tasks but is by no means essential for basic troubleshooting.

  I would recommend a good used Tektronix (Tek) or Hewlett Packard (HP) scope over a new scope of almost any other brand. You will usually get more scope for your money and these things last almost forever. Until recently, my 'good' scope was the militarized version (AN/USM-281A) of the HP180 lab scope. It has a dual channel 50 MHz vertical plugin and a delayed sweep horizontal plugin. I have seen these going for under $300 from surplus outfits. For a little more money, you can get a Tek 465 or 465B (newer version but similar specifications) 100 Mhz scope ($200 to $600, sometimes cheaper on eBay or elsewhere but there is more risk than buying from a reputable dealer). I have now acquired a Tek 465B and that's what I use mostly these days. The HP-180 is still fine but I couldn't pass up a really good deal. :) The Tek 465/B or other similar model will suffice for all but the most demanding (read: RF or high speed digital) repairs.

• A video signal source - depending on what type of monitor you are repairing, you may need both computer and television signals.

  Computer Monitors - a test PC is useful as a video source. Of course, it will need to support whatever scan rates and video types the monitor is designed to accept. Software programs are available to display purity, convergence, focus, color, and other test patterns. Or create your own test patterns using a program like Windows Paint. See the section: Using a PC as a monitor test pattern generator.

  Studio monitors - a baseband video source like a VCR or camcorder is useful in lieu of a test pattern generator. These will allow you to you to control the program material. In fact, making some test tapes using a camcorder or video camera to record static test patterns will allow you full control of what is being displayed and for how long.

• Color bar/dot/crosshatch signal generator. This is a useful piece of equipment if you are doing a lot of TV or studio monitor repair and need to perform CRT convergence and chroma adjustments. However, there are alternatives that are almost as good: a VHS recording of these test patterns will work for TVs. A PC programmed to output a suitable set of test patterns will be fine for monitors (and TVs if you can set up the video card to produce an NTSC/PAL signal. This can be put through a VCR to generate the RF (Channel 3/4) input to your TV if it does not have direct video inputs (RCA jacks).

  Sophisticated (and expensive) universal test pattern generators are available that will handle any possible monitor scan rate.

Incredibly handy widgets

• Series light bulb for current limiting during the testing of TVs, monitors, switching power supplies, audio power amplifiers, etc. I built a dual outlet box with the outlets wired in series so that a lamp can be plugged into one outlet and the device under test into the other. For added versatility, add a regular outlet and 'kill' switch using a quad box instead. The use of a series load will prevent your expensive replacement part like a horizontal output transistor from blowing if there is still some fault in the circuit you have failed to locate.

• A Variac. It doesn't need to be large - a 2 A Variac mounted with a switch, outlet and fuse will suffice for most tasks. However, a 5 amp or larger Variac is desirable. If you will be troubleshooting 220 VAC equipment in the US, there are Variacs that will output 0-240 VAC from a 115 VAC line (just make sure you don't forget that this can easily fry your 115 VAC equipment.) By varying the line voltage, not only can you bring up a newly repaired monitor gradually to make sure there are no problems; you can also evaluate behavior at low and high line voltage. This can greatly aid in troubleshooting power supply problems. Warning: a Variac is not an isolation transformer and does not help with respect to safety. You need an isolation transformer as well.

• Isolation transformer. This is very important for safely working on live chassis equipment. Since nearly all modern monitors utilize line connected switchmode power supply or line connected deflection circuits, it is essential. You can build one from a pair of similar power transformers back-to-back (with their highest rated secondaries connected together. I built mine from a couple of similar old tube type TV power transformers mounted on a board with an outlet box including a fuse. Their high voltage windings were connected together. The unused low voltage windings can be put in series with the primary or output windings to adjust voltage. Alternatively, commercial line isolation transformers suitable for TV troubleshooting are available for less than $100 - well worth every penny.

• Variable isolation transformer. You don't need to buy a fancy combination unit. A Variac can be followed by a normal isolation transformer. (The opposite order also works. There may be some subtle differences in load capacity.).

CAUTION: Keep any large transformer of this type well away from your monitor or TV. The magnetic field it produces may cause the picture to wiggle or the colors to become messed up - and you to think there is an additional problem!

• Degaussing coil. Make or buy. The internal degaussing coil salvaged from a defunct color TV or monitor doubled over to half it original diameter to increase its strength in series with a 200 W light bulb for current limiting will work just fine. Or, buy one from a place like MCM Electronics for about $15-$30 that will be suitable for all but the largest TVs and monitors. Also, see the section: Degaussing (demagnetizing) a CRT.

Safe discharging of capacitors in TVs and video monitors

This doesn't mean that every one of the 250 capacitors in your TV need to be discharged every time you power off and want to make a measurement. However, the large main filter capacitors and other capacitors in the power supplies should be checked and discharged if any significant voltage is found after powering off (or before any testing - the CRT capacitance in a TV or video monitor, for example, can retain a dangerous or at least painful charge for days or longer!)

The technique I recommend is to use a high wattage resistor of about 100 ohms/V of the working voltage of the capacitor. This will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage).

Then check with a voltmeter to be double sure. Better yet, monitor while discharging (not needed for the CRT - discharge is nearly instantaneous even with multi-M ohm resistor).

Obviously, make sure that you are well insulated!

• For the main capacitors in a TV or monitor power supply which might be 400 uF at 200 V, this would mean a 5K, 10W resistor. RC = 2 seconds. 5RC = 10 seconds. A lower wattage resistor can be used since the total energy in not that great. If you want to be more high tech, you can build the capacitor discharge circuit outlined in the companion document: Capacitor Testing, Safe Discharging, and Other Related Information. This provides a visible indication of remaining charge and polarity.

• For the CRT, use a several M ohm resistor good for 30 kV or more (or a string of lower value resistors to obtain this voltage rating). A 1/4 watt job will just arc over! Discharge to the chassis ground connected to the outside of the CRT - NOT SIGNAL GROUND ON THE MAIN BOARD as you may damage sensitive circuitry. The time constant is very short - a ms or so. However, repeat a few times to be sure, then use a shorting clip as these capacitors have a way of recovering a painful charge if left alone - there have been too many stories of painful experiences from charge developing for whatever reasons ready to bite when the HV lead is reconnected.

  Note that if you are touching the little board on the neck of the CRT, you may want to discharge the HV even if you are not disconnecting the fat red wire - the focus and screen (G2) voltages on that board are derived from the CRT HV.

  WARNING: Most common resistors - even 5 W jobs - are rated for only a few hundred volts and are not suitable for the 25 kV or more found in modern TVs and monitors. Alternatives to a long string of regular resistors are a high voltage probe or a known good focus/screen divider network. However, note that the discharge time constant with these may be a few seconds. Also see the section: Additional information on discharging CRTs.

  If you are not going to be removing the CRT anode connection, replacing the flyback, or going near the components on the little board on the neck of the CRT, I would just stay away from the fat red wire and what it is connected to including the focus and screen wires. Repeatedly shoving a screwdriver under the anode cap risks scratching the CRT envelope which is something you really do not want to do.

Again, always double check with a reliable voltmeter!

Reasons to use a resistor and not a screwdriver to discharge capacitors:

1. It will not destroy screwdrivers and capacitor terminals.

2. It will not damage the capacitor (due to the current pulse).

3. It will reduce your spouse's stress level in not having to hear those scary snaps and crackles.

Additional information on discharging CRTs

(From: Asimov (mike.ross@juxta.mnet.pubnix.ten).)

'Dag' is short for Aquadag. It is a type of paint made of a graphite pigment which is conductive. It is painted onto the inside and outside of picture tubes to form the 2 plates of a high voltage filter capacitor using the glass in between as dielectric. This capacitor is between .005uF and .01uF in value. This seems like very little capacity but it can store a substantial charge with 25,000 volts applied.

The outside "Dag" is always connected to the circuit chassis ground via a series of springs, clips, and wires around the picture tube. The high voltage or "Ultor" terminal must be discharged to chassis ground before working on the circuit especially with older TV's which didn't use a voltage divider to derive the focus potential or newer TV's with a defective open divider.

(From: Sam)

CAUTION: The Dag coating/springs/clips/etc. may not be the same as signal ground on the mainboard. Discharging to that instead could result in all sorts of expensive blown components. Discharging between the CRT anode cap and Dag should be low risk though it is best to use a HV probe or properly rated high value resistor.

For more details, see the document: TV and Monitor CRT (Picture Tube) Information.

Removing the CRT HV connector

The rubber part is usually not glued down so it can be lifted rather easily. However, there may be some silicone type grease between the rubber boot (that looks like a suction cup) and the CRT glass to seal out dust.

A metal clip with a spring keeping it spread out attaches inside the button.

While there are a variety of types of clips actually used, pushing the connector to one side and/or squeezing it in the appropriate direction (peel up one side of the rubber to inspect) while gently lifting up should free it. Probably :-).

The clip (when removed) and CRT button look sort of like this:

                       ||======= HV Cable
                       /\
               Clip   |  |
          (Removed)  _|  |_
                               (No DAG coating in vicinity of HV connector)
        ____________.-    -.___________
   CRT  ____________|______|___________ Glass
                  Metal Button

This isn't rocket science and excessive force should not be needed! :-)

The series light bulb trick

Actually using a series load - a light bulb is just a readily available cheap load - is better than a Variac (well both might be better still) since it will limit current to (hopefully) non-destructive levels.

What you want to do is limit current to the critical parts - usually the horizontal output transistor (HOT). Most of the time you will get away with putting it in series with the AC line. However, sometimes, putting a light bulb directly in the B+ circuit will be needed to provide adequate protection. In that location, it will limit the current to the HOT from the main filter capacitors of line connected power supplies. This may also be required with some switchmode power supplies as they can still supply bursts of full (or excessive) current even if there is a light bulb in series with the AC line.

Actually, an actual power resistor is probably better as its resistance is constant as opposed to a light bulb which will vary by 1:10 from cold to hot. The light bulb, however, provides a nice visual indication of the current drawn by the circuit under test. For example:

• Full brightness: short circuit or extremely heavy load - a fault probably is still present.

• Initially bright but then settles at reduced brightness: filter capacitors charge, then lower current to rest of circuit. This is what is expected when the equipment is operating normally. There could still be a problem with the power circuits but it will probably not result in an immediate catastrophic failure.

• Pulsating: power supply is trying to come up but shutting down due to overcurrent or overvoltage condition. This could be due to a continuing fault or the light bulb may be too small for the equipment.

Note: for a TV or monitor, it may be necessary (and desirable) to unplug the degauss coil as this represents a heavy initial load which may prevent the unit from starting up with the light bulb in the circuit.

The following are suggested starting wattages:

• 40 W bulb for VCR or laptop computer switching power supplies.
• 100 W bulb for small (i.e., B/W or 13 inch color) monitors or TVs.
• 150-200 W bulb for large color monitors or projection TVs.

A 50/100/150 W (or similar) 3-way bulb in an appropriate socket comes in handy for this but mark the switch so that you know which setting is which!

Depending on the power rating of the equipment, these wattages may need to be increased. I have had to go to a 300 W light bulb for some computer monitors. However, start low. If the bulb lights at full brightness, you know there is still a major fault. If it flickers or the TV (or other device) does not quite come fully up, then it should be safe to go to a larger bulb. Resist the temptation to immediately remove the bulb at this point - I have been screwed by doing this. Try a larger one first. The behavior should improve. If it does not, there is still a fault present.

Note that some TVs and monitors simply will not power up at all with any kind of series load - at least not with one small enough (in terms of wattage) to provide any real protection. The microcontroller apparently senses the drop in voltage and shuts the unit down or continuously cycles power. Fortunately, these seem to be the exceptions.

Getting inside a monitor

Getting into a monitor is usually quite simple requiring the removal of 2-10 Philips or 1/4" hex head screws - most around the edge of the cabinet or underneath, a couple perhaps in the rear. Disconnect the input and power cables first as it they stay with catch on the rear cover you are detaching. Reconnect whatever is needed for testing after the cover is removed. Set the screws aside and make notes if they are not all of the same length and thread type - putting a too long screw in the wrong place can short out a circuit board or break something else, for example. A screw that is too short may not be secure.

Once all visible screws are out, try to remove the cover. There still may be hidden catches or snaps around the edges or seam or hidden beneath little plastic or rubber cosmetic covers. Sometimes, the tilt-swivel base will need to be removed first. If no snaps or catches are in evidence, the cover may just need a bit of persuasion in the form of a carefully placed screwdriver blade (but be careful not to damage the soft plastic). A 'splitting' tool is actually sold for this purpose.

As you pull the cover straight back (usually) and off, make sure that no other wires are still attached. Often, the main circuit board rests on the bottom of the cover in some slots. Go slow as this circuit board may try to come along with the back. Once the back is off, you may need to prop the circuit board up with a block of wood to prevent stress damage and contact with the work surface.

Most - but not all - monitors can be safely and stably positioned either still on the tilt-swivel base or on the bottom of the frame. However, some will require care as the circuit board will be vulnerable.

Larger monitors are quite heavy and bulky. Get someone to help and take precautions if yours is one of the unstable variety. If need be, the monitor can usually safely be positioned on the CRT face if it is supported by foam or a folded blanket.

Once the cover is off, you will find anywhere from none to a frustratingly large number of sheetmetal (perforated or solid) shields. Depending on which circuit boards need to be accessed, one or more of these shields may need to be removed. Make notes of which screws go where and store in a safe place. However, manufacturers often place holes at strategic locations in order to access ad