Contents:
Notes on the Troubleshooting and Repair of Computer and Video Monitors
Copyright (c) 1994, 1995, 1996, 1997, 1998
All Rights Reserved
Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:
In the early days of small computers, a 110 baud teletype with a personal paper tape reader was the 'preferred' input-output device (meaning that this was a great improvement over punched cards and having to deal with the bozos in the computer room. Small here, also meant something that would comfortably fit into a couple of 6 foot electronics racks!) The earliest personal computers didn't come with a display - you connected them to the family TV. You and your kids shared the single TV and the Flintstones often won out. The Commodore 64 would never have been as successful as it was if an expensive monitor were required rather than an option. However, as computer performance improved, it quickly became clear that a dedicated display was essential. Even for simple text, a TV can only display 40 characters across the screen with any degree of clarity. When the IBM PC was introduced, it came with a nice 80x25 green monochrome text display. It was bright, crisp, and stable. Mono graphics (MGA or MDA) was added at 720x350, CGA at a range of resolutions from 160x200 to 640x200 at 2 to 16 colors, and EGA extended this up to a spectacular resolution of 640x350. This was really fine until the introduction of Windows (well, at least once Windows stayed up long enough for you to care). All of these displays used digital video - TTL signals which coded for a specific discrete number of possible colors and intensities. Both the video adapter and the monitor were limited to 2, 4, or 16 colors depending on the graphics standard. The video signals were logic bits - 0s and 1s. With the introduction of the VGA standard, personal computer graphics became 'real'. VGA and its successors - PGA, XGA, and all of the SVGA (non) standards use analog video - each of the R, G, and B signals is a continuous voltage which can represent a continuous range of intensities for each color. In principle, an analog monitor is capable of an unlimited number of possible colors and intensities. (In practice, unavoidable noise and limitations of the CRT restricts the actual number to order of 64-256 distinguishable intensities for each channel.) Note that analog video was only new to the PC world. TVs and other video equipment, workstations, and image analysis systems had utilized analog signals for many years prior to the PC's 'discovery' of this approach. In all fairness, both the display adapter and monitor are more expensive so it is not surprising that early PCs did not use analog video. Most of the information in the document applies to color computer video monitors and TV studio monitors as well as the display portions of television sets. Black and white, gray scale, and monochrome monitors use a subset of the circuitry (and generally at lower power levels) in color monitors so much of it applies to these as well. For most descriptions of symptoms, testing, diagnosis, and repair, an auto-scan PC SVGA monitor is assumed. For a fixed frequency workstation monitor, studio video monitor, or closed circuit TV monitor, only a subset of the possible faults and procedures will apply. Note: we use the term 'auto-scan' to describe a monitor which accepts a wide (and possibly continuous) range of scan rates. Usually, this refers mostly to the horizontal frequency as the vertical refresh rate is quite flexible on many monitors of all types. Fixed scan or fixed frequency monitors are designed to work with a single scan rate (though a 5% or so variation may actually be accepted). Multi-scan monitors sync at two or more distinct scan rates. While not very common anymore, multi-scan monitors may still be found in some specific applications.
See the manuals on "Troubleshooting and Repair of Small Switchmode Power Supplies" and "Troubleshooting and Repair of Television Sets" for additional useful pointers. Since a monitor must perform a subset of the functions of a TV, many of the problems and solutions are similar. For power related problems the info on SMPSs may be useful as well. If you are considering purchasing a monitor or have one that you would like to evaluate, see the companion document: "Performance Testing of Computer and Video Monitors".
Note: throughout this document, we use the term 'raster' to refer to the entire extent of the scanned portion of the screen and the terms 'picture', 'image'. or 'display', to refer to the actual presentation content. Monitors designed for PCs, workstations, and studio video have many characteristics in common. Modern computer monitors share many similarities with TVs but the auto-scan and high scan rate deflection circuitry and more sophisticated power supplies complicates their servicing. Currently, most computer monitors are still based on the Cathode Ray Tube (CRT) as the display device. However, handheld equipment, laptop computers, and the screens inside video projectors now use flat panel technology, mostly Liquid Crystal Displays - LCDs. These are a lot less bulky than CRTs, use less power, and have better geometry - but suffer from certain flaws. First, the picture quality in terms of gray scale and color is generally inferior to a decent analog monitor. The number of distinct shades of gray or distinct colors is a lot more limited. They are generally not as responsive as CRTs when it comes to real-time video which is becoming increasingly important with multimedia computers. Brightness is generally not as good as a decent CRT display. And last but not least, the cost is still much much higher due both to the increased complexity of flat panel technology and lower production volumes (though this is certainly increasing dramatically). It is really hard to beat the simplicity of the shadow mask CRT. For example, a decent quality active matrix color LCD panel may add $1000 to the cost of a notebook computer compared to $200 for a VGA monitor. More of these panels go into the dumpster than make it to product due to manufacturing imperfections. However, a variety of technologies are currently competing for use in the flat panel displays of the future. Among these are advanced LCD, plasma discharge, and field emission displays. Only time will tell which, if any survives to become **the** picture-on-the-wall or notepad display - at reasonable cost. Projection - large screen - TVs and monitors, on the other hand, may be able to take advantage of a novel development in integrated micromachining - the Texas Instruments Inc. Digital Micromirror Device (DMD). This is basically an integrated circuit with a tiltable micromirror for each pixel fabricated on top of a static memory - RAM - cell. This technology would permit nearly any size projection display to be produced and would therefore be applicable to high resolution computer monitors as well as HDTV. Since it is a reflective device, the light source can be as bright as needed. This is still not a commercial product but stay on line.
The following describe the capabilities which characterize a display:
1. Resolution - the number of resolvable pixels on each line and the
number of scanning lines. Bandwidth of the video source, cable, and
monitor video amplifiers as well as CRT focus spot size are all critical.
However, maximum resolution on a color CRT is limited by the dot/slot/line
pitch of the CRT shadow/slot mask or aperture grille.
2. Refresh rate - the number of complete images 'painted' on the screen
each second. Non-interlaced or progressive scanning posts the entire
frame during each sweep from top to bottom. Interlaced scanning posts
1/2 of the frame called a field - first the even field and then the
odd field. This interleaving reduces the apparent flicker for a given
display bandwidth when displaying smooth imagery such as for TV. It is
usually not acceptable for computer graphics, however, as thin horizontal
lines tend to flicker at 1/2 the vertical scan rate. Refresh rate is the
predominant factor that affects the flicker of the display though the
persistence of the CRT phosphors are also a consideration. Long persistence
phosphors decrease flicker at the expense of smearing when the picture
changes or moves. Vertical scan rate is equal to the refresh rate for
non-interlaced monitors but is the twice the refresh rate for interlaced
monitors (1 frame equals 2 fields). Non-interlaced vertical refresh rates
of 70-75 Hz are considered desirable for computer displays. Television
uses 25 or 30 Hz (frame rate) interlaced scanning in most countries.
3. Horizontal scan rate - the frequency at which the electron beam(s) move
across the screen. The horizontal scan rate is often the limiting factor
in supporting high refresh rate high resolution displays. It is what may
cause failure if scan rate speed limits are exceeded due to the component
stress levels in high performance deflection systems.
4. Color or monochrome - a color monitor has a CRT with three electron
guns each associated with a primary color - red, green, or blue.
Nearly all visible colors can be created from a mix of primaries
with suitable spectral characteristics using this additive color
system.
A monochrome monitor has a CRT with a single electron gun. However,
the actual color of the display may be white, amber, green, or whatever
single color is desired as determined by the phosphor of the CRT selected.
5. Digital or analog signal - a digital input can only assume a discrete
number of states depending on how many bits are provided. A single bit
input can only produce two levels - usually black or white (or amber,
green, etc.). Four bit EGA can display up to 16 colors (with a color
monitor) or 16 shades of gray (with a monochrome monitor).
Analog inputs allow for a theoretically unlimited number of possible gray
levels or colors. However, the actual storage and digital-to-analog
convertors in any display adapter or frame store and/or unavoidable
noise and other characteristics of the CRT - and ultimately, limitations
in the psychovisual eye-brain system will limit this to a practical
maximum of 64-256 discernible levels for a gray scale display or for
each color channel.
However, very high performance digital video sources may have RAMDACs (D/A
convertors with video lookup tables) of up to 10 or more bits of intensity
resolution. While it is not possible to perceive this many distinct gray
levels or colors (per color channel), this does permit more accurate tone
scale ('gamma') correction to be applied (via a lookup table in the RAMDAC)
to compensate for the unavoidable non-linearity of the CRT phosphor
response curve or to match specific photometric requirements.
Monitors can be classified into three general categories: 1. Studio video monitors - Fixed scanning rate for the TV standards in the country in which they are used. High quality, often high cost, utilitarian case (read: ugly), underscan option. Small closed circuit TV monitors fall into the class. Input is usually composite (i.e., NTSC or PAL) although RGB types are available. 2. Fixed frequency RGB - High resolution, fixed scan rate. High quality, high cost, very stable display. Inputs are analog RGB using either separate BNC connectors or a 13W3 (Sun) connector. These often have multiple sync options. The BNC variety permit multiple monitors to be driven off of the same source by daisychaining. Generally used underscanned for computer workstation (e.g., X-windows) applications so that entire frame buffer is visible. There are also fixed frequency monochrome monitors which may be digital or analog input using a BNC, 13W3, or special connector. 3. Multi-scan or auto-scan - Support multiple resolutions and scan rates or multiple ranges of resolutions and scan rates. The quality and cost of these monitors ranges all over the map. While cost is not a strict measure of picture quality and reliability, there is a strong correlation. Input is most often analog RGB but some older monitors of this type (e.g., Mitsubishi AUM1381) support a variety of digital (TTL) modes as well. A full complement of user controls permits adjustment of brightness, contrast, position, size, etc. to taste. Circuitry in the monitor identifies the video scan rate automatically and sets up the appropriate circuitry. With more sophisticated (and expensive) designs, the monitor automatically sets the appropriate parameters for user preferences from memory as well. The DB15 high density VGA connector is most common though BNCs may be used or may be present as an auxiliary (and better quality) input.
Thank IBM. Since the PC has evolved over a period of 15 years, display adapters have changed and improved a number of times. With an open system, vendors with more vision (and willing to take more risks) than IBM were continuously coming up with improved higher resolution display adapters. With workstations and the Apple MacIntosh, the primary vendor can control most aspects of the hardware and software of the computer system. Not so with PCs. New improved hardware adapters were being introduced regularly which were not following any standards for the high resolution modes (but attempted to be backward compatible with the original VGA as well as EGA and CGA (at least in terms of software)). Vast numbers of programs were written that were designed to directly control the CGA, EGA, and VGA hardware. Adapter cards could be designed to emulate these older modes on a fixed frequency high resolution monitor (and these exist to permit high quality fixed scan rate workstation monitors to be used on PCs) However, these would be (and are) much more expensive than basic display adapters that simply switch scan rates based on mode. Thus, auto-scan monitors evolved to accommodate the multiple resolutions that different programs required. Note: we will use the generic term 'auto-scan' to refer to a monitor which automatically senses the input video scan rate and selects the appropriate horizontal and vertical deflection circuitry and power supply voltages to display this video. Multi-scan monitors, while simpler than true auto-scan monitors, will still have much of the same scan rate detection and selection circuitry. Manufacturers use various buzz words to describe their versions of these monitors including 'multisync', 'autosync','panasync', 'omnisync', as well as 'autoscan' and 'multiscan'. Ultimately, the fixed scan rate monitor may reappear for PCs. Consider one simple fact: it is becoming cheaper to design and manufacture complex digital processing hardware than to produce the reliable high quality analog and power electronics needed for an auto-scan monitor. This is being done in the specialty market now. Eventually, the development of accelerated chipsets for graphics mode emulation may be forced by the increasing popularity of flat panel displays - which are basically similar to fixed scan rate monitors in terms of their interfacing requirements.
There are two aspects of monitor design that can be described in terms of analog or digital characteristics: 1. The video inputs. Early PC monitors, video display terminal monitors, and mono workstation monitors use digital input signals which are usually TTL but some very high resolution monitors may use ECL instead. 2. The monitor control and user interface. Originally, monitors all used knobs - sometimes quite a number of them - to control all functions like brightness, contrast, position, size, linearity, pincushion, convergence, etc. However, as the costs of digital circuitry came down - and the need to remember settings for multiple scan rates and resolutions arose, digital - microprocessor control - became an attractive alternative in terms of design, manufacturing costs, and user convenience. Now, most better quality monitors use digital controls - buttons and menus - for almost all adjustments except possibly brightness and contrast where knobs are still more convenient. Since monitors with digital signal inputs are almost extinct today except for specialized applications, it is usually safe to assume that 'digital' monitor refers to the user interface and microprocessor control.
Whether a monitor runs interlaced or non-interlaced is almost always strictly a function of the video source timing. The vertical sync pulse is offset an amount equal to 1/2 the line time on alternate fields (vertical scans - two fields make up a frame when interlaced scanning is used). Generally, a monitor that runs at a given resolution non-interlaced can run at a resolution with roughly twice the number of pixels interlaced at the same horizontal scan rate. For example, a monitor that will run 1024x768 non-interlaced at 70 Hz will run 1280x1024 interlaced at a 40 Hz frame rate. Whether the image is usable at the higher resolution of course also depends on many other factors including the dot pitch of the CRT and video bandwidth of the video card and monitor video amplifiers, as well as cable quality and termination. The flicker of fine horizontal lines may also be objectionable.
The ultimate perceived quality of your display is influenced by many aspects of the total video source/computer-cable-monitor system. Among them are: 1. Resolution of the video source. For a computer display, this is determined by the number of pixels on each visible scan line and the number of visible scan lines on the entire picture. 2. The pitch of the shadow mask or aperture grille of the CRT. The smallest color element on the face of the CRT is determined by the spacing of the groups of R, G, and B colors phosphors. The actual conversion from dot or line pitch to resolution differs slightly among dot or slot mask and aperture grille CRTs but in general, the finer, the better - and more expensive. Typical television CRTs are rather coarse - .75 mm might be a reasonable specification for a 20 inch set. High resolution computer monitors may have dot pitches as small as .22 mm for a similar size screen. A rough indication of the maximum possible resolution of the CRT can be found by determining how many complete phosphor dot groups can fit across the visible part of the screen. Running at too high a resolution for a given CRT may result in Moire - an interference pattern that will manifest itself as contour lines in smooth bright areas of the picture. However, many factors influence to what extent this may be a problem. See the section: "Contour lines on high resolution monitors - Moire". 3. Bandwidth of the video source or display card - use of high performance video amplifiers or digital to analog convertors. 4. Signal quality of the video source or display card - properly designed circuitry with adequate power supply filtering and high quality components. 5. High quality cables with correct termination and of minimal acceptable length without extensions or switch boxes unless designed specifically for high bandwidth video. 6. Sharpness of focus - even if the CRT dot pitch is very fine, a fuzzy scanning beam will result in a poor quality picture. 7. Stability of the monitor electronics - well regulated power supplies and low noise shielded electronics contribute to a rock solid image.
WARNING: No monitor is perfect. Running comprehensive tests on your
monitor or one you are considering may make you aware of deficiencies you
never realized were even possible. You may never be happy with any monitor
for the rest of your life!
Note: the intent of these tests is **not** to evaluate or calibrate a monitor
for photometric accuracy. Rather they are for functional testing of the
monitor's performance.
Obviously, the ideal situation is to be able to perform these sorts of
tests before purchase. With a small customer oriented store, this may
be possible. However, the best that can be done when ordering by mail
is to examine a similar model in a store for gross characteristics and
then do a thorough test when your monitor arrives. The following should
be evaluated:
* Screen size and general appearance.
* Brightness and screen uniformity, purity and color saturation.
* Stability.
* Convergence.
* Edge geometry.
* Linearity.
* Tilt.
* Size and position control range.
* Ghosting or trailing streaks.
* Sharpness.
* Moire.
* Scan rate switching.
* Acoustic noise.
The companion document: "Performance Testing of Computer and Video Monitors"
provides detailed procedures for the evaluation of each of these criteria.
CAUTION: since there is no risk free way of evaluating the actual scan
rate limits of a monitor, this is not an objective of these tests. It
is assumed that the specifications of both the video source/card and the
monitor are known and that supported scan rates are not exceeded. Some
monitors will operate perfectly happily at well beyond the specified range
or will shut down without damage. Others will simply blow up instantly and
require expensive repairs.
Unlike PC system boards where any disasters are likely to only affect your pocketbook, monitors can be very dangerous. Read, understand, and follow the set of safety guidelines provided later in this document whenever working on TVs, monitors, or other similar high voltage equipment. If you do go inside, beware: line voltage (on large caps) and high voltage (on CRT) for long after the plug is pulled. There is the added danger of CRT implosion for carelessly dropped tools and often sharp sheetmetal shields which can injure if you should have a reflex reaction upon touching something you should not touch. In inside of a TV or monitor is no place for the careless or naive. Having said that, a basic knowledge of how a monitor works and what can go wrong can be of great value even if you do not attempt the repair yourself. It will enable you to intelligently deal with the service technician. You will be more likely to be able to recognize if you are being taken for a ride by a dishonest or just plain incompetent repair center. For example, a faulty picture tube CANNOT be the cause of a color monitor only displaying in black-and-white (this is probably a software or compatibility problem). The majority of consumers - and computer professionals - may not know even this simple fact. This document will provide you with the knowledge to deal with a large percentage of the problems you are likely to encounter with your monitors. It will enable you to diagnose problems and in many cases, correct them as well. With minor exceptions, specific manufacturers and models will not be covered as there are so many variations that such a treatment would require a huge and very detailed text. Rather, the most common problems will be addressed and enough basic principles of operation will be provided to enable you to narrow the problem down and likely determine a course of action for repair. In many cases, you will be able to do what is required for a fraction of the cost that would be charged by a repair center. Should you still not be able to find a solution, you will have learned a great deal and be able to ask appropriate questions and supply relevant information if you decide to post to sci.electronics.repair. It will also be easier to do further research using a repair text such as the ones listed at the end of this document. In any case, you will have the satisfaction of knowing you did as much as you could before taking it in for professional repair. With your new-found knowledge, you will have the upper hand and will not easily be snowed by a dishonest or incompetent technician.
The following probably account for 95% or more of the common monitor ailments: * Intermittent changes in color, brightness, size, or position - bad connections inside the monitor or at the cable connection to the computer or or video source. * Ghosts, shadows, or streaks adjacent to vertical edges in the picture - problems with input signal termination including use of cable extensions, excessively long cables, cheap or improperly made video cables, improper daisychaining of monitors, or problems in the video source or monitor circuitry. * Magnetization of CRT causing color blotches or other color or distortion problems - locate and eliminate sources of magnetic fields if relevant and degauss the CRT. * Electromagnetic Interference (EMI) - nearby equipment (including and especially other monitors), power lines, or electrical wiring behind walls, may produce electromagnetic fields strong enough to cause noticeable wiggling, rippling, or other effects. Relocate the monitor or offending equipment. Shielding is difficult and expensive. * Wiring transmitted interference - noisy AC power possibly due to other equipment using electric motors (e.g., vacuum cleaners), lamp dimmers or motor speed controls (shop tools), fluorescent lamps, and other high power devices, may result in a variety of effects. The source is likely local - in your house - but could be several miles away. Symptoms might include bars of noise moving up or down the screen or diagonally. The effects may be barely visible as a couple of jiggling scan lines or be broad bars of salt and pepper noise, snow, or distorted video. Plugging the monitor into another outlet or the use of a line filter may help. If possible, replace or repair the offending device. * Monitor not locking on one or more video scan ranges - settings of video adapter are incorrect. Use software setup program to set these. This could also be a fault in the video source or monitor dealing with the sync signals. * Adjustments needed for background brightness or focus - aging CRT reduces brightness. Other components may affect focus. Easy internal (or sometimes external) adjustments. * Dead monitor due to power supply problems - very often the causes are simple such as bad connections, blown fuse or other component.
If you need to send or take the monitor to a service center, the repair could easily exceed half the cost of a new monitor. Service centers may charge up to $50 or more for providing an initial estimate of repair costs but this will usually be credited toward the total cost of the repair (of course, they may just jack this up to compensate for their bench time). Some places offer attractive flat rates for repairs involving anything but the CRT, yoke, and flyback. Such offers are attractive if the repair center is reputable. However, if by mail, you will be stuck with a tough decision if they find that one of these expensive components is actually bad. Monitors become obsolete at a somewhat slower rate than most other electronic equipment. Therefore, unless you need the higher resolution and scan rates that newer monitors provide, repairing an older one may make sense as long as the CRT is in good condition (adequate brightness, no burn marks, good focus). However, it may just be a good excuse to upgrade. If you can do the repairs yourself, the equation changes dramatically as your parts costs will be 1/2 to 1/4 of what a professional will charge and of course your time is free. The educational aspects may also be appealing. You will learn a lot in the process. Thus, it may make sense to repair that old clunker for your 2nd PC (or your 3rd or your 4th or....).
A computer or video monitor includes the following functional blocks:
1. Low voltage power supply (some may also be part of (2)). Most of the lower
voltages used in the TV may be derived from the horizontal deflection
circuits, a separate switching power supply, or a combination of the two.
Rectifier/filter capacitor/regulator from AC line provides the B+ to the
switching power supply or horizontal deflection system. Auto-scan
monitors may have multiple outputs from the low voltage power supply
which are selectively switched or enabled depending on the scan rate.
Degauss operates off of the line whenever power is turned on (after
having been off for a few minutes) to demagnetize the CRT. Better
monitors will have a degauss button which activates this circuitry
as well since even rotating the monitor on its tilt-swivel base can
require degauss.
2. Horizontal deflection. These circuits provide the waveforms needed to
sweep the electron beam in the CRT across and back at anywhere from
15 KHz to over 100 KHz depending on scan rate and resolution. The
horizontal sync pulse from the sync separator or the horizontal sync
input locks the horizontal deflection to the video signal. Auto-scan
monitors have sophisticated circuitry to permit scanning range of
horizontal deflection to be automatically varied over a wide range.
3. Vertical deflection. These circuits provide the waveforms needed to
sweep the electron beam in the CRT from top to bottom and back at
anywhere from 50 - 120 or more times per second. The vertical sync
pulse from the sync separator or vertical sync input locks the vertical
deflection to the video signal. Auto-scan monitors have additional
circuitry to lock to a wide range of vertical scan rates.
4. CRT high voltage (also part of (2)). A modern color CRT requires
up to 30 KV for a crisp bright picture. Rather than having a totally
separate power supply, most monitors derive the high voltage (as well
as many other voltages) from the horizontal deflection using a special
transformer called a 'flyback' or 'Line OutPut Transformer (LOPT) for
those of you on the other side of the lake. Some high performance
monitors use a separate high voltage board or module which is a self
contained high frequency inverter.
5. Video amplifiers. These buffer the low level inputs from the computer
or video source. On monitors with TTL inputs (MGA, CGA, EGA), a resistor
network also combines the intensity and color signals in a kind of poor
man's D/A. Analog video amplifiers will usually also include DC restore
(black level retention, back porch clamping) circuitry stabilize the
black level on AC coupled video systems.
6. Video drivers (RGB). These are almost always located on a little
circuit board plugged directly onto the neck of the CRT. They boost
the output of the video amplifiers to the hundred volts or so needed
to drive the cathodes (usually) of the CRT.
7. Sync separator. Where input is composite rather than separate H and
V syncs, this circuit extracts the individual sync signals. Output is
horizontal and vertical sync pulses to control the deflection circuits.
This is not needed on a monitor that only uses separate sync inputs.
8. System control. Most higher quality monitors use a microcontroller
to perform all user interface and control functions from the front panel
(and sometimes even from a remote control). So called 'digital monitors'
meaning digital controls not digital inputs, use buttons for everything
except possibly user brightness and contrast. Settings for horizontal
and vertical size and position, pincushion, and color balance for each
scan rate may be stored in non-volatile memory. The microprocessor
also analyzes the input video timing and selects the appropriate scan
range and components for the detected resolution. While these circuits
rarely fail, if they do, debugging can be quite a treat.
Most problems occur in the horizontal deflection and power supply sections.
These run at relatively high power levels and some components run hot.
This results in both wear and tear on the components as well as increased
likelihood of bad connections developing from repeated thermal cycles.
The high voltage section is prone to breakdown and arcing as a result
of hairline cracks, humidity, dirt, etc.
The video circuitry is generally quite reliable. However, it seems that
even after 15+ years, manufacturers still cannot reliably turn out circuit
boards that are free of bad solder connections or that do not develop them
with time and use.
The books listed in the section: "Suggested references" include additional information on the theory and implementation of the technology of monitors and TV sets.
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.
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.
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).
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.
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 rated for continuous operation and may overheat. 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 termister which heats up and cuts off the current. If you push the button twice in a row, that thermister 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 thermister 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.
These are not a defect - they are a 'feature'. 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. 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.
Proper care of a monitor does not require much. Following the recommendations below will assure long life and minimize repairs: * 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.
Monitors normally are positioned horizontally or via the limits of their tilt swivel bases out in the open on a table or desktop. However, for use in exhibits or for custom installations, it may be desirable to mount a monitor in a non-standard position and/or inside an enclosure. (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 for a monitor is pretty simple - just keep the case clean and free of obstructions. Clean the CRT screen with a soft cloth just dampened with water and mild detergent or isopropyl alcohol. This will avoid damage to normal as well as antireflection coated glass. DO NOT use anything so wet that liquid may seep inside of the monitor around the edge of the CRT. You could end up with a very expensive repair bill when the liquid decides to short out the main circuit board lurking just below. Then dry thoroughly. Use the CRT sprays sold in computer stores if you like but again, make sure none can seep inside. If you have not cleaned the screen for quite a while, you will be amazed at the amount of black grime that collects due to the static buildup from the CRT high voltage supply. In really dusty situations, periodically vacuuming inside the case and the use of contact cleaner for the controls might be a good idea but realistically, you will not do this so don't worry about it. (From: 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: 4. 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.
(From: Bob Myers (myers@fc.hp.com)). 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.
TVs and computer or video monitors are among the more dangerous of consumer electronics equipment when it comes to servicing. (Microwave ovens are probably the most hazardous due to high voltage at flesh frying and cardiac arresting high power.) 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 red 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.
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 insulating cup at the end of the fat red 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!
Many problems have simple solutions. Don't immediately assume that your problem is some combination of esoteric complex convoluted failures. For a monitor, it may just be a bad connection or blown fuse. Remember that the problems with the most catastrophic impact on operation like a dead monitor usually have the simplest solutions. The kind of problems we would like to avoid at all costs are the ones that are intermittent or difficult to reproduce: the occasional jitter or a monitor that blows its horizontal output transistor every six months. If you get stuck, sleep on it. Sometimes, just letting the problem bounce around in your head will lead to a different more successful approach or solution. Don't work when you are really tired - it is both dangerous (especially with respect to monitors) and mostly non-productive (or possibly destructive). Whenever working on complex equipment, make copious notes and diagrams. You will be eternally grateful when the time comes to reassemble the unit. Most connectors are keyed against incorrect insertion or interchange of cables, but not always. Apparently identical screws may be of differing lengths or 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! See the document: "Troubleshooting and Repair of Consumer Electronics 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.
Don't start with the electronic test equipment, start with some analytical thinking. Your powers of observation (and a little experience) will make a good start. Your built in senses and that stuff between your ears represents the most important test equipment you have. However, some test equipment will be needed: * Multitester (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 multitester 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 or 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. My 'good' scope is the militarized version (AN/USM-281A) of the HP180 lab scope. This 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 465 100 Mhz scope ($400-700) which 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.
These are the little gadgets and homemade testers that are useful for many repair situations. Here are just a few of the most basic: * 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.). * 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".
It is essential - for your safety and to prevent damage to the device under test as well as your test equipment - that large or high voltage capacitors be fully discharged before measurements are made, soldering is attempted, or the circuitry is touched in any way. Some of the large filter capacitors commonly found in line operated equipment store a potentially lethal charge. 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 - some capacitors (like the high voltage of the CRT in a TV or video monitor) will 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 switching power supply which might be 100 uF at 350 V this would mean a 5K 10W resistor. RC=.5 second. 5RC=2.5 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: "Testing capacitors with a multimeter and safe discharge". This provides a visible indication of remaining charge and polarity. * For the CRT, use a high wattage (not for power but to hold off the high voltage which could jump across a tiny 1/4 watt job) resistor of a few M ohms discharged 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. (Using a shorting clip lead may not be a bad idea as well while working on the equipment - 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 25KV 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.
You may hear that it is only safe to discharge from the Ultor to the Dag. So, what the @#$% are they talking about? :-). (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. For more details, see the document: "TV and Monitor CRT (Picture Tube) Information.
When powering up a monitor (or any other modern electronic devices with expensive power semiconductors) that has had work done on any power circuits, it is desirable to minimize the chance of blowing your newly installed parts should there still be a fault. There are two ways of doing this: use of a Variac to bring up the AC line voltage gradually and the use of a series load to limit current to power semiconductors. 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 provide better protection as it will limit the current out of the main filter capacitors to the HOT. 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.
You will void the warranty - at least in principle. There are usually no warranty seals on a monitor so unless you cause visible damage or mangle the screws or plastic, it is unlikely that this would be detected. You need to decide. A monitor still under warranty should probably be returned for warranty service for any covered problems except those with the most obvious and easy solutions. Another advantage of using warranty service is that should your problem actually be covered by a design change, this will be performed free of charge. And, you cannot generally fix a problem which is due to poor design! 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 adjustments - check for these before going to a lot of unnecessary bother. Note: sheetmetal usually has sharp edges. Take care. Reassemble in reverse order. Getting the circuit board to slide smoothly into its slots may take a couple of attempts but otherwise there should be no surprises.
Both electrical and mechanical dangers lurk: * Main filter capacitor(s). This is the most dangerous (not the HV as you would expect). Fortunately, these capacitors will normally discharge in a few minutes or less especially if the unit is basically working as the load will normally discharge the capacitors nearly fully as power is turned off. With TVs, the main filter capacitor is nearly always on the mainboard. Monitors are more likely to have a separate power supply module. However, you should check across this capacitor - usually only one and by far the largest in the unit - with a voltmeter and discharge as suggested in the section: "Safe discharging of capacitors in TVs and video monitors" if it holds more than a few volts (or wait longer) before touching anything. Some of these are as large as 1,000 uF charged to 160 V - about 13 w-s or a similar amount of energy as that stored in an electronic flash. This is enough to be potentially lethal under the wrong circumstances. * High Voltage capacitor formed by the envelope of the CRT. It is connected to the flyback transformer by the fat (usually red) wire at the suction cup (well, it looks like one anyhow) attached to the CRT. This capacitor can hold a charge for quite a while - weeks in the case of an old tube type TV! If you want to be doubly sure, discharge this also. However, unless you are going to be removing the HV connector/flyback, it should not bother you. The energy stored is about 1 w-s but if you touch it or come near to an exposed terminal, due to the high voltage, you will likely be handed *all* the energy and you *will* feel it. The danger is probably more in the collateral damage when you jump ripping flesh and smashing your head against the ceiling. Some people calibrate their jump based on voltage - about 1 inch/V. :-). There will be some HV on the back of the circuit board on the neck of the CRT but although you might receive a tingle but accidentally touching the focus or screen (G2) pins, it is not likely to be dangerous. * CRT implosion risk. Don't hammer on it. However, it is more likely that you will break the neck off the tube since the neck is relatively weak. This will ruin your whole day and the TV or monitor but will likely not result in flying glass everywhere. Just, don't go out of your way to find out. * Sharp sheet metal and so forth. This is not in itself dangerous but a reflex reaction can send your flesh into it with nasty consequences.
The first thing you will notice when you remove the cover is how super dusty everything is. Complements to the maid. You never dreamed there was that much dust, dirt, and grime, in the entire house or office building! Use a soft brush (like a new paintbrush) and a vacuum cleaner to carefully remove the built up dust. Blowing off the dust will likely not hurt the unit unless it gets redeposited inside various controls or switches but will be bad for your lungs - and will spread dirt all over the room. Don't turn anything - many critical adjustments masquerade as screws that just beg to be tightened. Resist the impulse for being neat and tidy until you know exactly what you are doing. Be especially careful around the components on the neck of the CRT - picture tube - as some of these are easily shifted in position and control the most dreaded of adjustments - for color purity and convergence. In particular, there will be a series of adjustable ring magnets. It is a good idea to mark their position in any case with some white paint, 'white out', or a Magic Marker so that if they do get moved - or you move them deliberately, you will know where you started.
There are times when it is desirable to remove the chassis or mainboard and work on it in a convenient location without having to worry about the attachments to the CRT and cabinet circuitry. My approach is usually to do as much work as possible without removing the main board and not attempt to power it up when disconnected since there are too many unknowns. Professionals will plug the chassis into a piece of equipment which will simulate the critical functions but this is rarely an option for the doit-yourselfer. Note that if you have a failure of the power supply - blown fuse, startup, etc., then it should be fine to disconnect the CRT since these problems are usually totally unrelated. Tests should be valid. However, if you really want to do live testing with the main board removed, here are some considerations. There are usually several connections to the CRT and cabinet: * Deflection yoke - since the horizontal coils are part of the horizontal flyback circuit, there could be problems running without a yoke. This could be anything from it appearing totally dead to an overheating or blown horizontal output transistor. There may be no problems. Vertical and any convergence coils may or may not be problems as well. * CRT video Driver board - pulling this should not usually affect anything except possibly video output and bias voltages. * CRT 2nd anode - without the CRT, there will be no capacitor to filter the high voltage and you would certaily want to insulate the HV connector **real** well. I do not know whether there are cases where damage to flyback could result from running in thie manner, however. * Front panel controls - disconnecting these may result in inability to even turn the unit on, erratic operation, and other unexpected behavior. * Degauss - you just won't have this function when disconnected. But who cares - you are not going to be looking at the screen anyhow. * Remote sensor - no remote control but I doubt that the floating signals will cause problems. * Speakers - there will be no audio but this should not cause damage. If you do disconnect everything, make sure to label any connectors whose location or orientation may be ambiguous. Most of the time, these will only fit one way but not always.
For general viewing, subdued lighting is preferred. Avoid backlighting and direct overhead lighting if possible. Display an image with a variety of colors and the full range of brightness from deep shadows to strong highlights. For PCs, a Windows desktop is generally satisfactory. An outdoor scene on a sunny day is excellent for studio monitors. Alternatively, use a test pattern specially designed for this purpose. Turn the BRIGHTNESS and CONTRAST controls (or use the buttons) all the way down. Increase the BRIGHTNESS until a raster is just visible in the darkest (shadow) areas of the picture. Increase the CONTRAST until the desired intensity of highlights is obtained. Since BRIGHTNESS and CONTRAST are not always independent, go back and forth until you get the best picture. On monitors with a color balance adjustment, you may want to set this but unless you are doing photorealistic work, using the manufacturer's defaults will be fine unless you need to match the characteristics of multiple monitors located side-by-side.
One of the most common complaints is that the monitor is not as crisp as it used to be - or just not as sharp as expected. Assuming that the focus has just been gradually getting worse over time, tweaking the internal focus control may be all that is needed. Some monitors have the focus adjustment accessible through a (possibly unmarked) hole in the side or rear of the case. If there is a single hole, it is almost certainly for overall focus. If there are two holes, one may be the screen (G2 - master brightness) or the two adjustments may be for different aspects of focus (e.g., horizontal and vertical). Just carefully observe what happens when each adjustment is moved a little so that you can return it to its original setting if you turned the wrong one. Use a thin insulated screwdriver - preferably with a plastic blade. As a extra precaution, determine of the screwdriver will mate easily with the adjustment with the monitor **off** (don't turn anything, however). Where there are two adjustment knobs on the flyback transformer, the top one is generally for focus and the bottom one is for G2. Most inexpensive monitors have only what is known as static focus - a constant voltage derived from the HV power supply is applied to the focus grid of the CRT. This does not allow for optimal focus across the screen and any setting is just a compromise between central and edge sharpness. Better monitors will have (in addition) H and V focus controls. These are for dynamic focus adjustments. There may be some interaction between the static and dynamic adjustments. If either of these controls has n