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Table of ContentsPrefaceAuthor and CopyrightDISCLAIMERAcknowledgements
IntroductionScope of This DocumentRelated DocumentsAdditional Information on CRTs
CRT Safety IssuesElectrical SafetySafe Discharging of Capacitors in TVs and Video MonitorsAdditional Information on Discharging CRTsAbout Charge Reappearing on Discharged CRTWarning about disconnecting CRT neck boardCRT Implosion Risk?Picture Tube Implosion IS Possible – But You Really Need Work at It!Risks from CRT Scratches?Disposing of Dead TVs or Monitors (CRTs and Charged HV Capacitors
CRT Construction and CharacteristicsWhy is the CRT Still Dominant?Comparison of CRT TypesColor CRT ConstructionAssembly of Color CRTsCRT Fine TuningNorthern/Southern Hemisphere Corrections and AdjustmentsTubes for All NationsSo What Does It Mean to Have a Trinitron CRT?Why are There Fine Lines Across My Trinitron Monitor or TVDifferences between Trinitron and Diamondtron CRTsSome History of In-Line Gun CRTsHow Far is the Shadow Mask from the Phosphor ScreenHow is the Shadow Mask Mounted Inside the CRT?Why is the Shadow Mask or Aperture Grill Made of a Magnetic Material?Why do CRTs Use Red, Green, and Blue rather than Red, Yellow, Blue?Purpose of a Separate CRT FaceplateLeaded Glass and CRT CoatingsFlat Versus Non-Flat CRTs
Resolution, Dot Pitch, and Other CRT SpecificationsColor CRT Resolution – Focus and Dot/Slot/Line PitchA Discussion of Issues Relating to Monitor and CRT ResolutionAbout the Quality of Monitor FocusHow to Compute Effective Dot PitchDot Pitch of TV CRTsCRT Aspect RatioCRT Deflection AngleCRT Contrast Ratio
Effects of External Magnetic Fields on CRTsMagnetic Interference and ShieldingComments on Speaker ShieldingWhy Magnetic Fields May Cause the Picture to RotateBest Direction to Face a Monitor?Ways Around North/South or Other Sensitivity to Magnetic FieldsAdditional Comments/Summary on Northern/Southern Hemisphere IssuesOrientation Considerations for Projection TVs
Picture Quality and Appearance IssuesWhy Does the Intensity Appear So Non-Uniform in Bright Areas?Comments On Color Purity, Set Orientation, and DomingDifference in Color Rendition Between CRTsContour Lines on High Resolution Monitors – MoireMoire and Shadow Mask Dot PitchIsolated Spots on DisplayPurple Blob – or WorseMagnet Fix for Purity Problems – If Duct Tape Works, Use It!How Much Tilt is Acceptable?What is Doming?Afterglow – Phantom Patterns on CRT After ShutoffDiscussion on the causes of color flare
Magnetic Fields and DegaussingDegaussing (Demagnetizing) a CRTHow Often to DegaussUltra Cheap Degaussing CoilBob Myers Notes on DegaussingDegaussing after lightning strikeDegaussing Humor – If it Works, Use It!Can a Really Strong Magnet Permanently Damage the CRT?WARNING about degaussing late model Sony Trinitron CRTs
CRT Related AdjustmentsPrinciples of Purity and Convergence AdjustmentDetailed Purity and Static Convergence Adjustment ProcedureTonys Notes on Setting Convergence on Older Delta Gun CRTsJerrys Comments on Convergence and Other Advanced AdjustmentsCRTs with No Purity or Static Convergence RingsProjection Set Convergence Adjustment PrinciplesMonitor Tune-Up?A Discussion on Correction Magnets
CRT and CRT Related Maintenance and RepairPreventive Maintenance – Care and CleaningCRT?Shorts in a CRTProviding Isolation for a CRT H-K ShortRescuing a Shorted CRTDetermining if Your CRT is up to AirScratches or Other Damage to the CRT Face
CRT DegradationCRT Aging – Effects on Electrical Characteristics and PerformanceCRT Age Resulting in Dark PictureBrightening an Old CRTChecking the Age of the CRT
CRT RejuvenationWhat is CRT Rejuvenation?CRT Degradation and Rejuvenation?More Comments on CRT RejuvenationHome-Made CRT Rejuvenator 1Home-Made CRT Rejuvenator 2
Items of InterestLifespans of MonitorsMonitor Life, Energy Conservation, and LazinessThernal Cycling and Component LifeWhy are Prices of Video Monitors So High Compared to TVs?Expected Life of TV CRTsProblems with Designing a Combination TV and Computer MonitorPicture Tube Disassembly for DemonstrationTurning a Large CRT Faceplate into the Side of a Fish TankWhy do TVs Overscan?What is Aquadag?Why are Indirectly Heated Cathodes Used in CRTsFrequency Response of CRTs
CRT Service InformationHow to Read CRT Part NumbersTypical Color CRT PinoutCRT SubstitutionCRT Replacement Worth It?Rebuilt CRTsWhat Does It Take to be a Picture Tube Rebuilder, Really?Shipping Damage: Why Monitors are Like Basketballs
Back toCRT FAQ Table of Contents.PrefaceAuthor: Samuel M. Goldwasser
For contact info, please see theSci.Electronics.Repair FAQ Email Links Page.
Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:
1.This notice is included in its entirety at the beginning.
2.There is no charge except to cover the costs of copying.We will not be responsible for damage to equipment, your ego, blown parts, county wide power outages, spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury that may result from the use of this material.Special thanks to Bob Myers ) and Jeroen Stessen (Jeroen.) for their contributions to this document through their newsgroup postings and private email.
Back toCRT FAQ Table of Contents.IntroductionThis document contains a collection of information relating to CRT (picture tube) construction, characteristics, problems, maintenance, troubleshooting, and repair. This was originally from the TV and monitor repair guides of theSci.Electronics.Repair FAQbut has been moved here due to its being of general interest.
Most new CRT related information originating on thesci.electronics.repair,comp.sys.ibm.pc.hardware.video, or other USENET newsgroups will be included here rather than in those other documents.The following may be of interest and cover many relavent topics related to CRT based equipment:Safety Guidelines for High Voltage and/or Line Powered Equipment.Notes on the Troubleshooting and Repair of Computer and Video Monitors.Notes on the Troubleshooting and Repair of Television Sets.Performance Testing of Computer and Video Monitors.Notes on Approaches to using Fixed Frequency Monitors on PCs.Notes on Video Conversion.ThePC Technology Guidehas some information with nice diagrams on both CRT and flat panel displays. This site is well worth visiting to get an idea of the construction, operation, and problems for a variety of display technologies.
Ive seen a few such pictures and I was fortunate enough to find a book on color CRTs that explained quite a few things:Color Television Picture Tubes
ISBN 0-12-022151-0.Im not sure if its still in print but you might check out your local university library.)
If you are lucky enough to see The Secret Life of Machines on The Learning Channel (or was, last time I saw it), theres an episode on the secret life of the TV. Its excellent! The creator and presenter, Tim Hunkin, has a weird sense of humor but hes very well informed and quite gifted in the way he demonstrates difficult-to-explain concepts. In the opening scene, he showed off a TV that he sawed in half, showing the CRT construction very clearly. (He must have let the air into the tube, then used a diamond saw to cut it; thats the only way it could be done without glass everywhere!)
(Of course, he may not *actually* have cut a TV in half – manufacturers no doubt maintain props of this sort!)
Back toCRT FAQ Table of Contents.CRT Safety IssuesTVs and computer or video monitors are among the more dangerous of consumer electronic 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 dont 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.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 doesnt 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:Capacitor Testing, Safe Discharging, and Other Related Information. 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. More below.) 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!T
Reasons to use a resistor and not a screwdriver to discharge capacitors:It will not destroy screwdrivers and capacitor terminals.
It will not damage the capacitor (due to the current pulse).
It will reduce your spouses 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? :-).
BTW, dont wash your CRTs even if the Maid complains about the filth until you have confirmed that your Dag isnt water soluble (maybe thats why it has aqua in the name!). It may all come off! Wipe off the dirt and dust with a cloth (and stay away from the HV connector or make sure it is discharged first!).
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 TVs which didnt use a voltage divider to derive the focus potential or newer TVs with a defective open divider.(From: Joseph Gwinn ().)
The issue with CRTs is that the glass dielectric, having been kept at many tens of kilovolts for years, will store charge deep in the glass, and this charge cannot be eliminated quickly. The phenomena is called dielectric adsorption or soakage. One can short such a CRT for a week, remove the short, and see the voltage magically spring back. So leave it shorted.
Big capacitors can do this as well, especially the big oil-paper capacitors used in HV power supplies. These can store a lethal jolt.
This same phenomena is used in Electret microphones, where a thin layer of Teflon stores the HV charge needed to make the microphone work.Some manufacturers warn against powering a TV or monitor CRT without the CRT neck board connected. Apparently, without something – anything – to drain the charge resulting from the current flow due to residual gas ions inside the CRT, the shortest path may be through the glass neck of the tube to the yoke or from the pins outside the CRT to whatever is nearby. There arent many ions in a modern CRT but I suppose a few here, a few there, and eventually they add up to enough to cause a major disaster at least on some CRTs.
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 wont hurt to take appropriate precautions with video and computer monitors as well.Also see the section:Disposing of Dead TVs or Monitors (CRTs and Charged HV Capacitors).
I have checked with our CRT expert and he thinks that any normal type of scratch does not pose any danger. Usual disclaimer applies … (what is normal?)
The front of the tube is much thicker and stronger than the rear. It has to be, to withstand the air pressure, because the curvature radius is so much larger. You wont break it by throwing a slipper at it. The neck is in fact very easy to break, usually without causing injuries to anyone.
Normally, if the tube should implode, the rimband (the tensioned steel band around the rim of all modern CRTs of any size) prevents the glass from flying outward too far. Every tube type has to pass tests in which it is deliberately imploded and it is checked whether any large shrapnel flies too far out.
What *is* very dangerous is a CRT with its rimband missing, or a CRT which never had a decent rimband in the first place (like some dubious Russian-made samples we once saw). Such a tube should not be handled at all. NEVER ever attempt to remove the rimband for and reason!
I just saw a picture tube that was broken due to dropping the (entire) TV on one corner. In the cone (the backside) there are open cracks of some 3 feet length in total. Nevertheless all the glass is still in its original place and it looks as if no glass has flown outward. The faceplate is still intact. So in this case nobody would have got hurt. I remember reading about Americans (who else?) who tried to shoot CRTs with smaller rifles, with little or no success.
Does this comfort you? Get out the shotgun and have a go at it!
Our 6 month old 20 SGI color monitor (model GDM-20D11) lost a fight with a fork lift. The case is intact, the CRT probably still has a vacuum, but the outer layer of glass on the screen is shattered.As noted elsewhere in this document, picture tube implosion is a hazard but under normal conditions, quite unlikely. Someone wrote:I heard somewhere that in the early days of TV, the tubes had a tendency to implode at the drop of a hat. (Due to poor design?) In order to prevent flying glass, the sets had a plastic sheet in front of the screen. Obviously, modern sets no longer have this. How safe are modern CRT screens in terms of impact damage etc?Well, it isnt quite as simple as that….. However, even if CRT implosion is one of those highly unlikely events, the downside is that should it occur in just the wrong way, the consequences can be disastrous. So, I wouldnt depend on the experiences below to guide you! Treat a CRT about the same way you would an armed nuclear bomb. OK, well maybe just 10 sticks of dynamite. 🙂
In high school, our electronics teacher did a demo for each class. He saved out an old black-and-white tube for each class and set up a place to break it. Put the tube on the ground by a brick wall, with a hammer suspended on a wire from the top of the wall. Did it on the driveway so that the glass would be easier to pick up. The tube was placed image-side down.
First he pulled the hammer back about 20 feet and just let it go. It bounced off the tube. This was to show that such tubes are pretty tough. Then he pulled the hammer back and gave it a pretty good shove, turning his back to the tube and moving quickly away from it. (Lets face it, the guy could probably have found a safer way to do this.)
Palm sized chunks of glass flew 50 feet. The noise was quite impressive. The thickness of the image plate of the tube was also quite impressive. Kind of looked like a porthole on a submarine. This was from the tube of a small black-and-white TV, about 14 inches or so. One of the larger colour models might be a LOT more violent.
If I was handling these things in such a way as to have the possibility of dropping one, Id insist on body armor and face protection. And if it involves a picture tube, I insist on competent trained professionals for service.
(From: Matthias Meerwein (Matthias.h.de).)
They ARE quite safe. Ive got several TVs and computer monitors in for repair that had been dropped. None of them had an imploded CRT. The damage encountered ranged from:Broken circuit boards, often around the flyback transformer (the most heavy weight part on the board) – This is quite easily repairable.
Shadow mask inside the tube knocked out of position (mostly in trinitron tubes due to their heavy aperture grille construction) – this renders the tube (and thus usually the set) a dumpster candidate.
Neck of tube broken of (usually when the set hit the floor back end first) – obviously junk.Furthermore, I did some experimentation with junk sets:26 inch color TV with back panel removed placed face-down under a bridge. Dropped a ~10 pound brick from top of the bride (about 10 ft high) into the glass funnel of the tube. Result: Funnel of tube shattered, faceplate intact. All glass shards (most of them rather large) were lying inside the sets cabinet – no flying glass.
14 inch B/W computer monitor tube dropped from the second story onto concrete floor, hitting the ground faceplate-first. Result: tube shattered into thousands of small glass particles (the largest ones were about one inch in size), but all debris was located on one heap – none of them traveled farther than about three feet.Conclusion: According to my experience, spectacular picture tube implosions are something like cars in movies that explode upon roll-over, hitting a tree or driving down the cliffs: an urban legend.
With todays tubes, thats more or less true (although walking through a picture tube plant can be instructive as you hear the exploding tubes). With older tubes it was a hazard. With pre-1960 tubes it was a big one. My old boss in the TV service, who I trusted not to exaggerate about such things, told me stories of setting a picture tube near a second-floor window, having them fall to the sidewalk and literally blow a hole in the sidewalk. I can tell you factually and first-person that although he took few precautions with other things, when he had to pop a picture tube in the dumpster he never ever ever did it without safety glasses, a shield and a six-foot piece of heavy pipe. (I stopped working there around 1973.)A really deep long scratch or gouge on the CRT face should be considered a serious safety hazard as it may reduce the structural integrity and increase the risk of implosion. However, you would likely need a hammer and chissel or diamond tipped tool to make scratches that deep. It is very unlikely that such scratches could come from any reasonable normal use. Dropping it from a cliff, deliberate use of a glass cutter, the use of a really really BIG hammer, or 12 gauge shotgun, might perhaps be sufficient.
This is more of a concern for modern CRTs that usually have integral implosion protection – that steel rimband around the outside near the front. Older CRTs used either (1) a separate safety shield – that laminated glass plate in front of your grandmoms TV – or (2) a second contoured glass panel bonded to the actual tube face. In both of these cases, the second panel is protective and cosmetic but is not part of the structure of the CRT. Therefore, any damage to it does not significantly compromise the tube. In the case of modern CRTs, the steel band in conjunction with the basic tube envelope is used to maintain the integrity of the overall CRT. In addition should implosion occur as a result of catastrophic damage, the rimband will reduce the range and velocity of flying debris.
Also see the section:CRT Implosion Risk?.
BTW, scratches in the CRT have absolutely no effect on X-ray emission. X-rays are blocked long before they come anywhere near the surface and glass has very little effect on their direction. Any scratch deep enough to have any detectable effect on X-ray emission (actually, it would need to be an inch deep gouge) would have caused the tube to implode.I dont know what the law says, but for safety, here is my recommendation:
Treat the CRT with respect – the implosion hazard should not be minimized. A large CRT will have over 10 tons of air pressure attempting to crush it. Wear eye protection whenever dealing with the CRT. Handle the CRT by the front – not the neck or thin funnel shaped envelope. Dont just toss it in the garbage – it is a significant hazard. The vacuum can be safely released (Let out? Sucked in? What does one do with an unwanted vacuum?) without spectacular effects by breaking the glass seal in the center of the CRT socket (may be hidden by the indexing plastic of the socket). Cover the entire CRT with a heavy blanket when doing this for additional protection. Once the vacuum is gone, it is just a big glass bottle though there may be some moderately hazardous materials in the phosphor coatings and of course, the glass and shadow mask will have many sharp edges if it is broken.
In addition, there could be a nice surprise awaiting anyone disconnecting the high voltage wire – that CRT capacitance can hold a charge for quite a while. Since it is being scrapped, a screwdriver under the suction cap HV connector should suffice.
The main power supply filter caps should have discharged on their own after any reasonable length of time (measured in terms of minutes, not days or years).
Of course around here, TVs and monitors (well, wishful thinking as I have yet to see a decent monitor on the curb) are just tossed intact which is fortunate for scavengers like me who would not be happy at all with pre-safed equipment of this type!
We have a procedure for disposing of used CRTs. The vacuum must be broken to avoid future implosion, like when it will be crushed by the dumpster truck press. Thats NOT funny! One method is to punch or drill a small hole in the anode contact, which is made of a soft metal. But take care of the electrical discharge of the aquadag capacitance first!!!
The other method is to break the stem in the centre of the socket pins. This is the stem through which the tube was pumped empty during manufacturing. It breaks off easily (after you have removed the plastic part around the pins).
You want to avoid making too large holes, like for example from chopping off the entire neck in one blow with a hammer.
Back toCRT FAQ Table of Contents.General CRT Construction and CharacteristicsCurrently, most TVs and computer monitors are still based on the Cathode Ray Tube (CRT) as the display device. However, many hand-held TVs, portable 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, color, and brightness 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 do to manufacturing imperfections.
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.
At least one company is about to introduce a 42 inch diagonal HDTV format flat plasma panel multisystem color TV/monitor which will accept input from almost any video or computer source. Its price at introduction will be more than that of a typical new automobile – about $15,000! 🙂 Thus, at first, such sets will find their way into business conference rooms and mansions rather than your home theater but prices will drop over time.
Projection – large screen – TVs and monitors, on the othe