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(From: David Moisan (dmoisan@shore.net).)
I've seen a few such pictures and I was fortunate enough to find a book on color CRTs that explained quite a few things:
If you are lucky enough to see "The Secret Life of Machines" on The Learning Channel (or was, last time I saw it), there's an episode on the secret life of the TV. It's excellent! The creator and presenter, Tim Hunkin, has a weird sense of humor but he's 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; that's 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!)
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.
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!
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.
Reasons to use a resistor and not a screwdriver to discharge capacitors:
BTW, don't wash your CRTs even if the Maid complains about the filth until you have confirmed that your 'Dag isn't water soluble (maybe that's 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!).
(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.
This is probably not a problem on small CRTs but for large ones with high high voltages and high deflection angles where the glass of the neck is very thin to allow for maximum deflection sensitivity, the potential does exist for arcing through the glass to the yoke to occur, destroying the CRT.
There is really no way to know which models will self destruct but it should be possible to avoid such a disaster by providing a temporary return path to the DAG ground of the CRT (NOT SIGNAL GROUND!!) via the focus or G2 pins preferably through a high value high voltage rated resistor just in case one of these is shorted.
This probably applies mostly to large direct-view TVs since they use high deflection angle CRTs but it won't hurt to take appropriate precautions with video and computer monitors as well.
(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)
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 won't 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 CRT's with smaller rifles, with little or no success.
Does this comfort you? Get out the shotgun and have a go at it!
Or, perhaps, the following:
(From: Ren Tescher (ren@rap.ucar.edu).)
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.
"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 isn't 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 wouldn't 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. :-)
(From: Dan Evens (dan.evens@hydro.on.ca).)
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. (Let's 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, I'd 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.Meerwein@rt.bosch.de).)
They ARE quite safe. I've 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:
(From: Clifton T. Sharp Jr. (agent150@spambusters.ml.org).)
With today's tubes, that's 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.)
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 grandmom's 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.
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. Don't 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!
(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)
We have a procedure for disposing of used CRT's. The vacuum must be broken to avoid future implosion, like when it will be crushed by the dumpster truck press. That's 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.
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 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 a reflective medium is used in this device, the light source can be as bright as needed. Commercial products based on the DMD are beginning to appear.
"Could someone please help to elucidate the comparative advantages of each technology? I know how they work but do not know which is advantageous and why."(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)
Trinitron is Sony technology. The shadow mask (called the aperture grille) consists of vertical wires under tension. The mask is always straight in the vertical direction and curved in the horizontal direction, thus the shape is a cylinder. The tube surface is also cylindrical, which causes some strange effects, particularly funny mirror reflections of yourself. Because the wires are under a lot of tension, the internal tube structure must be very strong and thus relatively heavy. Because the glass surface is cylindrical instead of spherical, the glass must be thicker and heavier too, to withstand atmospheric pressure.
Heavier always equates to more expensive!
The electron gun construction is also different: there are still 3 guns (not one as some may thing but the 3 guns share one main lens. (The assembly of focusing grids is called a lens, in analogy to the optical principle.) There are still 3 cathodes and 3 G1s, as usual. The large diameter lens has the advantage of less spherical aberration (and thus a sharper spot) but the disadvantage of large physical length which means a deeper cabinet.
In the deflection coil design another compromise is found between spot quality, purity and convergence. As a result horizontal convergence must be helped by an auxiliary dynamic convergence waveform (on an extra convergence coil?). This adds to cost and can occasionally give an interesting failure of the horizontal convergence.
The best non-Trinitron (or clone) CRTs use a conventional shadow mask made of Invar - originally Matsushita technology; Philips uses it too. The shadow mask is of the standard shape (spherical metal plate with holes in it) but it is made of a special alloy with a 7 times lower coefficient of thermal expansion than regular iron. This allows a brighter picture with less purity errors.
The problem with regular shadow masks is 'doming'. Due to the inherent principle of shadow masks, 2/3 or more of all beam energy is dissipated in the mask. Where static bright objects are displayed, it heats up several hundred degrees. This causes thermal expansion, with local warping of the mask. The holes in the mask move to a different place and the projections of the electron beams will land on the wrong colours: purity errors. The use of invar allows about 3 times more beam current for the same purity errors. See the section: What is Doming?.
Combating purity errors is a necessity due to 2 trends:
To summarize: Trinitron monitors are probably heavier, larger, more expensive, maybe better on purity, and maybe better on focus than other monitors, with or without invar shadow masks. There are excellent monitors other than Trinitron too... I suppose the Coke-Pepsi comparison is true.
All the color CRTs found in TVs and computer and video monitors 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 (doming) of the shadow mask.
However, there are some disadvantages of the aperture grille design:
The following is a greatly simplified description of the general process of color (shadow or slot mask) CRT construction. Trinitrons should be basically similar.
The screen and envelope glass pieces are molded separately and then glued (Epoxied?) together as one of the last steps of assembly prior the baking and evacuation. (You will note this seam if you examine the envelope of a color CRT near the front.)
The shadow mask is manufactured through a photo etching process. No, there are no workers responsible for punching all those holes! Since a position error of even a tiny fraction of a mm would result in purity errors, each shadow mask is unique for its faceplate. They are not interchangeable. To facilitate the following steps, it can easily be mounted and removed (essentially clicked in place) during tube production. Registration pins assure precise alignment.
At least one manufacturer adds some steps for the Superbright tubes. They put 3 different colour filters between the glass and the phosphor. In terms of contrast that tube is a definite killer.
Using the shadow mask repeatedly in this manner guarantees close registration. How else would you lay down a million individual dots in exactly the right place - paint by numbers? :-).
Then, an aluminum overcoat is deposited over the phosphor/black matrix. This has several functions:
The tube is evacuated through the thin stem that is located in the middle of the socket. That takes several hours at the vacuum pumps. The stem is then sealed by heating and melting.
The getter - part of the electron gun assembly - is then 'activated' via induction heating from a coil external to the next of the CRT. This vaporizes and deposits a highly active metal on the interior of the glass of the neck. The getter material adsorbs much of any remaining gas molecules left over from the evacuation of the tube. The getter material is normally silvery - if it changes to red or milky white, the tube is probably gassy or up to air.
When the tube is ready it is matched with a deflection coil that provides optimum purity. It takes some ingenuity to get a good match between using a light for exposure which matches the behaviour of the future electron optical system, in order to get good purity.
Amazingly, this basic process has not changed in any fundamental way since the invention of the shadow mask CRT!
However, Computer Aided Design (CAD) has had a major impact on the design of the electron optics. The working of the electron gun and deflection system is now much more predictable thanks to advanced computer simulation. This has reduced the number of active correction circuits for focus, geometry and convergence to almost zero.
(From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)
They still need to match the finished tube with a deflection coil that will give adequate purity performance and then they need to fiddle with magnets (multipole rings around the neck and sometimes other magnets all over the cone) to improve it further. And even then many tubes need active correction for convergence and/or geometry.
Only after all that correction can you call the yield high. (But you should see their scrap yard, good thing that glass recycles well...)
For monochrome monitors and B/W TVs, this will result only in a slight shift in position or rotation of the picture depending on the orientation of the CRT with respect to the earth's magnetic field. For the most part such effects will not be significant enough to be objectionable.
However, for high resolution color monitors and even some color TVs, the result of transporting the unit from the hemisphere from which it was manufactured or set up to a location in the opposite hemisphere may be uncorrectable purity problems or excessive sensitivity to local magnetic fields.
Note that is it quite possible that you will never encounter any of these problems. The extent to which your particular monitor or TV is affected depends on many factors - many of which you have no control over.
(From: Bob Myers (myers@fc.hp.com).)
For many monitors - especially the larger sizes, such as 21" - there is a subtle difference in the CRT itself which may mean that a unit with the wrong tube could NOT be adjusted to be within specifications when used in the 'wrong' hemisphere.
(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)
There are two types of adjustments:
Depending on the sophistication of the circuitry in the (television or monitor) set, the setmaker can adjust geometry and sometimes convergence (if there is a set of convergence coils present). If there is a rotation coil present then this may also improve the landing a bit.
In the 'digital monitors' there are flexible waveform generators to adjust the corrections. There may be further adjustments possible for the uniformity of the colour point and brightness. This gives a place-dependent modulation of the 3 beam currents, it does nothing to improve the landing.
The most expensive monitors (large screen, fine phosphor pitch, very critical on landing) may have active magnetic field compensation in all 3 directions with electronic magnetic field sensors for automatic adjustment. These monitors should be mostly insensitive to the earth magnetic field. (This technology was originally invented for the use of CRT displays on board of jet fighter planes, which tend to turn relative to the earth...)
All other monitors will degrade picture quality when the degaussing is not able to completely compensate for the earth magnetic field. With a tube built for the wrong hemisphere it is possible that the effect of the vertical component of the earth magnetic field will give a residual landing error. This can not be corrected by turning any of the available adjustments, digital or not. Re-alignment might become a very costly job.
CRT Manufacturers actually make different versions of their tubes for TV's for the northern and southern hemisphere, and sometimes a 3rd neutral type. These are so-to-say precorrected for the uncompensated field. (Note that the term 'tube' here includes much of the convergence hardware as well - not just what is inside the glass.)
I remember when we exported projection televisions from Belgium to Australia, a couple of years ago. They all had to be opened on arrival to re-adjust the rotation settings on the convergence panel, due to the different magnetic field in Australia. Projection TV's don't have degaussing (there is nothing to degauss), and the customer can only adjust red and blue shift, not rotation.
Our CRT application group has a "magnetic cage". This is a wooden cube (approx. 2 meter long sides) with copper coils around each of the 6 surfaces. With this they can simulate the earth magnetic field for every place on earth (as indicated on a map on the wall).
During production and adjustment of the tube, the beam landing is optimized for the field condition in which it will be used later. There may be different tube specifications for north, south and equator ("neutral"). If you choose to use it in different conditions then the landing reserve will be diminished and you will suffer sooner from colour purity errors. I'm not so sure if the convergence would be a primary problem, maybe yes.
With a dotted shadow mask, also the horizontal component of the field matters, which is bad because it also depends on which direction you orient the display. This too will eat away from your landing reserve. How critical it all is depends on tube size (bigger is worse) and on dot pitch (smaller is worse). Workstation monitors are most critical.
Using a Helmholtz cage you can test or optimize for a particular place on earth. The most expensive monitors come with their own built-in Helmholtz cage and magnetic sensors to always create a field-free space.
Another interesting bit of trivia:
B&O (Bang & Olufsen of Danmark) use Philips picture tubes in their beautifully designed cabinets. In order to facilitate a more narrow styling they decided to mount the tube upside-down, so they don't need safety clearance for the EHT on top. As a consequence they needed a southern-hemisphere tube for the northern hemisphere! So here is a hint for a solution to you all...
(From the editor).
In light of the above discussion, the following makes perfect sense:
(From: Nigel Morgan (nigel@wycombe.demon.co.uk).
When I was in the TV trade some 20 years ago, I was introduced to a model with a PYE badge on which differed in one significant detail: on all TV sets I'd seen to that date the tube had the blue gun uppermost and the EHT connector at the top of the tube. Thorn TV sets mounted the tube upside-down for some reason so that the EHT connector was at the bottom along with the blue gun, but these PYE sets had the blue gun at the bottom, but the EHT connector was at the top! When I asked about this, I was told that the tubes used in the PYE sets were 'Southern Hemisphere tubes. I never could decide whether this was genuine or BS!
(From: Terry DeWick (dewickt@esper.com).)
The magnetic field for South America is about 0 to -100 mG while the U.S. runs 400 to 500 mG (milli Gauss). For a CRT to set up correctly the gun is offset 1 to 1.5 mm left of center for the 500mG field and 1 mm to the right for 0 mG this way the purity will be centered and the yoke tilt will be centered making setup easy during production. A North American CRT can be set up in South America but there is a chance that it will not set up well with excessive purity correction and or wedging set to the extremes.
You can recognize a Trinitron tube by the fact that the picture is made up of fine vertical stripes of red, green, and blue rather than dots or slots. The shadow mask in all other kinds of common CRTs are made up of either dots (nearly all good non-Trinitron computer monitors) or slots (many television sets). The Trinitron equivalent is called an aperture grill and is made of around a thousand vertical wires under tension a fraction of an inch behind the glass faceplate with its phosphor stripes.
Several photos of a disemboweled Trinitron aperture grille can be found at James Sweet's Sony/Trinitron Directory along with some screen shots showing the symptoms resulting from a monitor falling on its face. :(
Since the aperture grill wires run the full height of the tube, there are 1 or 2 stabilizing wires to minimize vibration and distortion of the aperture grill. These may be seen by looking closely 1/3 and/or 2/3 of the way down the tube. The larger size tubes will have 2 while those under 17 inch (I think) will only have a single wire. Many have complained about these or asked if they are defects - no they are apparently needed. You can be sure that Sony would have eliminated them if it were possible.
Another noticeable characteristic of Trinitrons is the nearly cylindrical faceplate. The radius in the vertical direction is very large compared to the horizontal. This is both a requirement and a feature. Since the aperture grill wires are under tension, they cannot follow the curve of the glass as a normal shadow mask may. Therefore, the glass must be flat or nearly flat in the vertical direction. As a selling point, this is also an attractive shape.
In the final analysis, the ultimate image quality on a monitor depends as much on other factors as on the CRT. There are many fine monitors that do not use Trinitrons as well as many not-so-great monitors which do use Trinitron tubes.
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.
Mitsubishi makes the Diamondtron under license from Sony - the subtle differences (according to Mitsubishi) are improvements in the electron gun design for spot uniformity over the CRT face. Also, for the time being, Mitsubishi has tried to introduce Diamondtron tubes in sizes which are not available as Trinitrons - to keep from directly competing, and (ostensibly) to address niches which other sizes can't address.
In order to properly evaluate a monitor, one must consider more than the tube alone - as many readers know, Trinitrons are finding their way into various manufacturer's sets, but they don't all perform the same. In todays market, it's quite possible to find a dot mask design which performs as well as (or better in some cases) the aperture grill design - IMHO every critical monitor purchase should be made by personally examining the monitor to be bought, under the intended application(s).
(BTW, all color tubes use 3 guns, including the Trinitron. Sony used to talk about a "unitized gun", but that only refers to the cathode structure. It's classical use of a misleading term to gain market awareness (looks like it works).)
(From: Someone who wishes to remain anonymous.)
I have found other differences between the Trinitron and Diamondtron tubes. Most noticeable is the grill pitch. The 21" Sony GDM-F520 is 0.22 mm. The 22" Mitsubishi (Cornerstone P1750) is 0.25 mm. For high resolution screens, this makes a difference.
I have also noticed that in a room full of Dell Trinitron monitors, no two monitors have the same color. This is not just a setup issue, the actual tubes have different colors when they are off. The darkness of the black changes.
My gut feeling is that the Dells use a Mitsubishi tube, and that the quality control is not up to Sony's. It is just a feeling, I have not done any research on this.
From what little I know, if you want the very best, you will have to pay for it, (or you get what you pay for).
GE's first set was a 10 or 11 inch " "PortaColor" TV which, to the best of my memory, was introduced in the mid-60s. It was a tube chassis that made use of space saving Compactron multifunction tubes. A solid state version followed some years later I believe. If I remember correctly, the color circuit used a novel method to generate the local 3.58 MHz color signal: it used the recovered color burst to 'ring' a series crystal to produce a continuous carrier. I remember reading about all this in one of the late great "Radio-Electronics" Annual Color TV issues that I looked forward to each year back then as color TVs were dynamically evolving from many US companies.
The GE CRT did indeed use 3 in-line guns aimed at a conventional shadow mask triad phosphor screen. This simplified convergence and the CRT neck components needed. Sony uses one gun with a large common cathode to emit 3 electron beams which focus through a single large electrostatic 'lens' instead of 3 smaller ones like the GE and others used.
One last stroll down memory lane: Does anyone remember the forerunner of the Sony Trinitron? It began as the "Lawrence Tube" (named after its U.S. inventor Dr. Lawrence) then was demonstrated as the "Chromatron" (I think Paramount had some stake in it then). I don't know how the concept became Sony's property so if anyone can corroborate or correct any of my recollections, I would enjoy hearing about it. Thanks.
(From: Andy Cuffe (baltimora@psu.edu).)
I read about Sony's development of the Trinitron. Apparently Sony actually manufactured a 17" TV with a Chromatron CRT in the early 60's. It was only sold in Japan and used a very unreliable tube chassis. According to the book they all ended up being returned and Sony lost a lot of money on it. Later Sony took ideas from the GE in-line tube and the Cromatron to invent the Trinitron. They used the 3 in-line cathodes of the GE tube with the vertical phosphor stripe screen of the chromatron. The common focusing lens was a way to stay as close as possible to the single electron gun design of the chromatron. The tone of the book suggested that Sony bet the whole company on the success of the Trinitron. Apparently they were very close to licensing the shadow mask design from RCA because of the amount of money they were losing by developing their own color CRT. If anyone is interested I think the title of the book was "Sony Vision". It also had chapters on the Betamax and the development of the first solid state TV.
This is simple geometry - similar triangles (at least for a good approximation).
It is easy to do the calculations based on the distance between the electron guns and the horizontal stripe pitch of the CRT (assuming slot mask or Trinitron - just a little more trouble for dot mask to convert the dot pitch).
Dot pitch: 0.3 mm
| |
___________________________________ Phosphor screen
G B R G B R G B R G B R G B ^
\|/ 15 mm
- ----- ----- ----- ----- --------- Shadow mask
/|\ ^
/ | \ |
/ | \ 350 mm
/ | \ |
/ | \ v
B-gun G-gun R-gun ---------------- Electron guns (center of deflection)
| |
| |
Gun pitch: 7 mm
(Cool diagram based on efforts of Jeroem Stessen.)
Be aware that both face-plate and shadow-mask are curved and that the radius of curvature is much larger than the distance to the guns. The screen is relatively flat. This too has consequences for the calculation. Oh, heck.
At the center of the screen, we have:
Distance between E-guns (R-G) Slot pitch (R-G)
----------------------------------------- = ------------------
Distance from deflection center to mask Mask to screen
For a typical 25 inch TV CRT with a .9 mm slot pitch (.3 mm between adjacent
stripes) and 7 mm between adjacent guns we have a ratio of about 23:1.
For a distance of 350 mm between the center of deflection and mask, this gives us about 15 mm (~.6 inches) between the mask and the screen.
The shadow mask is mounted in a diaphragm. The diaphragm is mounted to the inside of the tube with 4 metal springs. In the old days these were bimetal springs. They have an important role for colour purity: they allow the mask to move forward as it expands due to self-heating.
Remember: it must dissipate a lot of power and there is no cool air in there...
During production the mask is mounted and removed many times to allow for etching of the phosphors. A point light source is precisely positioned at the deflection center of each gun in-turn to expose the photoresist used in laying down the phosphor dots. (I know, you thought they were painted on one spot at a time! :-)
The mask is never fastened permanently, only clicked in to place just prior to having the envelope glued to the front assembly.
As no two masks are identical, each tube is always paired with its own mask.
(From: David Moisan (dmoisan@shore.net).)
From pictures I've seen, the best way to describe the shadow mask is that it is like a picture inside its frame: The glass face is the frame and the mask is the picture it holds, so to speak. The mask is carefully designed in a frame of its own, with spring clips around the edges, so that it won't distort under the heating it gets from the electron beams (not to mention during manufacturing). There's also a magnetic shield around the inside of the bell in some tubes.
The question often arises: Well, if magnetization and the need for degauss is a problem, why not make the shadow mask or aperture grille from something that is non-magnetic?
The shadow mask *must* be made of magnetic material! This may seem to be undesirable or counterintuitive but read on:
Together with the internal shielding hood it forms sort of a closed space in which it is attempted to achieve a field-free space. The purpose of degaussing is *not* to demagnetize the metal, but to create a magnetization that compensates for the earth's magnetic field. The *sum* of the two fields must be near zero! Degaussing coils create a strong alternating magnetic field that gradually decays to zero. The effect is that the present earth magnetic field is "frozen" into the magnetic shielding and the field inside the shielding will be (almost) zero. Non-zero field will cause colour purity errors.
Now you will understand why a CRT must be degaussed again after it has been moved relative to the earth's magnetic field. This will also explain why expensive computer monitors on a swivel pedestal have a manual degaussing button, you must press it every time after you have rotated the monitor.
The axial component of the magnetic field is harder to compensate by means of degaussing. Better compensation may be achieved by means of a "rotation coil" (around the neck or around the screen), this requires an adjustment that depends on local magnetic field. CRT's for moving vehicles (like military airplanes) may be equipped with 6 coils to achieve zero magnetic field in all directions. They use magnetic field sensors and active compensation, thus they don't need any degaussing function. This is too expensive for consumer equipment.
Nearly any color that we can perceive can be made from some combination of primary colors. There are two types - additive and subtractive.
RGB are primary additive colors - anything that emits light will use these.
The three types of cone (color) recepters in the retina of the human eye have peaks (roughly) sensitive to these primary colors.
Those red, yellow, and blue primaries you used to create your works of art should actually not have been red, yellow, blue but rather magenta, yellow, cyan - close but no cigar. Red, yellow, and blue are approximations good enough for basic painting or printing but are not capable of reproducing the widest range of colors.
CMY (cyan, magenta, yellow) are subtractive colors. Printing processes and color photography use these because layers of ink or dye absorb light. Basically, each of CMY removes a single color from (RGB).
Warning: A CRT that is supposed to have a rimband but where it is missing or damaged is a serious hazard since the possibility of implosion is greatly increased and the effects of such an implosion will be more severe. However, such a situation is virtually impossible to occur on its own since the rimband is part of the mounting bracket assembly. Don't be tempted to remove the rimband for any reason unless the vacuum has been let out (in, whatever one does with a vacuum) of the CRT! Spontaneous implosion is even possible. See below for an example.
In some cases, there will be a separate faceplate. Older TVs usually had either a totally separate laminated glass plate in front of the CRT or a contoured glass panel bonded (glued) to the CRT itself. Part of its purpose is protective. It would prevent damage to the CRT in the event of a blow from a thrown object like an ashtray or shoe! In addition, it would contain the debrie in the unlikely event of an implosion resulting from some really catastrophic event.
However, the separate or bonded glass plate can also be used for cosmetic purposes to:
I got my User ID from the metal band. :) Anyway, a friend of mine decided to cut the rimband off a picture tube. I wasn't there, he told me about it. This was a 25" RCA tube he wanted to fit into a Zenith TV (don't ask me why). What happened in the next few seconds after he cut the rimband, the picture tube imploded in his face, embedding the neck and yoke assembly in the ceiling, he came out with a cut about half an inch above his right eye that needed 6 stitches to close. Had that shard of glass been half an inch lower, he would be wearing an eye patch or have a glass eye for the rest of his life.
I told him what an idiot he was, he's lucky he didn't kill himself or blind himself, and also told him NEVER cut the rimband off a picture tube that has vacuum. I just wanted to add that!:)
"Is it really true that they put lead in the CRT glass for X-ray shielding? What is the transparent conductive coating on the front of the CRT made of?"(From: Bob Myers (myers@fc.hp.com).)
First - yes, the glass is leaded (or contains other "impurities") to reduce emissions. In short, it's not just straight sand. :-)
There are various proprietary formulas used to make the faceplate coating, which often acts both as a conductive layer to reduce low-frequency electric fields and as a glare-reduction layer, but one of the most popular materials for making a transparent conductive layer is indium-tin oxide, a.k.a. "ITO". Such transparent conductors are also used in LCDs and other flat-panel technologies - at least the top layer of electrodes (row or column lines) has to be transparent! As conductors go, these things aren't THAT conductive - the age of see-through power lines or Star Trek's "transparent aluminum" is not upon us (and for certain theoretical reasons CAN'T be) - but they get the job done.
Compensation for the geometry and brightness problems becomes much more challenging and it's never perfect. Even a well adjusted CRT will often have a very detectable, if not obvious, variation in brightness from center to edges and corners. Scan linearity and pincushion correction require most complex and carefully adjusted circuits. The thicker faceplate means a heavier CRT and monitor.
The net effect is that for a given screen size, cost will be greater. At a normal viewing distance, the perceived advantages may be minimal. Some people may find (after having gotten used to a moderately spherical CRT) that they actually like a flat one less especially if the deficiencies are easily seen. Note that Sony Trinitron (and clone) CRTs are nearly flat in the vertical direction and curved in the horizontal direction. To get used to this geometry may take some time as well.
The CRT is primarily responsible for the latter two.
The focus or sharpness of the spot or spots that scan across the screen is a function of the design of the electron gun(s) in the CRT and the values of the various voltages which drive them. Focus may be adjustmented but excellent focus everywhere on the screen is generally not possible.
Sharp focus is a difficult objective - the negatively charged electrons repel each other and provide an inherent defocusing action. However, increasingly sharp focus would not be of value beyond a certain point as the ultimate resolution of a color CRT is limited by the spacing - the pitch - of the color phosphor elements. (For monochrome displays and black-and-white TVs, CRT resolution is limited primarily by the electron beam focus.)
One of three approaches are used to ensure that only the proper electron beam strikes each color phosphor. All perform the same function:
Dot pitches as small as .22 mm are found in high resolution CRTs. Very inexpensive 14" monitors - often bundled with a 'low ball' PC system - may have a dot pitch as poor as .39 mm. This is useless for any resolution greater than VGA. Common SVGA monitors use a typical dot pitch of .28 mm. TVs due to their lower resolution have pitches (depending on screen size) as high as .75 mm - or more.
Obviously, with smaller screens and higher desired video source resolutions, CRT pitch becomes increasingly important. However, it isn't a simple relationship like the size of a pixel should be larger than the size of a dot triad or triple, for example. Focus is important. All other factors being equal, a smaller pitch is generally preferred and you will likely be disappointed if the pitch is larger than a pixel. As the pixel size approaches the phosphor triad or triple size, Moire becomes more likely. However, the only truly reliable way to determine whether Moire will be a problem with your monitor is to test it at the resolutions you intend to use.
(From: Bob Niland (rjn@csn.net).)
Dot pitch is the major component in the actual resolution of the monitor. Most monitor vendors quote the highest resolution signal their monitor will sync to irrespective of whether or not the tube can resolve it. Indeed, it often cannot resolve the highest (and even second highest) claimed display resolution.
(From: Bob Myers (myers@fc.hp.com).)
Very true. On the other hand, things may not be quite as bad as what the numbers appear to say, sometimes.
(From: Bob Niland (rjn@csn.net).)
It's no accident that monitor size is specified in inches, and dot pitch in mm. The vendors don't want to make it easy for you to know what the geometry of their phosphor triads actually is, i.e. how many RGB dot triplets there are across and down the screen."
(From: Bob Myers (myers@fc.hp.com).)
Well, I wouldn't want to accuse the tube industry of deception. Expressing diagonal sizes in inches comes from long-standing tradition. Expressing pitch in millimeters is actually a relatively new practice in comparison, and isn't too unusual when you realize that most tube manufacturers - esp. those in the Far East - actually spec their tube diagonals in metric terms. For instance, Matsushita (Panasonic) has listed their "15 inch visual" color CRTs as "420xxxx" models, 420 being the overall diagonal in mm (16.54")
(From: Bob Niland (rjn@csn.net).)
Here's how to figure it out. You need first to know:
Trinitron (aperture grille) tubes will never have the pitch specified as a diagonal measurement, since they have vertical stripes of phosphor. Conventional (flat-square) models will, and probably the safest conversion between diagonal and horizontal for these is to mlutiply by the cosine of 30 degrees (0.866), unless you know for sure the angle to horizontal at which the diagonal measurement was made. (It varies for different tube designs.) See the section: How to Compute Effective Dot Pitch.
(From: Bob Niland (rjn@csn.net).)
This is the Active Picture Horizontal size (APH) in inches.
This is the APH in mm (APHmm).
This is the useful horizontal resolution of the monitor.
Notice that this number probably does not precisely match any common (640, 800, 1024, 1152, 1280 or 1600) resolution in use, and that it is probably *less* than what the vendor claimed.
(From: Bob Myers (myers@fc.hp.com).)
Here is where some words of explanation are in order.
What many people fail to realize is that the phosphor triads of the screen *do not* correspond to pixels in the image; they are not kept in alignment with the image pixels/lines/whatever, nor is there are reason for them to be. The phosphor dot pitch IS a limiting factor in resolution, but we need to look a little further to determine whether or not a given tube will be usable for a given format (what most people mistakenly call a "resolution".)
The true resolution capabilities of a CRT are limited primarily by the dot pitch AND the spot size. For practically all CRTs and monitors in the PC market, the spot size is considerably larger than the dot pitch - up to 2X or so at the corners, if the tube is at or near its specification limits. This doesn't necessarily cause a problem with the image quality, however, since you aren't really resolving individual "pixels" in any case - what you need to resolve are the *differences* between adjacent pixels, or pixel/line pairs. And, oddly enough, it doesn't take a dot pitch of equal or greater size than a logical pixel to do this to most people's satisfaction. In fact, display types sometimes talk about a 'Resolution/Addressibility Ratio', or RAR, which is in effect the ratio of the actual size of a spot on the display to the size of a "logical" pixel in the image. And for best perceived appearance, this is generally going to be GREATER than 1:1 - say, 1.5:1 or even 2:1. (Too high, and the image is blurred; but too low, and the image takes on a grainy appearance that most people find objectionable.)
Bob is absolutely correct in stating that most displays, when run at the highest support addressibility or format (or, if you insist, "resolution") wind up with the "pixel size" being smaller than the dot pitch. But what is also correct, if somewhat counterintuitive, is that this is OK, and can still result in an image that will be acceptable (and even perceived as 'sharp') to the user.
You can certainly exceed the resolution capabilities of a tube and/or monitor (monitors differ from simple tubes by also having a video amp to worry about!). For instance, you probably won't be really happy with 1600 x 1200 on a 17" 0.28 mm CRT. But 1280 x 1024 on an 0.31 mm 20-21" tube can look very good, even though the numbers don't appear to work out.
(From: Bob Niland (rjn@csn.net).)
While not stated above, I would speculate that this is due to various human visual system factors, particularly that humans have more visual acuity in luminance (B&W) than in chrominance (color). If a CRT can actually illuminate less than a full phosphor triad, its luminance resolution can exceed the dot pitch. There will be some color fringing, but the eye may not notice.
(From: Bob Myers (myers@fc.hp.com).)
That's a good bit of it. Whether or not you're going to be satisfied with a given dot pitch versus addressibility ("resolution") basically has to do with what you think "resolve" means.
The fact that we don't generally have the same spatial acuity for color - in other words, you won't really see small details based on differences in color alone, there has to be a difference in brightness - is a big part of this. And you will be able to see such variations acceptably even when the size of the logical pixel is somewhat under the dot pitch size. When this occurs, you don't get constant color pixels - you don't even get constant *luminance* pixels - but you do perceive acceptable levels of detail to call the image 'sharp'.
"I have 2 identical monitors. One is razor sharp from edge to edge. The other is blurred at the corners- not from convergence problems, but just plain out of focus. In this monitor, the focus adjustment on the flyback can improve the focus at the edges, but then the center of the screen becomes worse..My question is : Is this a problem in the electronics and presumably a fixable flaw or is it caused by variance in the picture tube itself and not correctable ? Or is it some other issue?"(From: Bob Myers (myers@fc.hp.com).)
The adjustment on the flyback sets the "static" focus voltage, which is a DC voltage applied to the focus electrode in the CRT. However, a single fixed focus voltage will not give you the best focus across the whole CRT screen, for the simple reason that the distance from the gun to the screen is different at the screen center than it is in the corners. (The beam SHAPE is basically different in the corners, too, since the beam strikes the screen at an angle there, but that's another story.) To compensate for this, most monitors include at least some form of "dynamic" focus, which varies the focus voltage as the image is scanned. The controls for the dynamic focus adjustment will be located elsewhere in the monitor, and will probably have at LEAST three adjustments which may to some degree interact with one another. Your best bet, short of having a service tech adjust it for you, would be to get the service manual for the unit in question.
It is also possible that the dynamic focus circuitry has failed, leaving only the static focus adjust.
As always, DO NOT attempt any servicing of a CRT display unless you are familiar with the correct procedures for SAFELY working on high-voltage equipment. The voltages in even the smallest CRT monitor can be lethal.
The usual arrangement of phoshpor dots on the screen of a dot mask type CRT is shown below:
B R G B R G B R G B R G B R R --- G --- B --- R
R G B R G B R G B R G B R G B Magnified -> / |
B R G B R G B R G B R G B R / |
R G B R G B R G B R G B R G B G --- B -+- R --- G --- B
(Portions from: Jac Jamar (jamar@comp.snads.philips.nl).)
For a dot mask type CRT, normally the nominal pitch (also called the Hexagonal Pitch or HexP) is defined as the distance between one phosphor dot to the next same colored one in the 'hexagonal' direction (i.e. in the direction 30 degrees above the horizontal).
The calculations below follow from simple geometry:
SCHDP = HexP * sqrt(3) (sqrt(3) = 1.732 or 2 * cos(30 degrees))
This is the distance between one phosphor dot and the next dot of the same
color on the same horizontal line.
In general, the dot/slot/line pitch of TV CRTs is very large compared to even mediocre computer monitors. Here are some typical values which I measured very precisely (!!) by putting a machinest's scale against the screen. These are all slot mask type CRTs:
(From: Bob Myers (myers@fc.hp.com).)
The physical shape of the tubes themselves came through this evolution, but the aspect ratio assumed for the original transmission format specs WAS 4:3, as driven by Hollywood. Where did you think the shape of the masks came from?
The desired 4:3 aspect ratio standard was known right from the start, and the early TV designers DID realize that the use of round tubes to display this was a literal case of a "square peg in a round hole". Rectangular CRTs for TV use had been developed as early as 1939, with the first American rectangular tube to enter production in late 1949.
(See Peter Keller's very excellent "The Cathode Ray Tube: Technology, History, and Applications" for all the details.)
This is the maximum angle the beam makes with respect to the gun axis to fill the screen.
Apparently CRTs have made quite an increase lately. Years ago when I looked into it, CRTs were not much better than about 20:1. Now, folks are claiming well over 40:1.
One thing to watch, though. The phosphor has two decay curves, a rapid one followed by a slow one. A change in scene can lower contrast ratio of a bar chart that appears in a region that was a large white area.
(From: Steve Eckhardt (skeckhardt@mmm.com).)
This comment makes me curious about the claims made by manufacturers of video projectors. Visually, their images have lots of resolution but mediocre contrast at large scale. A video monitor looks a lot better in contrast. The manufacturers, however, claim contrast ratios of 100:1 or better.
Are the numbers simply marketing hyperbole or have I missed something interesting?
There are several methods for arriving at the advertised numbers for contrast. One is to simply advertise the number for the imager and ignore the degradation due to the rest of the system. Another is to measure the illuminance of a white screen compared to a black screen. The best way is to use the ANSI method and advertise ANSI contrast, which is the practice at 3M. We really do sell projectors that achieve 100:1 contrast when measured by the ANSI standard. This is, however, a relatively recent achievement. LCD projectors are improving at a rapid rate.
CRT projectors are an alternate technology that I know little about, but they have characteristics that allow them to produce very high localized contrast. This can make displays and projectors based on CRT's look superior to anything an LCD can produce.
(From: Don Stauffer (stauffer@htc.honeywell.com).)
One big problem with LCD displays, projection or otherwise, is view angle. In order to cut off the light completely, polarization needs to be controlled to a couple of degrees. The LCD works by affecting the rotation, so many degrees per unit distance through the crystal. But the total path through the crystal depends on view angle. So max contrast may be only over a small field angle. Now, games can be played with this in projection optics, but it is hard.
Advantages: simple (at least in priniple), doesn't care if conditions change (within specified field strength limits). Mu-metal can be very effective if used properly - but see below.
Disadvantages: expensive and often ugly. The cost of a complete functional but not aesthetic enclosure for use of a color monitor near an MRI scanner was about $2,000 a couple of years ago when we needed to provide this for one of our customers. Check out MuShield specifically under "Monitor Enclosures" if you're curious. Less EMF, Inc. sells Mu-metal foil by the foot but what they have listed is rather thin - I don't know how well it would work for CRT shielding.
Advantages: can be built inside the monitor using small coils in some cases.
Disadvantages: must be engineered for each situation. Change almsot anything and it will no longer be effective even if feedback is used. Complex in practice since interfering field geometry is often not well behaved.
Advantages: will reduce interference for all monitors in the vicinity.
Disadvantages: shielding location may not be readily accessible. Geometry offending device may not lend itself to a reasonable size or shape shield.
You can buy commercial Mu-metal screening cans and yes they are a complete enclosure, with small holes for the I/O wires.
Mu-metal is very expensive and not easy to work but will solder, especially with acid flux.
I suggest you try a dummy run first with some mild steel to get the design sorted and to test if it looks worth it.
You never know your luck, mild steel may do the job anyway and you may not want to deal with mu-metal (--- sam):
"Just got my 10' sheet of mu-metal delivered today. It came very well packaged sandwiched between two pieces of wood so that it would not bend during shipment."(From: James P. Meyer (jimbob@acpub.duke.edu).)
One of the reasons it came so well packaged was the fact that the magnetic properties are degraded if the material is bent or stressed in any way. Once you fabricate anything out of the mu-metal, you have to go through a high temperature annealing process to remove the stress and restore its full magnetic properties. If you don't do that, you are no better off with Mu-metal than you would be with tin-can stock.
Shielding of conventional speakers may also be possible:
(From: Lionel Wagner (ck508@freenet.carleton.ca).)
Put a Tin can over the magnet. This will reduce the external field by about 50%. If more shielding is desired, put additional cans over the first, in layers, like Russian dolls. (Note: a Tin can is actually made nearly entirely of steel - the term 'Tin' is historical. --- sam)
(From: Nicholas Bodley (nbodley@tiac.net).)
While both electrostatic and electromagnetic (E/M) fields can affect the paths of the electron beams in a CRT, only E/M fields are likely to be strong enough to be a problem.
Magnetic shields have existed for about a century at least. Some decades ago, a tradenamed alloy called Mu-Metal became famous, but it lost its effectiveness when bent or otherwise stressed. Restoring it to usefulness required hydrogen annealing, something rarely done in a home shop (maybe one or two in the USA).
More-recent alloys are much less fussy; tradenames are Netic and Co-Netic.
Magnetic shields don't block lines of force; they have high permeability, vastly more than air, and they guide the magnetism around what they are shielding; they make it bypass the protected items.
I have been around some shielded speakers recently, but never saw any disassembled. They looked conventional, must have had the "giant thick washer" (my term) magnet, and seemed to have a larger front polepiece than usual.
They had a shielding can around the magnet; there was a gap between the front edge of the can and the polepiece. I suspect that a second internal magnet was placed between the rear of the main magnet and the rear (bottom) of the can, so there would be minimal flux at the gap between the can and the front polepiece. Holding pieces of steel close to the gap between the can and the polepiece showed very little flux there.
Modern magnets are not easy to demagnetize, in general.
(From: Dave Roberts (dave@aasl.demon.co.uk).)
The *good* so-called magnetically screened speakers rely on two means of
controlling stray flux. The static field from the magnet on the speaker
(which would cause colour purity problems) is minimized by the design of the
magnet. This is often at the expense of gap field linearity, leading to
greater distortion - not that most users seem to worry about that...
The mains varying field is minimized by use of a toroidal mains transformer,
but the more recent mains powered speakers seem to be coming with *plug top*
PSUs, which take the problem further away.
Magnetic fields don't really 'pull' on charged particles, they result in a
force at right angles to the field lines with a direction dependent on the
charge (negative for electrons, positive for protons) and field (North or
South). The magnitude of the effect also depends on the energy/speed of the
particles and their mass.
For the case of a CRT:
The rotation knob or setting ion some TVs and monitors varies the current in a
coil wrapped around the CRT bell just beyond the neck which has its axis
parallel to the CRT's axis and adds a magnetic field to counteract the
component of the ambient field along that direction.
(From: Bob Myers (myers@fc.hp.com).)
No, it's not nonsense. The fields generated by the deflection coils, etc.,
ARE much greater in magnitude than the Earth's field, but they're AC fields.
The DC offset of these fields is relatively small, and the Earth's field (also
DC) IS sufficient to cause a visible shift in the position of the raster and
affect the beam landing, etc.. This is why, for instance, there ARE often
problems when trying to use a "Northern hemisphere" monitor in the Southern
hemisphere.
Having said that, however, this isn't really something the average user needs
to worry about. In the detailed specs for any monitor, there generally ARE a
set of specific ambient conditions under which certain performance specs are
intended to be checked. These usually include the ambient magnetic fields
(which also tells you what magnetic environment was used at the factory for
adjustment), and the orientation of the monitor within those fields. For the
vast majority of monitors, the specified ambient conditions simulate average
magnetic fields in the U.S. or Europe (which are very similar), and the
monitor is specified as facing east or west within those fields. Strictly
speaking, one has to establish those conditions (and so match the factory
adjustment environment) in order to evaluate the monitor for compliance with
these specifications.
Monitors are aligned in whatever field the manufacturer (or large OEM
customer) SPECIFIES. This USUALLY involves an east or west alignment, as this
results in no field component in the CRT's Z-axis (the axis "down the throat"
of the CRT, perpendicular to the center of the screen).
However, this doesn't necessarily mean that optimum performance at YOUR
location will be obtained with the unit facing east or west, as local fields
can vary considerably from the specified nominal field. The field identified
in the specs is just that - it is part of the conditions under which those
specifications are to be checked.
But the *specific* conditions for a given installation can vary considerably
from the nominal, and so the only advice I can give the individual user is
that if you're happy with the performance, don't worry about it. If you DO
think that a local DC field (the Earth's field or any other) is causing a
problem, THEN try to move or rotate the unit to see if you can find a better
orientation or location. Of course, *AC* fields, such as those from a nearby
power line or electrical equipment, are something else entirely.
They use magnetic field compensation for the professional types. This is too
expensive for us mortals, so we get a CRT that has been optimized for one field
condition only: North, South or Neutral. Not all displays are CRTs. LCDs for
instance are not sensitive to the earth magnetic field. And not all CRTs use
a shadow mask for colour selection. For instance, in Tektronix colour
oscilloscope they use a white CRT with a colour LCD shutter in front of it.
That too would not be affected too much by the earth magnetic field.
You see, plenty of ways out for aircraft, ships, and the Space Shuttle.
For this case, you might have some problems with:
Bang & Olufsen once made a compact style television where they wanted the
anode contact to be away from the top of the cabinet, so the back cover
could fit tighter. So they mounted the tube upside-down. Consequently they
had to order southern hemisphere tubes for a northern hemisphere TV set.
That works, of course.
Yes, it is true.
It makes a difference whether you talk about a front or rear projector. Front
projectors are expensive and critical enough that they will be converged after
installation, so that takes care of any convergence errors. Purity errors are
of course no issue with 3 separate CRTs...
Rear projectors are converged in the factory, the customer does only the
static convergence (4 pots) after he has decided which direction the set will
face. This takes care of problems due to the horizontal component of the earth
agnetic field.
In a rear projector the CRTs are mounted almost vertically. The vertical
component of the earth magnetic field causes a rotation error. Normally this
is not an issue because that component does not depend on the orientation of
the set and it is more or less constant over the entire continent.
It makes a biiiig difference though if you manufacture PTVs in Belgium and
then export them to Australia... That means opening the cabinet and
re-adjusting for rotation.
A front projector has its tubes mounted horizontally. The rotation error will
depend on the direction the set is facing. This is easily adjusted through the
convergence.
(From: Bob Myers (myers@fc.hp.com).)
It is extremely difficult for any CRT display to maintain perfect
brightness and color uniformity across the entire image. Just the geometry
of the thing - the change distance from the gun to the screen as the beam
is scanned, the changing spot size and shape, etc. - makes this nearly
impossible, and there can also be variations in the phosphor screen, the
thickness of the faceplate, etc.. Typical brightness-uniformity specs
are that the brightness won't drop to less than 70% or so of the center
value (usually the brightest spot on the screen).
On color tubes, the lack of perfect brightness uniformity is aggravated
by the lack of perfect *color* uniformity and purity. What appear to be
"dark spots" on a solid gray image may actually be beam mislanding (color
purity) problems, which may to some degree be remedied by degaussing
the monitor.
Again, *some* variation is normal; if you think you're seeing too much, you
can try degaussing the thing and seeing if that helps. If it doesn't,
then the question is whether or not the product meets its published specs,
and that is something you'll have to discuss with the manufacturer or
distributor.
What are the possible causes of doming? I have noticed that the magnitude of
the doming effect varies with TV orientation even after degaussing several
times at the new orientation. Does this help identify the cause of the
doming in my case?"
The problem with regular shadow masks is 'doming'. Due to the inherent
principle of shadow masks, 2/3 or more of all beam energy is dissipated
in the mask. Where static bright objects are displayed, it heats up several
hundred degrees. This causes thermal expansion, with local warping of the
mask. The holes in the mask move to a different place and the projections
of the electron beams will land on the wrong colours: purity errors.
The use of invar allows about 3 times more beam current for the same
purity errors.
Both local doming and magnetic fields compete for the remaining landing
reserve. Due to improper degaussing, the doming problem may be more visible.
And applying a tube designed for the wrong hemisphere may very well increase
the doming complaints. It is possible to deliberately offset the nominal
landing in order to get more doming reserve (the shift due to doming is
always to the outside of the tube). You would do this using spoiler magnets
put in the right places.
Permanently setting the contrast lower is not a real cure because the customer
might not like such a dark picture. A better picture tube (Invar shadow mask)
*is* a good cure (in most cases) but there is the cost price increase. (This
is mainly due to the fact that Invar metal is harder to etch.)
Also see the section: What is Doming?.
There can be several reasons why primary colours (especially red) may
look different between picture tube brands:
One cause of these lines is moire (interference patterns) between the
raster and the dot structure of the CRT. Ironically, the better the focus
on the tube, the worse this is likely to be. Trinitrons, which do not
have a vertical dot structure should be immune to interference of this sort
from the raster lines (but not from the horizontal pixel structure).
You can test for moire by slowly adjusting the vertical size. If it is moire,
you should see the pattern change in location and spatial frequency as slight
changes are made to size. Changes to vertical position will move the patterns
without altering their structure - but they will not remain locked to
the moving image.
If they are due to the raster line structure - your focus is too good - the
patterns will remain essentially fixed in position on the face of the CRT
for horizontal size and position adjustments - the patterns will remain
fixed under the changing image.
How to eliminate it? If moire is your problem, then there may be no easy
answer. For a given resolution and size, it will either be a problem or
not. You can try changing size and resolution - moire is a function
of geometry. Ironically, I have a monitor which is nicer in this respect
at 1024x768 interlaced than at 800x600 non-interlaced.
Some monitors have a 'Moire Reduction Mode' switch, control, or mode. This
may or may not be of help. One way to do this is - you guessed it - reduce
the sharpness of the beam spot and make the picture fuzzier! You might
find the cure worse than the disease.
Another cause of similar problems is bad video cable termination
creating reflections and ghosting which under certain conditions can be so
severe as to mimic Moire effects. This is unlikely to occur in all colors
with a VGA display since the termination is internal to the monitor and
individual resistors are used for each color (RGB).
I think it is ironic that some people will end up returning otherwise superb
monitors because of moire - when in many cases this is an indication of most
excellent focus - something many people strive for! You can always get rid of
it - the converse is not necessarily true!
The density of the holes in the shadow mask set an upper limit on the
resolution supported by that monitor. Lower resolutions work just fine;
there is no need to have the logical pixels in the image line up with the
physical holes in the mask (nor is there any mechanism to make this happen),
and so you can think of this as the "larger pixels" of the lower-res image
simply covering more than one hole or slot in the mask.
As the effective size of the pixels in the image approach the spacing of
the mask holes, individual pixels are no longer guaranteed to cover enough
phosphor dots on the screen to ensure that they are constant color or constant
luminance, but an image will still be displayed which ON AVERAGE (over a
reasonably large area) looks OK. Actually, the specified "top end"
format ("resolution") for most monitors usually is at or slightly beyond
this point - the effective pixel size is somewhat UNDER the dot pitch.
You can easily distinguish between video problems and CRT problems - missing
pixels due to the video source will move on the screen as you change raster
position. CRT defects will remain stationary relative to the screen and will
generally be much more sharply delineated as well.
There is a specification for the number and size of acceptable CRT blemishes
so you may have to whine a bit to convince the vendor to provide a replacement
monitor under warranty.
Is there any chance that someone waved a magnet hear the tube? Remove it
and/or move any items like monster speakers away from the set.
Was your kid experimenting with nuclear explosives - an EMP would magnetize
the CRT. Nearby lightning strikes may have a similar effect.
If demagnetizing does not help, then it is possible that something shifted
on the CRT - there are a variety of little magnets that are stuck on at the
time of manufacture to adjust purity. There are also service adjustments
but it is unlikely (though not impossible) that these would have shifted
suddenly. This may be a task for a service shop but you can try your
hand at it if you get the Sams' Photofact or service manual - don't attempt
purity adjustments without one.
If the monitor or TV was dropped, then the internal shadow mask of the CRT may
have become distorted or popped loose and you now have a hundred pound paper
weight. If the discoloration is slight, some carefully placed 'refrigerator'
magnets around the periphery of the tube might help. See the section:
Magnet Fix for Purity Problems - If Duct Tape Works, Use
It!.
It is even possible that this is a 'feature' complements of the manufacturer.
If certain components like transformers and loudspeakers are of inferior
design and/or are located too close to the CRT, they could have an effect
on purity. Even if you did not notice the problem when the set was new,
it might always have been marginal and now a discoloration is visible due
to slight changes or movement of components over time.
In any case, first, relocate those megablaster loudspeakers and that MRI
scanner with the superconducting magnets.
The addition of some moderate strength magnets carefully placed to reduce or
eliminate purity problems due to a distorted or dislocated shadow mask may be
enough to make the TV usable - if not perfect. The type of magnets you want
are sold as 'refrigerator magnets' and the like for sticking up notes on steel
surfaces. These will be made of ferrite material (without any steel) and will
be disks or rectangles. Experiment with placement using masking tape to hold
them in place temporarily. Degauss periodically to evaluate the status of
your efforts. Then, make the 'repair' permanent using duct tape or silicone
sealer or other household adhesive.
Depending on the severity of the purity problem, you may need quite a few
magnets! However, don't get carried away and use BIG speaker or magnetron
magnets - you will make the problems worse.
Also note that unless the magnets are placed near the front of the CRT, very
significant geometric distortion of the picture will occur - which may be a
cure worse than the disease.
WARNING: Don't get carried away while positioning the magnets - you will be
near some pretty nasty voltages!
(From: Mr. Caldwell (jcaldwel@iquest.net).)
I ended up with the old 'stuck on a desert island trick':
I duck taped 2 Radio Shack magnets on the case, in such a way
as to pull the beam back.!!!!
A $2 solution to a $200 problem. My friend is happy as heck.
RCA sells magnets to correct corner convergence, they are shaped like chevrons
and you stick them in the 'right' spot on the rear of the CRT.
(From: David Kuhajda (dkuhajda@mail.locl.net).)
A 1 degree tilt given the effect of the earth's magnetic field is well within
tolerance for a 27" TV set. The larger the picture tube, the more the tilt
effect of the earth's magnetic field is noticeable. Even a shielded speaker
may have just enough magnetic field to cause some slight tilt. 1 degreWhy Magnetic Fields May Cause the Picture to Rotate
One might think that the result of the Earth's or stray magnetic fields would
only be for the picture to shift position slightly. Why isn't this the case?
The actual direction of the Earth's magnetic field experienced by the CRT
depends on the latitude and includes both horizontal and vertical components -
horizontal at the equator and becoming progressively angled toward the poles
(with opposite polarities - N or S - depending on which hemisphere it is in).
This is the main reason that TVs and monitors really need to be set up slightly
differently depending on location (hemisphere and latitude). And, of course,
local magnetic conditions also affect this including geologic formations and
other equipment and wiring which produce magnetic fields.
Best Direction to Face a Monitor?
One would think that the magnetic field of the earth is inconsequential
compared to what is used to drive a CRT. While the reasons this is not true
should be obvious from other sections of this document, some would still call
worrying about such issues as the direction of the monitor nonsense.
Ways Around North/South or Other Sensitivity to Magnetic Fields?
(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)
Additional Comments/Summary on Northern/Southern Hemisphere Issues
(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)
Where you have a TV or monitor that was manufactured for a different location,
your options (apart from tossing it) are:
Orientation Considerations for Projection TVs
Projection TVs do have have CRTs with shadow masks or aperture grills but
nonetheless can be affected by magnetic fields. In fact, it is possible that
degaussing could even be needed if a strong magnet were somehow placed near
the set - but how would THAT happen? :-)
"Any truth to the rumor that how you position a projection TV in a room
(N,E,S,W) can affect the image quality? Does the Earth's magnetic field
truly have that much of an effect."
(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)
Picture Quality and Appearance Issues
Why Does the Intensity Appear So Non-Uniform in Bright Areas?
Actually, the intensity variation is likely to be even worse than you might
think - possibly as much as 2:1 from the center to the corners. In most cases
you do not notice it. With large deflection angle tubes, fewer electrons make
it to phosphor dots near the edge of the screen. It is simple geometry.
Comments On Color Purity, Set Orientation, and Doming
"The problem with my TV is that bright parts of the picture change color.
For example, white areas may shift towards yellow or blue depending on the
orientation of the set.
(Portions from: Jeroen Stessen (Jeroen.Stessen@philips.com).)
Difference in Color Rendition Between CRTs
(From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)
Contour Lines on High Resolution Monitors - Moire
These fall into the category of wavey lines, contour lines, or light and dark
bands even in areas of constant brightness. (Some people may refer to this
phenomenon as "focus or Newton's rings".) These may be almost as fine
as the dot pitch on the CRT or 1 or 2 cm or larger and changing across the
screen. If they are more or less fixed on the screen and stable, then
they are not likely to be outside interference or internal power supply
problems. (However, if the patterns are locked to the image, then there
could be a problem with the video board.)
Moire and Shadow Mask Dot Pitch
(From: myers@fc.hp.com (Bob Myers).)
Isolated Spots on Display
These could be a problem with the video source - bad pixels in the video
card's frame buffer or bad spots on a camcorder's CCD, for example.
Or, they could be dirt or dead phosphor areas in the CRT. Except for
problems with the on-screen character generator, it is unlikely that the
monitor's circuitry would be generating isolated spots.
Purple Blob - or Worse
Have you tried demagnetizing it? Try powering it off for a half hour, then
on. Repeat a couple of times. This should activate the internal degausser.
See the section: Degaussing (Demagnetizing) a CRT.
Magnet Fix for Purity Problems - If Duct Tape Works, Use It!
The approach below will work for slight discoloration that cannot be eliminated
through degaussing. However, performing the standard purity adjustments
would be the preferred solution. On the other hand, the magnets may be quick
and easy. And, where CRT has suffered internal distortion or dislocation of
the shadow mask, adjustments may not be enough.
How Much Tilt is Acceptable?
This was in reply to a concern that a 1 degree tilt on a 27" TV was a problem.
Yes, you may not like it, but unless there is a user tilt adjustment, the laws
of physics prevail!