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TV and Monitor CRT (Picture Tube) Information
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This document contains a collection of information relating to CRT (picture tube) construction, characteristics, problems, maintenance, troubleshooting, and repair from the TV and Monitor Repair FAQs and other sources. Most new CRT related information resulting from sci.electronics.repair, comp.sys.ibm.pc.hardware.video, or other newsgroups will be included here rather than those other documents. In the future, I may replace the 'CRT Basics' chapter of those documents with a link to this one instead. Related documents I have written: * 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. Special thanks to Bob Myers (myers@fc.hp.com) and Jeroen Stessen (Jeroen.Stessen@ehv.ce.philips.com) for their contributions to this document through their newsgroup postings and private email. As always, comments, corrections, and additions are welcome.
Currently, 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 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.
For an introductory on-line article about (mostly) CRTs, see: High Tech Tubes, Popular Mechanics, April 1997. 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: * Weight - a heavy support structure must be provided for the tensioned wires (like a piano frame). * Price (proportional to weight). * Always a cylindrical screen (this may be considered an advantage depending on your preference. * Visible stabilizing wires which may be objectionable or unacceptable for certain applications. Apparently, there is no known way around the need to keep the fine wires from vibrating or changing position due to mechanical shock in high resolution tubes and thus all Trinitron monitors require 1, 2, or 3 stabilizing wires (depending on tube size) across the screen which can be see as very fine lines on bright images. Some people find these wires to be objectionable and for some critical applications, they may be unacceptable (e.g., medical diagnosis).
(Portions from: Jeroen H. Stessen (Jeroen.Stessen@ehv.ce.philips.com)). 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. * For each of the phosphor colours (and optional black matrix) one phosphor layer is deposited followed by one photoresist layer. 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. * The shadow mask for that CRT (unique) is then mounted - clicked in place. * An intense point source of light is mounted at the location of the effective center of deflection for the electron gun associated with that phosphor. * The photoresist is exposed to light. * The shadow mask is removed and the excess resist (not exposed to light) and phosphor is washed away. These steps are repeated for the red, green, and blue phosphors, and the optional (but very common) black matrix surround. 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: * Provide the return path for the electron beam - connected to the EHT 2nd anode. * Reduces backscattering or secondary emission. Electrons that bounce back from either the shadow mask or the screen may hit a phosphor elsewhere and thus cause unwanted white light. That reduces contrast and colour purity. * A side benefit is that it blocks negative ions from residual air molecules from hitting the phosphors. These might result in an unsightly blemish in the center of the screen since they are much heavier (many thousands of times the mass) than electrons and are not deflected very much. (This was a problem in the early days of CRT production but apparently not with present high vacuums and getters to clean up whatever is left.) The shadow mask is then mounted for a final time and the faceplate, envelope (with its electron gun assembly already fused to it) are mated. At this point, it is ready for the final baking and evacuation. 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.
The ability to display fine detail involves many factors including the resolution of the video source, video bandwidth, sharpness of the electron beam(s), and the dot/slot/line pitch (color only) of the CRT. 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 mask - the phosphor screen consists of triads of R, G, and B, circular dots in a triangular arrangement. The shadow mask is a steel or InVar sheet filled with holes - one for triad. The dot mask has been used since the early days of color TV and is still popular today. The electron guns are also arrange in a triangular configuration. Slot mask - the phosphor screen consists of triples of vertically elongated R, G, and B, stripes (actually, these are usually full vertical stripes interrupted by narrow gaps). The shadow mask is a steel or InVar sheet filled with slots - one for each triple. Ideally, the metal between the slots vertically is as thin as possible to maintain the structural stability of the slot mask sheet. This type of tube seems to be very popular in TVs but also shows up in some computer monitors. The electron guns are in line which makes some of the setup adjustments less critical compared to the dot mask CRT. Aperture grille - the phosphor screen consists of triples of vertical R, G, and B, lines running the full height of the screen. The aperture grille is a series of tensioned steel wires running vertically behind the phosphor stripes - one for each triple. The aperture grille - until recently under patent protection and therefore only available in the Trinitron from Sony - is found in both TVs and monitors. The electron guns are also in line. The pitch of a color CRT refers to the spacing of phosphor triads or triples. For dot mask CRTs, this parameter is relevant in both the horizontal and vertical direction. For slot mask and aperture grille CRTs, the pitch is only relevant in the horizontal direction. 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.
When color CRTs must be operated in areas where the magnetic field causes unacceptable purity errors or distortion (either static or dynamic depending on whether the source is constant (as with the magnet in an MRI scanner or MegaBase(tm) loudspeaker) or changing (as with nearby motors, transformers, or even other monitors), there are several options (besides relocating): * Passive shielding - soft magnetic materials (those that are easily magnetized and don't retain their magnetism) can effectively block modest strength magnetic fields. The best known of these for shielding purposes is called 'Mu-Metal', an alloy of 76% nickel, 17% iron, 5% copper, and 2% chromium. (Nelson and Parker, A.L.Physics). 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. * Active compensation - a set of coils is energized with exactly the correct currents to cancel the effects of the interfering fields. 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. * Shielding can also sometimes be introduced at the source. See the document: "Notes on the Troubleshooting and Repair of Audio Equipment and other Miscellaneous Stuff", specifically, the section: "Comments on speaker shielding" 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. (From: Tony Williams (tonyw@ledelec.demon.co.uk)). 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.
Computer monitor specifications always include the dot pitch of the
CRT. However, this information is rarely available for TVs. Why?
The quick answer is that since TVs are always used at the same scan
rate (except for multisystem international TVs), this information is
not nearly as important for TVs as for high resolution multiscan
computer monitors.
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:
* 13" GE - .60 mm.
* 19" Samsung - .75 mm.
* 25" RCA - .9 mm.
Therefore, you can forget about trying to use one of these CRTs for your
1280x1024 high resolution PC or workstation. The dot/stripe pitch needed for
1280 pixels on a 25" tube would be about .3-.4 mm maximum. The pixels are
about .35 mm. Typical high resolution CRTs for high resolution computer
monitors have a dot/stripe pitch of .25 to .28 mm (I have heard of numbers
as low as .22 mm in commercially available monitors).
Many factors influence the effective resolution of a monitor but the CRT dot
or slot mask or aperture grill is the ultimate limit (though it may still be
possible to use a monitor at a resolution which exceeds the that of the CRT).
However, as the pixel spacing approaches that of the CRT, moire effects are
likely to be more of a problem.
(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:
1. The diagonal 'active picture" area (APD). If the vendor fails to specify
this, subtract 1 inch from the advertised monitor size. I.e. a '21 inch'
monitor will usually have about a 20-inch usable diagonal picture area.
('PC Inches' versus 'real inches' is a topic for another time. :-)
2. You need the horizontal dot pitch (HDP). The vertical and horizontal are
often different (with the vertical being a smaller number). If you have
been given only one number, it's probably the diagonal, and is misleading,
but it is all we have to work with.
(From: Bob Myers (myers@fc.hp.com)).
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)).
3. The monitor aspect ratio (AR). This is 4:3 (or 1.33:1) for any CRT
you are likely to be using.
To calculate useful horizontal resolution:
* Multiply the APD by .80 (4:3 tube).
This is the Active Picture Horizontal size (APH) in inches.
* Multiply APH by 25.4.
This is the APH in mm (APHmm).
* Divide the APHmm by the HDP.
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.
I use a Hitachi AccuVue UX4921D (aka HM-4921-D/A-HT01) 21-inch monitor.
It is a claimed 1600x1200 monitor, and having a .22 horizontal dot
pitch, actually has over 1800 phosphor triads across the screen. This
is rare. Most large monitors usually have 1280 or fewer triads across
the screen.
(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'.
We always see CRTs specififed in terms of dot pitch but what does this mean
with respect to actual useful horizontal and vertical dot pitch?
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 --- R -+- B --- G --- R
(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:
* The Vertical Dot Pitch (VDP) will be equal to: HexP * 1/2.
* The Same Color Horizontal Dot Pitch (SCHDP) will be:
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.
* The Horizontal Dot Pitch (HDP) is the distance between adjacent columns of
same color dots. This is equal to: SCHDP * 1/2.
* The distance between adjacent dots of different colors or Closest Dot Spacing
(CDS) is equal to: SCHDP * 1/3. A landing error of this magnitude (due to
improper manufacture, adjustment, inadequate degauss, external fields, or
doming) may completely shift the color from what it is supposed to be to one
of the other primary colors.
So, for a 0.28 mm dot pitch CRT, VDP = .14 mm, SCHDP = .485 mm, HDP = .242 mm,
and CDS = .16 mm.
(Portions from: Jeroen H. Stessen (Jeroen.Stessen@ehv.ce.philips.com)).
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.
(Portions from: Jeroen H. Stessen (Jeroen.Stessen@ehv.ce.philips.com)). 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.
"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.
Once the CRT is sealed, baked, evacuated, etc., the job is not yet done! (From: Jeroen H. Stessen (Jeroen.Stessen@ehv.ce.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...)
(The following includes material from:
Jeroen Stessen (Jeroen.Stessen@ehv.ce.philips.com)).
TVs are always set up to generate a picture which is 10-15 percent large
than the visible face of the CRT. Why?
In the early days of TV, this was probably done to make the design easier.
Component tolerances and power line voltage fluctuations would be masked even
if they caused changes in picture size.
There certainly is almost no reason today to have any more than a couple of
percent overscan. Most modern TVs have very well regulated power supplies
and component values do not really drift much.
Computer monitors, for example, are usually set up for no overscan at all
so that the entire image is visible.
We are constantly reminded of that, now that we are building TV's with
VGA inputs (PD5029C1 in the USA, US$ 2000). This mixed application has
overscan in TV mode and underscan in VGA mode. Geometry adjustment is
quite critical if you see border-on-border.
Unfortunately, TV's may be good but VCR's certainly are not. If you have
too little overscan and then put the VCR in any feature mode (like picture
search) then one (black) picture edge may become visible. Bad form.
Viewers do not like this.
While design considerations may have been the original reason for overscan,
now it has become accepted as a de facto standard, and broadcasters are
counting on the overscan being a certain percentage. One wonders whether
it will ever change or whether this really matters.
I suppose when we have true flat panel digitally addressed displays,
we might have 0% overscan.
At the Japan Electronics Show all the signs are pointed toward flat panel
displays so maybe I will not have to hold your breath for much longer.
Physically, as with an LCD display on a laptop computer, there will be
0% overscan (no need to build the extra pixels) but that doesn't mean
that all 480 lines will be visible.
(From: Jeroen Stessen (Jeroen.Stessen@ehv.ce.philips.com)). 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.
The vertical component of the earth's magnetic field varies in intensity and polarity (N/S) as one moves from the North pole over the equator and to the South pole. It is maximum at the poles and decreases to zero at the equator. The total strength is not large - after all it is less than the total magnitude of the earth's magnetic field of about .5 Gauss (.00005 Tesla). However, it is enough to affect the trajectory of the electron beam(s) slightly. 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@ehv.ce.philips.com)). There are two types of adjustments: * The passive ones that are done in the picture tube factory and * The active ones that are done by the setmaker a/o the customer. In the factory inside the neck of every (Philips) tube a metal ring is permanently magnetized to create a multipole correction field. Then each tube is matched with a deflection yoke to achieve optimum colour purity. It is possible that a couple of yokes must be tried in succession. This matching is done under specific ambient magnetic field conditions. On oriental tubes you will often see little permanent magnets added to achieve further fine correction of landing and/or convergence. When the tube is within landing specification it is shipped to the setmaker. 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.
(From: Jeroen Stessen (Jeroen.Stessen@ehv.ce.philips.com))
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.
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.
(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.
(From: Jeroen Stessen (Jeroen.Stessen@ehv.ce.philips.com)) 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.
(From: Jeroen Stessen (Jeroen.Stessen@ehv.ce.philips.com))
* The vertical component of the earth magnetic field varies as a function of
latitude, particularly between hemispheres a vertical magnetic field will
influence the color purity of a CRT.
* The magnetic shielding of a CRT will, after degaussing, not provide complete
compensation for the vertical field, especially for the space between shadow
mask and screen.
* That's why manufacturers produce different displays for different
hemispheres: northern, southern and neutral. They do this by adjusting for
optimum purity in a simulated magnetic field.
Where you have a TV or monitor that was manufactured for a different location,
your options (apart from tossing it) are:
* Re-adjust the purity, this involves moving the deflection coil, adjustment
magnets, adding more magnets, etcetera. This is a big job and success would
not be guaranteed.
* Simulate a southern hemisphere location by creating a vertical magnetic
field around the TV, put two big multi-turn wire loops (Helmholtz coils)
above and below the TV and run a DC current through them. Might be expensive
and certainly would provide a 'different' look!
* Replace the picture tube with a northern hemishpere type, this is very
expensive.
* Mount the picture tube upside-down inside the TV cabinet. Then reverse the
wires for the line (H) and field (V) deflection to put the picture correct
side up again.
For this case, you might have some problems with:
- The mounting nuts for the tube are hard to reach and may have left thread
(look carefully before turning!
- The wires to the inverted picture tube panel being too short, they can
probably be easily extended.
- The distance between high-voltage anode connection and the chassis
(circuits) being too short (safety!)
- Condensation dripping into the anode contact.
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.
Purity involves bending all 3 of the beams so that they cross the space
between shadow mask and screen at the proper angle and will land at a
different place on the phosphors. Convergence involves adjusting the aim of
1 or 2 of the beams at a different angle so that they all land at the same
place on the screen.
Dynamic convergence circuitry is now virtually non-existent, except in high
resolution monitor tubes and in Sony Trinitron tubes (they require a very
basic horizontal convergence). All other tubes have the convergence
correction built into the design of the tube and the coil. Sony has chosen a
different trade-off between price and performance (which includes also
sharpness).
Most CRTs have a series - usually 3 pairs - of ring magnets mounted on the
neck near the socket end. These are used for part of the purity adjustment
and static convergence. (Coarse purity is set by the position of the yoke and
dynamic convergence is set by the tilt of the yoke.) These rings consist of
multi-pole magnets which due to their field configuration are able to affect
the electron beams from the 3 guns in different ways.
(Some CRTs employ internal structures that are premagnetized at the factory
and cannot be adjusted in the field - though perhaps auxiliary magnet rings
could be added if the original magnetization were lost for reasons we won't go
into :-). This type of CRT will be obvious as there will be no adjustable
rings to mess screw up!)
The relative orientation of the rings in a pair affect the strength of the
effect.
In a nutshell, the electron guns in most modern CRTs are arranged in-line.
For example: GRB. Some of the ring adjustments are designed to affect them
all while others pretty much leave the center gun's beam alone and only
affect the outer ones. Various options then exist depending on the magnetic
field configuration.
The three sets of ring magnets are shown below along with the position of the
red (R), green (G), and blue (B) electron beams passing through them. Each is
actually a pair of rings which may be moved relative to one-another to control
the strength of the magnetic field. When the tabs are adjacent, the fields
from the two rings nearly cancel and the rings then have no effect. Two
typical orientations are shown (N and S are the poles of the ring magnets):
Orientation 1:
S S N
N R G B S N R G B N N R G B S
S S N
2-pole 4-pole 6-pole
(purity) (red-blue) (red/blue-green)
0 Degrees 0 Degrees 0 Degrees
Orientation 2:
N N S S
N N
R G B R G B R G B
S S
S S N N
2-pole 4-pole 6-pole
(purity) (red-blue) (red/blue-green)
90 Degrees 45 Degrees 30 Degrees
(My apologies if I have the direction of deflection reversed - I can never
remember the right hand rule for electrons moving in magnetic fields!)
* The 2-pole purity rings move the set of RGB beams more or less together to
fine tune the position of the center of deflection.
The field lines go generally across (at the orientation shown) between the
N and S poles.
Orientation 1, the RGB beams will be raised.
Orientation 2, the RGB beams will be moved to the right.
* The 4-pole red-blue rings move the R and B beams relative to the G beam but
leave the G beam alone.
The field lines go generally between adjacent N and S poles. On opposite
sides of the rings, the polarity/direction of the lines oppose and thus tend
to move the R and B beams in opposite directions. The G beam in the center
does not experience any deflection from the 4-pole ring magnets since all
the fields tend to cancel.
Orientation 1: The R beam will move up and the B beam will move down relative
to G.
Orientation 2: The R beam will move up and to the right and the B beam will
move down and to the left relative to G.
* The 6-pole red/blue-green rings move the RB beams with relative to the G
beam but leave the G beam alone.
The field lines go generally between adjacent N and S poles. On opposite
sides of the rings, the polarity/direction of the lines are the same and
thus tend to affect the R and B beams in the same direction. The G beam
in the center does not experience any deflection from the 6-pole ring
magnets since all the fields tend to cancel.
Orientation 1: The R and B beams will move up relative to G.
Orientation 2: The R and B beams will move up and to the right relative to G.
For purity to be perfect (or as good as possible), the electron beams must
originate from the same effective center of deflection as used in originally
laying down the phosphors. Moving the yoke forward and backward on the neck
of the CRT can precisely set the deflection center along the axis of the
neck. However, slight transverse errors may still exist due to imperfections
in the yoke windings or positions of the electron guns. This is affected
slightly by the earth's magnetic field as well. The purity magnet rings are
those closest to the yoke and provide the means for moving the electron beams
very slightly to compensate.
The adjustment procedures generally use the red gun for the setting purity.
Intuitively, one would think this should be the center (green) gun. However,
since the red beam current is the highest (red phosphor is least sensitive),
problems are likely to show up first with the red purity so it is used for
the adjustment. In any case, it is a good idea to check all three guns for
proper purity and tweak if needed before moving on to convergence.
In an in-line gun, the green gun is always in the middle. The only reason for
adjusting purity with the red beam is because it gives the greatest
sensitivity:
(From: Jeroen H. Stessen (Jeroen.Stessen@ehv.ce.philips.com)).
* The red beam current usually has the largest amplitude.
* A landing error of the red beam gives the best visible discoloration (much
better than green, better than blue).
* This makes the landing of the red beam the most critical.
I have a late model TV with a 13 inch tube with no static purity or convergence rings. I don't get to see that many modern tubes so this was a bit of a surprise or maybe I just hadn't noticed before on small CRTs if they didn't have purity/convergence problems. I do see it has wrapping of a rubber-ferrite-permalloy type material where the ring assembly would go. I assume that this is magnetized selectively at the factory to adjust purity/convergence? The yoke has the usual position and tilt adjustments. This one was an RCA/GE CRT. What this means is that if you were to accidentally bring a strong permanent magnet near the base of the CRT or a strong degaussing coil, there is a slight possibility of totally messing up this compensation requiring replacement of the CRT. I don't know how possible this is without really working at it! (From: Jeroen H. Stessen (Jeroen.Stessen@ehv.ce.philips.com)). Since eternity, Philips CRTs have not had external multipole magnet rings around the neck. There is an iron ring inside the neck, at the end of the electron gun assembly. This ring is permanently magnetized in the factory by a strong outside magnetic field at a later stage of the production. Further responsibility for purity, convergence and geometry is in the design of the coil windings distribution and some metal parts. Final purity adjustment is achieved by matching a tube with a coil and then shifting and tilting the coil slightly. This explains why Philips CRTs are always sold as a matched combination of tube and coil, contrary to some other brands.
(From: Jeroen H. Stessen (Jeroen.Stessen@ehv.ce.philips.com)). CRT projection displays require much convergence correction, especially since the 3 tubes aim at the screen under different angles. Generally the green Horizontal convergence coil is not driven because that is a geometry correction which is taken care of by the horizontal deflection circuit. The 3 vertical convergence coils usually also take care of vertical geometry correction (N-S corrrection) because the vertical deflection circuit is generally a standard direct-view type. Add to that a severe keystone correction for the Red and Blue tubes. The convergence waveforms used to be generated from an analog polynomial generator. The components are then weighted and summed to form a Taylor polynomial. Consider the adjustment of horizontal convergence, then typical polynomial components are: 1 (shift), x (amplitude), x^2 (linearity), y (rotation or tilt), y^2 (bow), x*y (keystone), x^2*y (dunno if it's used) x*y^2 (pin-cushion), x^3 (side linearity) x*y^4 (corner pin-cushion) Adjusting convergence is a highly iterative (read: costly) process because each potentiometer tends to influence the whole screen. Also, this method is not easily extendible to higher order adjustments for more accuracy. That's why better waveform generators have been designed, like digital look-up tables with D/A converters (which are quite expensive) and spline-like waveform generators (which are cheap and easy to adjust, the Philips design is called Convergence Spline Processor, it's digital too).
The shadow or slot mask inside the CRT is a thin sheet of steel or InVar positioned a half an inch or so behind the phosphor screen. The flatter the screen, the more susceptible it will be to thermal expansion effects: With individual phosphor dots spaced as as little as .13 mm apart (for a .22 mm dot pitch CRT), it doesn't take much inaccuracy in their position to result in a noticeable effect. (See the section: "How to compute effective dot pitch".) As a result, high resolution CRTs tend to be more susceptible to doming problems. (Portions from: Jac Jamar (jamar@comp.snads.philips.com)). 1. Doming is a deformation of the shadow mask or its support structure caused by heating and subsequent expansion in bright (high beam current) areas of the picture. This causes a shift in position of the finely spaced holes or slots in the mask. The result will be color purity problems - discoloration and brightness variations. For a .28 mm dot pitch CRT, a change of only .14 mm in the position of a hole or slot can totally shift the display from one of the primary colors to another. 2. InVar shadow masks can sustain a significantly higher current density than steel shadow masks (by as much as 3:1) without noticeable problems. Trinitrons are more resistant to local doming effects as long as the wires are under enough tension. However, expansion of the suspension components can still result in doming with an overall bright picture. 3. The onset and disappearance of color purity problems will generally lag the cause due to the thermal mass of the affected components. For local heating resulting from picture highlights, this will be only a few seconds since the thermal mass of a small area of the mask is not that great. However, for effects having to do with expeansion of the suspension or support structure, it may take up to 30 minutes to reach equilibrium. 4. The orientation of the TV or monitor with respect to the earth's magnetic field and even whether the CRT was set up for the Northern or Southern hemispheres may affect the resulting color shift. Thus, the picture may tend toward yellow while the monitor is facing one way and blue when rotated 180 degrees on its base (even if degaussed at each position). 5. Reducing the brightness/contrast or setting the brightness limiter will prevent doming but may result in an unacceptably dark picture. 6. Shadow mask doming in itself is not something that becomes defective and has to be repaired. It is a characteristic of the CRT assembly. However, shifts in the position of purity adjustments can results in increased sensitivity to slight doming. Purity problems resulting in discolouration and/or brightness variations are due to mislanding of the microscopic electron beams (the electron beams after the mask) on the red/green/blue phosphor stripes or dots. The mislanding is in general caused by: * Influences of ambient magnetic fields (such as the earth magnetic field). * Shadow mask doming. * Tolerances occurring in the production of CRTs. * Less than optimal setup of the purity adjustments (yoke position, rings on CRT neck, etc. Only when the sum of these influences exceeds the 'guardband' provided in the CRT design, discolouration (or brightness variations) becomes visible. If discolouration complaints arise, this will normally not be due to changes in doming behaviour, but to changes in shielding against magnetic fields. The ambient magnetic fields are shielded by means of iron components inside (or sometimes outside) the tube, which have to be 'degaussed' to give good shielding. For this in a set degaussing coils and circuits are provided. A discolouration complaint will thus often be due to insufficient degaussing. * TV sets and monitors which are kept in 'stand-by' mode for a long time may never be degaussed adequately because the degaussing circuit may only operate for a short time after the unit is switched on from cold - whether this is so with your unit depends on the design). In this case, they can pick up magnetic fields from magnets moved nearby or other equipment. The solution in this case is to switch the TV or monitor completely off or pull the plug if in doubt, let it cool down for half an hour or longer and switch it on again. If necessary this procedure can be repeated a few times (I had to do this at home once when my children had been playing with magnets). For monitors with degauss buttons, you can usually hear a hum when the degauss is activated. * Similarly, if the orientation of a unit with respect to the earth's magnetic field is changed, it requires degaussing. So if you put your TV in another corner of the room or rotate your computer monitor on its tilt-swivel base, you have to activate its degauss circuitry (by letting it cool down or in the case of a high-end monitor, using its degauss button) or degauss it manually (see the section: "Degaussing (demagnetizing) a CRT"). * The PTC resistor (thermistor or posistor) in the degaussing circuit can become defective. This prevents proper degaussing after switch-on. Since lower resolution CRTs are used for most TVs compared to similar size computer monitors, doming would not be nearly as much of a problem if they were both run at similar brightness (energy density) levels. However, TVs are very often used at higher brightness levels resulting in more of a thermal load on the mask which offsets the lower resolution.
These are not a defect - they are a 'feature'. All Trinitron (or clone) CRTs - tubes that use an aperture grille - require 1, 2, or 3 very fine wires across the screen to stabilize the array of vertical wires in the aperture grille. Without these, the display would be very sensitive to any shock or vibration and result in visible shimmering or rippling. (In fact, even with these stabilizing wires, you can usually see this shimmering if you whack a Trinitron monitor.) The lines you see are the shadows cast by these fine wires. The number of wires depends on the size of the screen. Below 15" there is usually a single wire; between 15" and 21" there are usually 2 wires; above 21" there may be 3 wires. Only you can decide if this deficiency is serious enough to avoid the use of a Trinitron based monitor. Some people never get used to the fine lines but many really like the generally high quality of Trinitron based displays and eventually totally ignore them.
(From: Bill Nott (BNott@Bangate.compaq.com)). 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).)
Trinitron is a CRT technology developed by Sony. The patent has recently expired and therefore other manufacturers are free to offer similar CRTs. The CRT uses a set of fine vertical wires called an aperture grill instead of a steel shadow mask to separate the R, G, and B electron beams and force them to strike only the appropriate colored phosphors. This in conjunction with an in-line set of electron guns is supposed to provide a brighter image with simpler convergence and purity adjustments. It should be brighter because the percentage of open space of the aperture grill is higher then that of a shadow mask. Other adjustments should be less critical in the vertical direction. In addition, since there is no imposed structure in the vertical direction, undesirable moire patterns caused by scan line pitch compared with the shadow mask dot pitch should be eliminated. 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. 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.
"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@ehv.ce.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: * Flatter picture tubes: flatter shadow masks are more sensitive to doming * Darker (glass) picture tubes: this gives more contrast but more beam current is needed for enough brightness The trinitron aperture grill shadow mask is inherently insensitive to doming as long as the tension in the wires remains positive. If the wires become too long then they become more sensitive to microphony (try tap the cabinet...). The vertical wires are connected in several places by thin horizontal wires. Some people complain about seeing faint shadows of these wires. 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.
(From: Thomas Maggio (staccato@gate.net)). 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.
Here are three reasons: 1. The cathode can be made of and/or coated with a material optimal for emitting electrons without regard to its performance as a heater. 2. The separate cathode and filament can be electrically isolated so that the filament voltage and cathode drive signal, if any, do not interfere. 3. The cathode can have an optimal shape for the application. This would be particularly significant for CRTs. The spot on the screen is a reduced focused image of the effective shape of the emitting portion of the cathode.
TVs and computer or video monitors are among the more dangerous of consumer electronics equipment when it comes to servicing. (Microwave ovens are probably the most hazardous due to high voltage at flesh frying and cardiac arresting high power.) There are two areas which have particularly nasty electrical dangers: the non-isolated line power supply and the CRT high voltage. Major parts of nearly all modern TVs and many computer monitors are directly connected to the AC line - there is no power transformer to provide the essential barrier for safety and to minimize the risk of equipment damage. In the majority of designs, the live parts of the TV or monitor are limited to the AC input and line filter, degauss circuit, bridge rectifier and main filter capacitor(s), low voltage (B+) regulator (if any), horizontal output transistor and primary side of the flyback (LOPT) transformer, and parts of the startup circuit and standby power supply. The flyback generates most of the other voltages used in the unit and provides an isolation barrier so that the signal circuits are not line connected and safer. Since a bridge rectifier is generally used in the power supply, both directions of the polarized plug result in dangerous conditions and an isolation transformer really should be used - to protect you, your test equipment, and the TV, from serious damage. Some TVs do not have any isolation barrier whatsoever - the entire chassis is live. These are particularly nasty. The high voltage to the CRT, while 200 times greater than the line input, is not nearly as dangerous for several reasons. First, it is present in a very limited area of the TV or monitor - from the output of the flyback to the CRT anode via the fat red wire and suction cup connector. If you don't need to remove the mainboard or replace the flyback or CRT, then leave it alone and it should not bite. Furthermore, while the shock from the HV can be quite painful due to the capacitance of the CRT envelope, it is not nearly as likely to be lethal since the current available from the line connected power supply is much greater.
It is essential - for your safety and to prevent damage to the device under test as well as your test equipment - that large or high voltage capacitors be fully discharged before measurements are made, soldering is attempted, or the circuitry is touched in any way. Some of the large filter capacitors commonly found in line operated equipment store a potentially lethal charge. This doesn't mean that every one of the 250 capacitors in your TV need to be discharged every time you power off and want to make a measurement. However, the large main filter capacitors and other capacitors in the power supplies should be checked and discharged if any significant voltage is found after powering off (or before any testing - some capacitors (like the high voltage of the CRT in a TV or video monitor) will retain a dangerous or at least painful charge for days or longer!) The technique I recommend is to use a high wattage resistor of about 100 ohms/V of the working voltage of the capacitor. This will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage). Then check with a voltmeter to be double sure. Better yet, monitor while discharging (not needed for the CRT - discharge is nearly instantaneous even with multi-M ohm resistor). Obviously, make sure that you are well insulated! * For the main capacitors in a switching power supply which might be 100 uF at 350 V this would mean a 5K 10W resistor. RC=.5 second. 5RC=2.5 seconds. A lower wattage resistor can be used since the total energy in not that great. If you want to be more high tech, you can build the capacitor discharge circuit outlined in the companion document: "Testing capacitors with a multimeter and safe discharge". This provides a visible indication of remaining charge and polarity. * For the CRT, use a high wattage (not for power but to hold off the high voltage which could jump across a tiny 1/4 watt job) resistor of a few M ohms discharged to the chassis ground connected to the outside of the CRT - NOT SIGNAL GROUND ON THE MAIN BOARD as you may damage sensitive circuitry. The time constant is very short - a ms or so. However, repeat a few times to be sure. (Using a shorting clip lead may not be a bad idea as well while working on the equipment - there have been too many stories of painful experiences from charge developing for whatever reasons ready to bite when the HV lead is reconnected.) Note that if you are touching the little board on the neck of the CRT, you may want to discharge the HV even if you are not disconnecting the fat red wire - the focus and screen (G2) voltages on that board are derived from the CRT HV. WARNING: Most common resistors - even 5 W jobs - are rated for only a few hundred volts and are not suitable for the 25KV or more found in modern TVs and monitors. Alternatives to a long string of regular resistors are a high voltage probe or a known good focus/screen divider network. However, note that the discharge time constant with these may be a few seconds. Also see the section: "Additional information on discharging CRTs". If you are not going to be removing the CRT anode connection, replacing the flyback, or going near the components on the little board on the neck of the CRT, I would just stay away from the fat red wire and what it is connected to including the focus and screen wires. Repeatedly shoving a screwdriver under the anode cap risks scratching the CRT envelope which is something you really do not want to do. Again, always double check with a reliable voltmeter!T Reasons to use a resistor and not a screwdriver to discharge capacitors: 1. It will not destroy screwdrivers and capacitor terminals. 2. It will not damage the capacitor (due to the current pulse). 3. It will reduce your spouse's stress level in not having to hear those scary snaps and crackles.
Preventive maintenance for a monitor is pretty simple - just keep the case clean and free of obstructions. Clean the CRT screen with a soft cloth just dampened with water and mild detergent or isopropyl alcohol. This will avoid damage to normal as well as antireflection coated glass. DO NOT use anything so wet that liquid may seep inside of the monitor around the edge of the CRT. You could end up with a very expensive repair bill when the liquid decides to short out the main circuit board lurking just below. Then dry thoroughly. Use the CRT sprays sold in computer stores if you like but again, make sure none can seep inside. If you have not cleaned the screen for quite a while, you will be amazed at the amount of black grime that collects due to the static buildup from the CRT high voltage supply. In really dusty situations, periodically vacuuming inside the case and the use of contact cleaner for the controls might be a good idea but realistically, you will not do this so don't worry about it.
Degaussing may be required if there are color purity problems with the display. On rare occasions, there may be geometric distortion caused by magnetic fields as well without color problems. The CRT can get magnetized: * if the TV or monitor is moved or even just rotated. * if there has been a lightning strike nearby. A friend of mine had a lightning strike near his house which produced all of the effects of the EMP from a nuclear bomb. * If a permanent magnet was brought near the screen (e.g., kid's magnet or megawatt stereo speakers). * If some piece of electrical or electronic equipment with unshielded magnetic fields is in the vicinity of the TV or monitor. Degaussing should be the first thing attempted whenever color purity problems are detected. As noted below, first try the internal degauss circuits of the TV or monitor by power cycling a few times (on for a minute, off for 30 minutes, on for a minute, etc.) If this does not help or does not completely cure the problem, then you can try manually degaussing. Commercial CRT Degaussers are available from parts distributors like MCM Electronics and consist of a hundred or so turns of magnet wire in a 6-12 inch coil. They include a line cord and momentary switch. You flip on the switch, and bring the coil to within several inches of the screen face. Then you slowly draw the center of the coil toward one edge of the screen and trace the perimeter of the screen face. Then return to the original position of the coil being flat against the center of the screen. Next, slowly decrease the field to zero by backing straight up across the room as you hold the coil. When you are farther than 5 feet away you can release the line switch. The key word here is ** slow **. Go too fast and you will freeze the instantaneous intensity of the 50/60 Hz AC magnetic field variation into the ferrous components of the CRT and may make the problem worse. It looks really cool to do this while the CRT is powered. The kids will love the color effects. Bulk tape erasers, tape head degaussers, open frame transformers, and the "ass-end" of a weller soldering gun can be used as CRT demagnetizers but it just takes a little longer. (Be careful not to scratch the screen face with anything sharp.) It is imperative to have the CRT running when using these whimpier approaches, so that you can see where there are still impurities. Never release the power switch until you're 4 or 5 feet away from the screen or you'll have to start over. I've never known of anything being damaged by excess manual degaussing though I would recommend keeping really powerful bulk tape erasers turned degaussers a couple of inches from the CRT. If an AC degaussing coil or substitute is unavailable, I have even done degaussed with a permanent magnet but this is not recommended since it is more likely to make the problem worse than better. However, if the display is unusable as is, then using a small magnet can do no harm. (Don't use a 20 pound speaker or magnetron magnet as you may rip the shadow mask right out of the CRT - well at least distort it beyond repair. What I have in mind is something about as powerful as a refrigerator magnet.) Keep degaussing fields away from magnetic media. It is a good idea to avoid degaussing in a room with floppies or back-up tapes. When removing media from a room remember to check desk drawers and manuals for stray floppies, too. It is unlikely that you could actually affect magnetic media but better safe than sorry. Of the devices mentioned above, only a bulk eraser or strong permanent magnet are likely to have any effect - and then only when at extremely close range (direct contact with media container). All color CRTs include a built-in degaussing coil wrapped around the perimeter of the CRT face. These are activated each time the CRT is powered up cold by a 3 terminal thermister device or other control circuitry. This is why it is often suggested that color purity problems may go away "in a few days". It isn't a matter of time; it's the number of cold power ups that causes it. It takes about 15 minutes of the power being off for each cool down cycle. These built-in coils with thermal control are never as effective as external coils.
Some monitor manufacturers specifically warn about excessive use of degauss, most likely as a result of overstressing components in the degauss circuitry which are designed (cheaply) for only infrequent use. In particular, there is often a thermister that dissipates significant power for the second or two that the degauss is active. Also, the large coil around the CRT is not rated for continuous operation and may overheat. If one or two activations of the degauss button do not clear up the color problems, manual degaussing using an external coil may be needed or the monitor may need internal purity/color adjustments. Or, you may have just installed your megawatt stereo speakers next to the monitor! You should only need to degauss if you see color purity problems on your CRT. Otherwise it is unnecessary. The reasons it only works the first time is that the degauss timing is controlled by a termister which heats up and cuts off the current. If you push the button twice in a row, that thermister is still hot and so little happens. One word of clarification: In order for the degauss operation to be effective, the AC current in the coil must approach zero before the circuit cuts out. The circuit to accomplish this often involves a thermister to gradually decrease the current (over a matter of several seconds), and in better monitors, a relay to totally cut off the current after a certain delay. If the current was turned off suddenly, you would likely be left with a more magnetized CRT. There are time delay elements involved which prevent multiple degauss operations in succession. Whether this is by design or accident, it does prevent the degauss coil - which is usually grossly undersized for continuous operation - to cool.
Pack Rat Trick #457384 Next time you scrap a computer monitor (or tv), save the degaussing coil (coil of wire, usually wrapped in black tap or plastic) mounted around the front of the tube. To adapt it for degaussing sets, wrap it into a smaller coil, maybe 4"-6". To limit the current to something reasonable, put it in series with a light bulb (60-100W). You need AC current to degauss, so just put the bulb in series with the coil and use the your local 120V outlet. BE VERY CAREFUL that you actually wired it in series, and that everything is properly insulated before you plug it in (A fuse would be a real good idea too!!) A few circles over the affected area will usually do it. Note that it will also make your screen go crazy for a little bit, but this will fade out within a minute or so. Just a couple of points for emphasis: 1. The coil as removed from the TV is not designed for continuous operation across the line as indicated above. In fact, it will go up in a mass of smoke without the light bulb to limit the current. The poor TV from which this organ was salvaged included additional circuitry to ramp the current to 0 in a few seconds after power is turned on. 2. Reducing the coil size by a factor of 2 or 3 will increase the intensity of the magnetic field which is important since we are limiting the current with the light bulb to a value lower than the TV used. You don't need to unwind all the magnet wire, just bend the entire assembly into a smaller coil. Just make sure that the current is always flowing in the same direction (clockwise or counterclockwise) around the coil. 3. Insulate everything very thoroughly with electrical tape. A pushbutton momentary switch rated for 2 amps at 115 volts AC would be useful so that you do not need to depend on the wall plug to turn it on and off.
A couple of comments: first of all, it makes no difference whatsoever if the display is on while it's being degaussed. (Oh, some people DO like to watch the psychodelic light show, but it really doesn't help anything for it to be on.) Actually, there is a very minor case to be made for degaussing while OFF, at least for the Trinitron and similar tubes. (The field of an external degauss coil CAN cause the grille wires to move slightly, and they're a bit more flexible when hot - so it is conceivable, although certainly unlikely, that you're running a higher risk of causing the grille wires to touch or cross and become damaged.) Secondly, a good practice for degaussing is to slowly back away from the monitor after giving the screen a good going over. Once you're about 5-6' away, turn the coil so it's a right angles to the CRT faceplate (which minimizes the field the monitor is seeing), and THEN turn to coil off. This is to reduce the possibility of the field transient caused by switching the coil off from leaving you once again with a magnetized monitor. The last point is to make sure that you DON'T leave the coil on too long. These things are basically just big coils of wire with a line cord attached, and are not designed to be left on for extended periods of time - they can overheat. (I like the kind with the pushbutton "on" switch, which turns off as soon as I release the button. That way, I can never go off and leave the coil energized.) Oh, one more thing - make sure your wallet is in a safe place. You know all those credit cards and things with the nice magnetic stripe on them? :-) (Actually, I've got a good story about that last. I was teaching a group of field service engineers how to do this once, and being the Big Deal Out of Town Expert, made VERY sure to place my wallet on a shelf far away from the action. Unfortunately, Mr. Big Deal Out of Town Expert was staying in a hotel which used those neat little magnetic-card gadgets instead of a "real" key. Ever try to explain to a desk clerk how it was that, not only would your keycard NOT let you into your room, it was no longer anything that their system would even recognize as a key? :-))
Even a magnet that can suspend your weight may still not have much range as they usually have metal pole pieces that concentrate the flux and work well only with a matching flat steel plate. The only thing in the guts of a TV or monitor (that is accessible from outside the cabinet) that can be damaged permanently is the shadow or slot mask of the CRT. If the magnet is strong enough to distort it, the CRT will be ruined. Even manual degaussing with a similarly powerful degaussing coil will then not totally clear up color purity problems. The shadow or slot mask is a very thin perforated steel or InVar sheet about 1/2 inch behind the glass of the CRT screen - which is itself about 1 inch thick or more. So, even up against the screen, your magnet is still at least 1-1/2 inches from the shadow mask. It would take a mighty powerful magnet to distort it. For Trinitron (or clone) CRTs with aperture grilles - tensioned fine wires in place of a shadow or slot mask, the force required would be even greater. No way to know without trying it :-(. (From: Jeff Mangas (jeff@edldisplays.com)). I work in a small monitor factory and some time ago we were using some very strong degaussing wands to remove magnetism from some of our chassis. We found that this caused a weakening of the shadow mask and it would take only a small shock/vibration to break the mask loose. We are not 100% sure that it was the degaussing that caused the problem but we only used these strong wands for a short time (lost several tubes while using them) and have not had any problems before or since.
Also see the adjustment information in the document: "Notes on the Troubleshooting and Repair of Computer and Video Monitors" (or Television
Sets).
(From: Alan McKinnon (alan.mck@pixie.co.za)).
The rearmost pair of magnets (seem from the service position behind the set
in other words furthest from you nearest the front of the tube) affects
purity. More on this later. The middle and front magnets are for convergence
and work on pairs of colours. The effects can most easily be seen on a cross
hatch test pattern (10 or so horizotal lines, 15 or so vertical lines).
But first, purity:
Without getting into the details of what happens inside the guns, I assume you
need to know how to do the adjustments. You need some means of generating an
evenly red screen. An (expensive) pattern generator is the preferred method.
Fiddle the rear purity rings to distort the screen by bringing green and blue
blobs into it. You will note that the magnets can be adjusted by moving both
together, and moving them aart relative to each other. The best advice here
is: adjust slowly and observe what happens. Once you have the screen evenly
red, move on to convergence, which is the trick of getting the red green and
blue beams to coincide on the screen to produce white, with the minimum of
colour shadowing.
With a cross hatch pattern on screen, you can see easily how convergence
works, and how the magnets affect the picture. Each tube type is different in
exactly how this is done, but the general idea is that one set of magnets
affects two specific colours only, moving them apart and bringing them nearer,
while leaving the third colour alone. The other set of magnets takes the
colours affected by the other set, and moves them together relative to the
third colour. Also, moving a pair of magnets together adjusts the colours in
one direction (vert or horiz) while moving the magents apart adjusts the other
direction. With all things in life, there is some overlap, so you need to
look carefully and see what happens mostly - the adjustments are not cut and
dried. Oh, and they are interactive to some degree. Keep checking purity after
you do convergence. All of this is best shown with a picture, the colours are
arbitrary, you may well find the details do not apply to your tv, but the
basic principles will. These initial converence adjustments apply only to the
centre of the screen by the way, the edges are done elsewhere:
Rotating one set of magnets together might move red and blue together till
they coincide vertically:
| | | | | | | |
| | | -----> | | | -----> | |
| | | | | | | |
R G B G R B G R&B
And moving them apart relative to each other might move red and blue together
horizontally:
R -----
R -------- R&B---------
G ----- -----> G -------- -----> G ---------
B --------
B -----
Moving the other set of magnets together might take the red and blue pair and
move them to coincide with the green, vertically:
| | | | |
| | -----> | | ------> |
| | | | |
G R&B G R&B R&G&B (=white)
And moving them apart relative to each other might move the red and blue pair
and move them to coincide with green horizontally:
R&B -------
R&B -------
-----> ----> ------- R&G&B
G ------- (=white)
G -------
Once the convergence is perfect in the centre of the screen (called static
convergence) it's time to handle the edges and corners (called dynamic
convergence for historical reasons). This is done by physically moving the
entire yoke that is clamped around the tube neck with the deflection coild on
it. It is anchored in place by a collar on the tube neck, loosen this
slightly, butnot enough so that the yoke can move backwards. It is also held
in place by rubber wedges glued or taped down. Take the wedges out. By
gripping the yoke and levering it up and down, left and right, you will change
he convergence in the corners. The effects don't work as you might at first
suppose - moving theyoke left affects the lower right corner, this type of
thing. Get the dynamic convergence right and stuff the wedges back under the
yoke to hold it precisely in place and glue them back down. The recheck
purity.
There you have it. Easy as pie. Some folk would have you believe no-one in
their right minds adjusts these things. Well, balls. Someone did it at the
factory, and they did it the way I just described. All you need is the right
tools (pattern generator), patience, and time.
(From: ard12@eng.cam.ac.uk (A.R. Duell)) The older delta-gun tubes (3 guns in a triangle, not in a line) can give **excellent** pictures, with very good convergence, provided: 1. You've set those 20-or-so presets correctly - a right pain as they interact to some extent. 2. The CRT is set up in the final position - this type of tube is more sensitive to external fields than the PIL type. Both my delta-gun sets (a B&O 3200 chassis and a Barco CDCT2/51) have very clearly set out and labeled convergence panels, and you don't need a service manual to do them. The instructions in the Barco manual are something like: "Apply crosshatch, and adjust the controls on the convergence board in the numbered order to converge the picture. The diagrams by each control show the effect". Here's a very quick guide to delta gun convergence where the settings are done using various adjustments on the neck of the CRT (if you don't have a service manual but do know what each control does, and where they all are - otherwise, follow the instructions in the service manual --- sam): 1. Apply a white crosshatch or dot pattern to the set. Don't try and converge on anything else - you'll go insane. It's useful to be able to switch between those 2 patterns. 2. Before you start, set the height, width, linearity, pincushion, etc. They will interact with the convergence. Also check PSU voltages, and the EHT voltage if it's adjustable. That's where you do need a service manual, I guess. 3. Turn off the blue gun using the A1 switch, and use the red and green static radial controls to get a yellow croshatch in the middle of the screen. These controls may be electrical presets, or may be movable magnets on the radial convergence yoke (the Y-shaped think behind the deflection yoke). 4. Turn on the blue gun and use the 2 blue static controls (radial and lateral) to align the blue and yellow crosshatches at the center of the screen. Some manufacturers recommend turning off the green gun when doing this, and aligning red with blue (using *only* the blue controls, of course), but I prefer to align blue with yellow, as it gives a check on the overall convergence of the tube. 5. Turn off the blue gun again. Now the fun starts - dynamic convergence. The first adjustments align the red and green crosshatches near the edges - I normally do the top and bottom first. There will be 2 controls for this, either a top and a bottom, or a shift and a linearity. The second type is a *pain* to do, as it's not uncommon for it to affect the static convergence. 6. Getting the red and green verticals aligned near the edges is a similar process. 7. You now have (hopefully) a yellow crosshatch over the entire screen. 8. Now to align the blue. This is a lot worse, although the principle is the same. Turn on the blue gun again, and check the static (center) convergence 9. To align the blue lines with the yellow ones, you'll find not only shift controls, but also slope controls. Use the shift controls to align the centers of the lines and the slope controls to get the endpoints right. These interact to some extent. You'll need to fiddle with the controls for a bit to work out what they do, even if you have the manual. The convergence over the entire screen should now be good.... A word of warning here... The purity is set by ring magnets on almost all colour CRTs, but on PIL tubes, there are other ring magnets as well - like static convergence. Make sure you know what you are adjusting.
(From: Jerry G. (jerryg@total.net)). Convergence alignment is not something you can do yourself unless you have the proper calibration instruments and skills. It takes lots of experience and time. There are published specs for most of the good monitors. Most of the time they are as follows: There is the 'A area', 'B area', and 'C area'. On a 15 inch monitor the A area would be a diameter of about 4 inches. The B area would be about 7.5 inches. The C area would be the outside areas including the corners. These numbers are approximate. There are actually standard specs for these areas. They are expressed in percentage of screen viewing area. Therefore the inches would vary with the CRT size. The higher the price (quality) of the monitor CRT, yoke, and scanning control circuits, the tighter the convergence can be aligned by the technician. For the A area on a good monitor, the maximum error should not exceed 0.1 mm. For the B area it should not exceed more than about 0.25 mm. And for the C area, it can be allowed up to about 0.3 mm. Most of the monitors that I have repaired, seen, and used did not meet these specs unless they were rather expensive. With these specs there would not be any real visible misconvergence unless you put your nose very close to the screen... A lot of the ones in the medium price range they were about 0.15 mm error in the A area, about 0.4 in the B and greater than in the C area. This also annoys me because I am very critical. If one has the skills and test gear he or she can do a better job on most monitors. It is a question of the time involved. To see the convergence errors a grating or crosshatch pattern is used. A full raster color generator is required for the purity adjustments as well. This is necessary to align the landing points of the CRT guns. The exact center reference and purity adjustments are done with the ring magnets on the CRT neck. The yoke position angle adjustments are also done for the side and top-bottom skewing as well. Everything interacts! The corners are done with various sorts of slip or edge magnets. As for corner convergence skewing, button magnets are used. The color purity will be effected as you go, and must be also corrected. These adjustments interact on one another, and the processes continues until the convergence and purity are good at the same time...! I don't recommend the amateur or hobbiest, or even the do-it-yourselfer to attempt this alignment procedure. The test gear would exceed the cost of a really good monitor anyways...!!! And without the proper skills required, he or she would only make it worse anyways... As for purity specs, the color change from any corner to any corner must not exceed an error of more than 200 degrees Kelvin. The error in the B area should not exceed 300 degrees kelvin. This applies to a white raster. Most of the monitors I see don't get better than about 300 degrees Kelvin. And some are even 1000 out! The purity errors are best checked with a full Red raster using 100 % saturation. Then the other color vector angles are checked with cyan, and then magenta. The color temperature stability should be the same in all aspects. A color spectrometer should be used to judge this error factor. As far as the eye is concerned, it will see a purity error of more than about 500 degrees Kelvin if the person knows what to look for... When changing the CRT, this alignment must be done completely. Most shops do not even employ people who are skilled to a proper alignment, or don't even own the instruments to do it right, and the poor customer get back a monitor that is not in specs...!
(The following from: Bob Myers (myers@fc.hp.com)). Most manufacturers will quote an MTBF (Mean Time Before Failure) of somewhere in the 30,000 to 60,000 hour range, EXCLUSIVE OF the CRT. The typical CRT, without an extended-life cathode, is usually good for 10,000 to 15,000 hours before it reaches half of its initial brightness. Note that, if you leave your monitor on all the time, a year is just about 8,000 hours. The only "tuneup" that a monitor should need, exclusive of adjustments needed following replacement of a failed component, would be video amplifier and/or CRT biasing adjustments to compensate for the aging of the tube. These are usually done only if you're using the thing in an application where exact color/brightness matching is important. Regular degaussing of the unit may be needed, of course, but I'm not considering that a "tuneup" or adjustment.
So you have this great deal on a used TV or monitor. How can you tell if the picture tube is about to die on you? (From: Andy Cuffe (baltimora@psu.edu)). The best way to tell is to look at the picture quality. There is no way to tell the exact number of hours. Also, the life of CRTs varies quite a bit. some will go down hill much faster than others. * It should be sharply focused over the entire screen and all 3 colors should be equally sharp. * Set the picture brightness and color to maximum. If you see any bleeding or smearing to the right of bright objects don't buy it. * When you first turn it on the picture should look normal in well under a minute. If it is dim, tinted, or blurry for more than a minute or two the CRT is getting weak. * A B/W picture should not be tinted. * The picture should have decent brightness with the picture at about mid range. Apart form that, if the overall picture is good the CRT is fine. CRTs usually fail very slowly. Even if it's starting to show it's age it probably has several years left. (Portiongs from: Jerry G. (jerryg50@hotmail.com)). You cannot tell the hours used by just looking or even measuring a tube. A tube can go at any time. There are no hour counters! Turn on the unit and see if there is any unusual bleeding of the image in the picture at high contrast levels. When turning the brightness up and down, the color temperature should not change, only the brightness. When turning the contrast up and down, the focus at the center should also be very stable. It may change only a little bit. When turning on the set, the color temperature should be stable within about 3 to 5 minutes. Look at the colors in the corners to see if the purity is good. Bad purity can be attributed to a miss-adjusted yoke assembly, to a bad shadow mask. To know the manufacture date of the unit, it us usually on the back with the model and serial number. Most TV sets are on about 5 to 8 hours a day if it is a family TV. If it is a bedroom TV the hours may be 1/2 that amount. Monitors may be on 24 hours a day - or much less. A good way to know if the emission of the CRT is up to specs is to get a CRT analyzer and measure the gun emission. Some service centers own one.
If performing adjustments of the internal background and/or screen controls still results in a dark picture even after a long warmup period (and the controls are having an effect - they are not faulty), the CRT may simply be near the end of its useful life. In the old days of TVs with short lived CRTs, the CRT brightener was a common item (sold in every corner drugstore, it seemed!). First confirm that the filaments are running at the correct voltage - there could be a marginal connection or bad resistor or capacitor in the filament power supply. Since this is usually derived from the flyback, it may not be possible to measure the (pulsed high frequency) voltage with a DMM but a service manual will probably have a waveform or other test. A visual examination is not a bad way to determine if the filaments are hot enough. They should be a fairly bright orange to yellow color. A dim red or almost dark filament is probably not getting its quota of electrons. It is not be the CRT since all three filaments are wired in parallel and for all three to be defective is very unlikely. If possible, confirm that the video output levels are correct. For cathode driven CRTs, too high a bias voltage will result in a darker than normal picture. CRT brighteners are available from parts suppliers like MCM Electronics. Some of these are designed as isolation transformers as well to deal with heater-to-cathode shorts. You can try a making a brightener. Caution: this may shorten the life of the CRT - possibly quite dramatically (like it will blow in a couple of seconds or minutes). However, if the monitor or TV is otherwise destined for the scrap heap, it is worth a try. The approach is simple: you are going to increase the voltage to the filaments of the electron guns making them run hotter. Hopefully, just hotter enough to increase the brightness without blowing them out. Voltage for the CRT filament is usually obtained from a couple of turns on the flyback transformer. Adding an extra turn will increase the voltage and thus the current making the filaments run hotter. This will also shorten the CRT life - perhaps rather drastically. However, if the monitor was headed for the dumpster anyhow, you have nothing to lose. You can just add a turn to an existing winding or make your own separate filament winding as outlined in the section: "Providing isolation for a CRT H-K short". In some monitors, there is a separate filament supply on the mainboard - this should be obvious once you trace the filament wires from the video driver board). In this case, it still may be possible to increase this output or substitute another supply but a schematic will be required. There are also commercial CRT rejuvenators that supposedly zap the cathodes of the electron guns. A TV or monitor service center may be able to provide this service, though it is, at best, a short term fix.
Where one or more electron guns in the CRT have deteriorated due to wear and tear, it is sometimes possible to give them a new, but possibly, temp