Preface

Author and Copyright

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Copyright &copy 1994-2003

All Rights Reserved

Reproduction of this document in whole or in part is permitted if both of the
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DISCLAIMER

We will not be responsible for damage to equipment, your ego, blown parts,
county wide power outages, spontaneously generated mini (or larger) black
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of this material.

Acknowledgements

Special thanks to Bob Myers
(myers@fc.hp.com) and Jeroen Stessen
(Jeroen.Stessen@philips.com)
for their contributions to this document through their newsgroup postings and
private email.

Introduction

Scope of This Document

Most new CRT related information originating on the
sci.electronics.repair,
comp.sys.ibm.pc.hardware.video,
or other USENET newsgroups will be included here rather than in those other
documents.

Related Documents

• Safety Guidelines for High Voltage and/or Line Powered Equipment.
  
• Notes on the Troubleshooting and Repair of Computer and Video Monitors.
  
• Notes on the Troubleshooting and Repair of Television Sets.
  
• Performance Testing of Computer and Video Monitors.
• Notes on Approaches to using Fixed Frequency Monitors on PCs.
  
• Notes on Video Conversion.
  

Additional Information on CRTs

(From: David Moisan (dmoisan@shore.net).)

I’ve seen a few such pictures and I was fortunate enough to find a book on
color CRTs that explained quite a few things:

Color Television Picture Tubes

Morell, Law, Ramberg, Harold

ISBN 0-12-022151-0.

If you are lucky enough to see “The Secret Life of Machines” on The
Learning Channel (or was, last time I saw it), there’s an episode on
the secret life of the TV. It’s excellent! The creator and
presenter, Tim Hunkin, has a weird sense of humor but he’s very well
informed and quite gifted in the way he demonstrates
difficult-to-explain concepts. In the opening scene, he showed off a
TV that he sawed in half, showing the CRT construction very clearly.
(He must have let the air into the tube, then used a diamond saw to cut
it; that’s the only way it could be done without glass everywhere!)

(Of course, he may not actually have cut a TV in half - manufacturers
no doubt maintain props of this sort!)

CRT Safety Issues

Electrical Safety

There are two areas which have particularly nasty electrical dangers: the
non-isolated line power supply and the CRT high voltage.

Major parts of nearly all modern TVs and many computer monitors are directly
connected to the AC line - there is no power transformer to provide the
essential barrier for safety and to minimize the risk of equipment damage.
In the majority of designs, the live parts of the TV or monitor are limited
to the AC input and line filter, degauss circuit, bridge rectifier and main
filter capacitor(s), low voltage (B+) regulator (if any), horizontal output
transistor and primary side of the flyback (LOPT) transformer, and parts
of the startup circuit and standby power supply. The flyback generates most
of the other voltages used in the unit and provides an isolation barrier so
that the signal circuits are not line connected and safer.

Since a bridge rectifier is generally used in the power supply, both
directions of the polarized plug result in dangerous conditions and an
isolation transformer really should be used - to protect you, your test
equipment, and the TV, from serious damage. Some TVs do not have any
isolation barrier whatsoever - the entire chassis is live. These are
particularly nasty.

The high voltage to the CRT, while 200 times greater than the line input,
is not nearly as dangerous for several reasons. First, it is present in a
very limited area of the TV or monitor - from the output of the flyback
to the CRT anode via the fat 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.

Safe Discharging of Capacitors in TVs and Video Monitors

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

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

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

Obviously, make sure that you are well insulated!

• For the main capacitors in a switching power supply which might be 100 uF at 350 V this would mean a 5K 10W resistor. RC=.5 second.
  5RC=2.5 seconds. A lower wattage resistor can be used since the total
  energy in not that great. If you want to be more high tech, you can
  build the capacitor discharge circuit outlined in the companion
  document: Capacitor
  Testing, Safe Discharging, and Other Related Information.
  This provides a visible indication of remaining charge and polarity.

• For the CRT, use a high wattage (not for power but to hold off the high voltage which could jump across a tiny 1/4 watt job) resistor of a few
  M ohms discharged to the chassis ground connected to the outside of the
  CRT - NOT SIGNAL GROUND ON THE MAIN BOARD as you may damage sensitive
  circuitry. The time constant is very short - a ms or so. However, repeat
  a few times to be sure. (Using a shorting clip lead may not be a bad idea
  as well while working on the equipment - there have been too many stories
  of painful experiences from charge developing for whatever reasons ready
  to bite when the HV lead is reconnected.) 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.
  

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

1. It will not destroy screwdrivers and capacitor terminals.

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

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

Additional Information on Discharging CRTs

BTW, don’t wash your CRTs even if the Maid complains about the filth until you
have confirmed that your ‘Dag isn’t water soluble (maybe that’s why it has
‘aqua’ in the name!). It may all come off! Wipe off the dirt and dust with
a cloth (and stay away from the HV connector or make sure it is discharged
first!).

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

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

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

Warning about disconnecting CRT neck board

This is probably not a problem on small CRTs but for large ones with high
high voltages and high deflection angles where the glass of the neck is
very thin to allow for maximum deflection sensitivity, the potential does
exist for arcing through the glass to the yoke to occur, destroying the CRT.

There is really no way to know which models will self destruct but it
should be possible to avoid such a disaster by providing a temporary return
path to the DAG ground of the CRT (NOT SIGNAL GROUND!!) via the focus or G2
pins preferably through a high value high voltage rated resistor just in
case one of these is shorted.

This probably applies mostly to large direct-view TVs since they use high
deflection angle CRTs but it won’t hurt to take appropriate precautions with
video and computer monitors as well.

CRT Implosion Risk?

(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)

I have checked with our CRT expert and he thinks that any ‘normal’ type of
scratch does not pose any danger. Usual disclaimer applies … (what is
‘normal’?)

The front of the tube is much thicker and stronger than the rear. It has to
be, to withstand the air pressure, because the curvature radius is so much
larger. You won’t break it by throwing a slipper at it. The neck is in fact
very easy to break, usually without causing injuries to anyone.

Normally, if the tube should implode, the rimband (the tensioned steel band
around the rim of all modern CRTs of any size) prevents the glass from flying
outward too far. Every tube type has to pass tests in which it is deliberately
imploded and it is checked whether any large shrapnel flies too far out.

What is very dangerous is a CRT with its rimband missing, or a CRT which
never had a decent rimband in the first place (like some dubious Russian-made
samples we once saw). Such a tube should not be handled at all. NEVER ever
attempt to remove the rimband for and reason!

I just saw a picture tube that was broken due to dropping the (entire) TV on
one corner. In the cone (the backside) there are open cracks of some 3 feet
length in total. Nevertheless all the glass is still in its original place
and it looks as if no glass has flown outward. The faceplate is still intact.
So in this case nobody would have got hurt. I remember reading about Americans
(who else?) who tried to shoot CRT’s with smaller rifles, with little or no
success.

Does this comfort you? Get out the shotgun and have a go at it!

Or, perhaps, the following:

(From: Ren Tescher (ren@rap.ucar.edu).)

Our 6 month old 20″ SGI color monitor (model GDM-20D11) lost a fight with a
fork lift. The case is intact, the CRT probably still has a vacuum, but the
outer layer of glass on the screen is shattered.

Picture Tube Implosion IS Possible - But You Really Need To Work at It!

“I heard somewhere that in the early days of TV, the tubes had a tendency to
implode at the drop of a hat. (Due to poor design?) In order to prevent flying
glass, the sets had a plastic sheet in front of the screen. Obviously, modern
sets no longer have this. How safe are modern CRT screens in terms of impact
damage etc?”

(From: Dan Evens (dan.evens@hydro.on.ca).)

In high school, our electronics teacher did a demo for each class. He saved
out an old black-and-white tube for each class and set up a place to break it.
Put the tube on the ground by a brick wall, with a hammer suspended on a wire
from the top of the wall. Did it on the driveway so that the glass would be
easier to pick up. The tube was placed image-side down.

First he pulled the hammer back about 20 feet and just let it go. It bounced
off the tube. This was to show that such tubes are pretty tough. Then he
pulled the hammer back and gave it a pretty good shove, turning his back to
the tube and moving quickly away from it. (Let’s face it, the guy could
probably have found a safer way to do this.)

Palm sized chunks of glass flew 50 feet. The noise was quite impressive. The
thickness of the image plate of the tube was also quite impressive. Kind of
looked like a porthole on a submarine. This was from the tube of a small
black-and-white TV, about 14 inches or so. One of the larger colour models
might be a LOT more violent.

If I was handling these things in such a way as to have the possibility of
dropping one, I’d insist on body armor and face protection. And if it involves
a picture tube, I insist on competent trained professionals for service.

(From: Matthias Meerwein (Matthias.Meerwein@rt.bosch.de).)

They ARE quite safe. I’ve got several TVs and computer monitors in for
repair that had been dropped. None of them had an imploded CRT. The
damage encountered ranged from:

• Broken circuit boards, often around the flyback transformer (the most heavy weight part on the board) - This is quite easily repairable.
  

• Shadow mask inside the tube knocked out of position (mostly in trinitron tubes due to their heavy aperture grille construction) - this renders the
  tube (and thus usually the set) a dumpster candidate.
  

• Neck of tube broken of (usually when the set hit the floor back end first) - obviously junk.
  

• 26 inch color TV with back panel removed placed face-down under a bridge. Dropped a ~10 pound brick from top of the bride (about 10 ft high) into the
  glass funnel of the tube. Result: Funnel of tube shattered, faceplate
  intact. All glass shards (most of them rather large) were lying inside the
  set’s cabinet - no flying glass.

• 14 inch B/W computer monitor tube dropped from the second story onto concrete floor, hitting the ground faceplate-first. Result: tube shattered
  into thousands of small glass particles (the largest ones were about one inch
  in size), but all debris was located on one heap - none of them traveled
  farther than about three feet.
  

(From: Clifton T. Sharp Jr. (agent150@spambusters.ml.org).)

With today’s tubes, that’s more or less true (although walking through a
picture tube plant can be instructive as you hear the exploding tubes).
With older tubes it was a hazard. With pre-1960 tubes it was a big one.
My old boss in the TV service, who I trusted not to exaggerate about such
things, told me stories of setting a picture tube near a second-floor
window, having them fall to the sidewalk and literally blow a hole in the
sidewalk. I can tell you factually and first-person that although he took
few precautions with other things, when he had to “pop” a picture tube in
the dumpster he never ever ever did it without safety glasses, a shield
and a six-foot piece of heavy pipe. (I stopped working there around 1973.)

Risks from CRT Scratches?

This is more of a concern for modern CRTs that usually have ‘integral implosion
protection’ - that steel rimband around the outside near the front. Older CRTs
used either (1) a separate safety shield - that laminated glass plate in front
of your grandmom’s TV - or (2) a second contoured glass panel bonded to the
actual tube face. In both of these cases, the second panel is protective
and cosmetic but is not part of the structure of the CRT. Therefore, any
damage to it does not significantly compromise the tube. In the case of modern
CRTs, the steel band in conjunction with the basic tube envelope is used to
maintain the integrity of the overall CRT. In addition should implosion
occur as a result of catastrophic damage, the rimband will reduce the range
and velocity of flying debris.

Also see the section: CRT Implosion Risk?.

BTW, scratches in the CRT have absolutely no effect on X-ray emission.
X-rays are blocked long before they come anywhere near the surface and
glass has very little effect on their direction. Any scratch deep enough
to have any detectable effect on X-ray emission (actually, it would need
to be an inch deep gouge) would have caused the tube to implode.

Disposing of Dead TVs or Monitors (CRTs and Charged HV Capacitors)

Treat the CRT with respect - the implosion hazard should not be minimized.
A large CRT will have over 10 tons of air pressure attempting to crush it.
Wear eye protection whenever dealing with the CRT. Handle the CRT by the
front - not the neck or thin funnel shaped envelope. Don’t just toss it
in the garbage - it is a significant hazard. The vacuum can be safely
released (Let out? Sucked in? What does one do with an unwanted vacuum?)
without spectacular effects by breaking the glass seal in the center of the
CRT socket (may be hidden by the indexing plastic of the socket). Cover the
entire CRT with a heavy blanket when doing this for additional protection.
Once the vacuum is gone, it is just a big glass bottle though there may be
some moderately hazardous materials in the phosphor coatings and of course,
the glass and shadow mask will have many sharp edges if it is broken.

In addition, there could be a nice surprise awaiting anyone disconnecting the
high voltage wire - that CRT capacitance can hold a charge for quite a while.
Since it is being scrapped, a screwdriver under the suction cap HV connector
should suffice.

The main power supply filter caps should have discharged on their own
after any reasonable length of time (measured in terms of minutes, not
days or years).

Of course around here, TVs and monitors (well, wishful thinking as I
have yet to see a decent monitor on the curb) are just tossed intact
which is fortunate for scavengers like me who would not be happy at
all with pre-safed equipment of this type!

(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)

We have a procedure for disposing of used CRT’s. The vacuum must be broken to
avoid future implosion, like when it will be crushed by the dumpster truck
press. That’s NOT funny! One method is to punch or drill a small hole in the
anode contact, which is made of a soft metal. But take care of the electrical
discharge of the aquadag capacitance first!!!

The other method is to break the stem in the centre of the socket pins. This
is the stem through which the tube was pumped empty during manufacturing. It
breaks off easily (after you have removed the plastic part around the pins).

You want to avoid making too large holes, like for example from chopping off
the entire neck in one blow with a hammer.

General CRT Construction and Characteristics

Why is the CRT Still Dominant?

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.

Comparison of CRT Types

“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.”

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
  

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.

Color CRT Construction

• High Tech Tubes, Popular Mechanics, April 1997.
  
• Display.

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).

Assembly of Color CRTs

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.
  

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 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.

CRT Fine Tuning

Once the CRT is sealed, baked, evacuated, etc., the job is not yet done!

(From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

They still need to match the finished tube with a deflection coil that will
give adequate purity performance and then they need to fiddle with magnets
(multipole rings around the neck and sometimes other magnets all over the
cone) to improve it further. And even then many tubes need active correction
for convergence and/or geometry.

Only after all that correction can you call the yield high. (But you should
see their scrap yard, good thing that glass recycles well…)

Northern/Southern Hemisphere Corrections and Adjustments

For monochrome monitors and B/W TVs, this will result only in a slight shift
in position or rotation of the picture depending on the orientation of the
CRT with respect to the earth’s magnetic field. For the most part such
effects will not be significant enough to be objectionable.

However, for high resolution color monitors and even some color TVs, the
result of transporting the unit from the hemisphere from which it was
manufactured or set up to a location in the opposite hemisphere may be
uncorrectable purity problems or excessive sensitivity to local magnetic
fields.

Note that is it quite possible that you will never encounter any of these
problems. The extent to which your particular monitor or TV is affected
depends on many factors - many of which you have no control over.

(From: Bob Myers (myers@fc.hp.com).)

For many monitors - especially the larger sizes, such as 21″ - there is a
subtle difference in the CRT itself which may mean that a unit with the
wrong tube could NOT be adjusted to be within specifications when used in
the ‘wrong’ hemisphere.

(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)

There are two types of adjustments:

• 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.

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.

Tubes for All Nations

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.

So What Does It Mean to Have a Trinitron CRT?

You can recognize a Trinitron tube by the fact that the picture is made
up of fine vertical stripes of red, green, and blue rather than dots or
slots. The shadow mask in all other kinds of common CRTs are made up of
either dots (nearly all good non-Trinitron computer monitors) or slots
(many television sets). The Trinitron equivalent is called an aperture
grill and is made of around a thousand vertical wires under tension a
fraction of an inch behind the glass faceplate with its phosphor stripes.

Several photos of a disemboweled Trinitron aperture grille can be found at
James Sweet’s Sony/Trinitron
Directory along with some screen shots showing the symptoms resulting
from a monitor falling on its face.

Since the aperture grill wires run the full height of the tube, there
are 1 or 2 stabilizing wires to minimize vibration and distortion of the
aperture grill. These may be seen by looking closely 1/3 and/or 2/3 of the
way down the tube. The larger size tubes will have 2 while those under 17
inch (I think) will only have a single wire. Many have complained about
these or asked if they are defects - no they are apparently needed.
You can be sure that Sony would have eliminated them if it were possible.

Another noticeable characteristic of Trinitrons is the nearly cylindrical
faceplate. The radius in the vertical direction is very large compared
to the horizontal. This is both a requirement and a feature. Since the
aperture grill wires are under tension, they cannot follow the curve
of the glass as a normal shadow mask may. Therefore, the glass must be
flat or nearly flat in the vertical direction. As a selling point, this
is also an attractive shape.

In the final analysis, the ultimate image quality on a monitor depends
as much on other factors as on the CRT. There are many fine monitors
that do not use Trinitrons as well as many not-so-great monitors which
do use Trinitron tubes.

Why are There Fine Lines Across My Trinitron Monitor or TV?

All Trinitron (or clone) CRTs - tubes that use an aperture grille - require
1, 2, or 3 very fine wires across the screen to stabilize the array of
vertical wires in the aperture grille. Without these, the display would
be very sensitive to any shock or vibration and result in visible shimmering
or rippling. (In fact, even with these stabilizing wires, you can usually
see this shimmering if you whack a Trinitron monitor.) The lines you see
are the shadows cast by these fine wires.

The number of wires depends on the size of the screen. Below 15″ there
is usually a single wire; between 15″ and 21″ there are usually 2 wires;
above 21″ there may be 3 wires.

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.

Differences between Trinitron and Diamondtron CRTs

Mitsubishi makes the Diamondtron under license from Sony - the subtle
differences (according to Mitsubishi) are improvements in the electron
gun design for spot uniformity over the CRT face. Also, for the time
being, Mitsubishi has tried to introduce Diamondtron tubes in sizes which
are not available as Trinitrons - to keep from directly competing, and
(ostensibly) to address niches which other sizes can’t address.

In order to properly evaluate a monitor, one must consider more than the
tube alone - as many readers know, Trinitrons are finding their way into
various manufacturer’s sets, but they don’t all perform the same. In todays
market, it’s quite possible to find a dot mask design which performs as well
as (or better in some cases) the aperture grill design - IMHO every critical
monitor purchase should be made by personally examining the monitor to be
bought, under the intended application(s).

(BTW, all color tubes use 3 guns, including the Trinitron. Sony used to
talk about a “unitized gun”, but that only refers to the cathode structure.
It’s classical use of a misleading term to gain market awareness (looks like
it works).)

(From: Someone who wishes to remain anonymous.)

I have found other differences between the Trinitron and Diamondtron tubes.
Most noticeable is the grill pitch. The 21″ Sony GDM-F520 is 0.22 mm. The
22″ Mitsubishi (Cornerstone P1750) is 0.25 mm. For high resolution screens,
this makes a difference.

I have also noticed that in a room full of Dell Trinitron monitors, no two
monitors have the same color. This is not just a setup issue, the actual
tubes have different colors when they are off. The darkness of the black
changes.

My gut feeling is that the Dells use a Mitsubishi tube, and that the
quality control is not up to Sony’s. It is just a feeling, I have not done
any research on this.

From what little I know, if you want the very best, you will have to pay
for it, (or you get what you pay for).

Some History of In-Line Gun CRTs

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.

How Far is the Shadow Mask from the Phosphor Screen?

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

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 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.

How is the Shadow Mask Mounted Inside the CRT?

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.

Why is the Shadow Mask or Aperture Grill Made of a Magnetic Material?

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.

Why do CRTs Use Red, Green, and Blue rather than Red, Yellow, Blue?

Nearly any color that we can perceive can be made from some combination of
primary colors. There are two types - additive and subtractive.

RGB are primary additive colors - anything that emits light will use these.

The three types of cone (color) recepters in the retina of the human eye
have peaks (roughly) sensitive to these primary colors.

Those red, yellow, and blue primaries you used to create your works of art
should actually not have been red, yellow, blue but rather magenta, yellow,
cyan - close but no cigar. Red, yellow, and blue are approximations good
enough for basic painting or printing but are not capable of reproducing
the widest range of colors.

CMY (cyan, magenta, yellow) are subtractive colors. Printing processes
and color photography use these because layers of ink or dye absorb light.
Basically, each of CMY removes a single color from (RGB).

• Cyan = (green+blue) and is the complement of red.
• Magenta = (red+blue) and is the complement of green.
• Yellow = (red+green) and is the complement of blue.

Purpose of a Separate CRT Faceplate

Warning: A CRT that is supposed to have a rimband but where it is missing
or damaged is a serious hazard since the possibility of implosion is greatly
increased and the effects of such an implosion will be more severe. However,
such a situation is virtually impossible to occur on its own since the rimband
is part of the mounting bracket assembly. Don’t be tempted to remove the
rimband for any reason unless the vacuum has been let out (in, whatever one
does with a vacuum) of the CRT! Spontaneous implosion is even possible. See
below for an example.

In some cases, there will be a separate faceplate. Older TVs usually had
either a totally separate laminated glass plate in front of the CRT or a
contoured glass panel bonded (glued) to the CRT itself. Part of its purpose
is protective. It would prevent damage to the CRT in the event of a blow
from a thrown object like an ashtray or shoe! In addition, it would contain
the debrie in the unlikely event of an implosion resulting from some really
catastrophic event.

However, the separate or bonded glass plate can also be used for cosmetic
purposes to:

• Improve contrast in a bright light by using a tinted glass.

• Reduce reflection by using an anti-reflection coating.

• Iron out the bumps by using a glass plate smoother than the CRT.

• Give the impression of a flatter display by using a glass plate with a larger radius of curvature than the CRT itself.
  

• Give the impression of a Sony Trinitron by using a cylindrical (plastic) plate in front of a real-flat rear-projection screen.
  

I got my User ID from the metal band. Anyway, a friend of mine decided to
cut the rimband off a picture tube. I wasn’t there, he told me about it. This
was a 25″ RCA tube he wanted to fit into a Zenith TV (don’t ask me why). What
happened in the next few seconds after he cut the rimband, the picture tube
imploded in his face, embedding the neck and yoke assembly in the ceiling, he
came out with a cut about half an inch above his right eye that needed 6
stitches to close. Had that shard of glass been half an inch lower, he would
be wearing an eye patch or have a glass eye for the rest of his life.

I told him what an idiot he was, he’s lucky he didn’t kill himself or blind
himself, and also told him NEVER cut the rimband off a picture tube that has
vacuum. I just wanted to add that!:)

Leaded Glass and CRT Coatings

“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?”

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.

Flat Versus Non-Flat CRTs

• Geometry correction: As the electron beam scans across a flat faceplate, its velocity increased near the edges and corners. Without
  compensation, the pixels will be stretched significantly in these areas.
  

• Brightness uniformity: Likewise, this means less time on each phosphor dot and lower brightness. In addition, the electron beam hits
  the screen at an increasingly steep angle which further decreases the
  brightness for a fixed dot size.
  

• Structural integrity: A totally flat faceplate would have to be much thicker to withstand the force due to the atmosphere with respect to
  the vacuum inside. So, most “flat” CRTs will still have a slight spherical
  shape.
  

Compensation for the geometry and brightness problems becomes much more
challenging and it’s never perfect. Even a well adjusted CRT will often
have a very detectable, if not obvious, variation in brightness from center
to edges and corners. Scan linearity and pincushion correction require
most complex and carefully adjusted circuits. The thicker faceplate means
a heavier CRT and monitor.

The net effect is that for a given screen size, cost will be greater. At a
normal viewing distance, the perceived advantages may be minimal. Some people
may find (after having gotten used to a moderately spherical CRT) that they
actually like a flat one less especially if the deficiencies are
easily seen. Note that Sony Trinitron (and clone) CRTs are nearly flat in
the vertical direction and curved in the horizontal direction. To get used
to this geometry may take some time as well.

Resolution, Dot Pitche, and Other CRT Specifications

Color CRT Resolution - Focus and Dot/Slot/Line Pitch

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:

1. 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.
   

2. 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.
   

3. 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.
   

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.

A Discussion of Issues Relating to Monitor and CRT Resolution

(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:

• 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.
  

• 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.
  

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).)

• The monitor aspect ratio (AR). This is 4:3 (or 1.33:1) for any CRT you are likely to be using.
  

• 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.
  

(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 × 1200 on a 17″ 0.28 mm CRT. But 1280 × 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’.

About the Quality of Monitor Focus

“I have 2 identical monitors. One is razor sharp from edge to edge. The
other is blurred at the corners- not from convergence problems, but just
plain out of focus. In this monitor, the focus adjustment on the flyback
can improve the focus at the edges, but then the center of the screen
becomes worse..My question is : Is this a problem in the electronics and
presumably a fixable flaw or is it caused by variance in the picture tube
itself and not correctable ? Or is it some other issue?”

The adjustment on the flyback sets the “static” focus voltage, which is
a DC voltage applied to the focus electrode in the CRT. However, a single
fixed focus voltage will not give you the best focus across the whole CRT
screen, for the simple reason that the distance from the gun to the screen
is different at the screen center than it is in the corners. (The beam
SHAPE is basically different in the corners, too, since the beam strikes
the screen at an angle there, but that’s another story.) To compensate
for this, most monitors include at least some form of “dynamic” focus, which
varies the focus voltage as the image is scanned. The controls for the
dynamic focus adjustment will be located elsewhere in the monitor, and
will probably have at LEAST three adjustments which may to some degree
interact with one another. Your best bet, short of having a service
tech adjust it for you, would be to get the service manual for the unit
in question.

It is also possible that the dynamic focus circuitry has failed, leaving
only the static focus adjust.

As always, DO NOT attempt any servicing of a CRT display unless you are
familiar with the correct procedures for SAFELY working on high-voltage
equipment. The voltages in even the smallest CRT monitor can be lethal.

How to Compute Effective 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 — B + R — G — B

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.
  

Dot Pitch of TV CRTs

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 inch GE - .60 mm.
• 19 inch Samsung - .75 mm.
• 25 inch RCA - .9 mm.

CRT Aspect Ratio

(From: Bob Myers (myers@fc.hp.com).)

The physical shape of the tubes themselves came through this evolution,
but the aspect ratio assumed for the original transmission format specs
WAS 4:3, as driven by Hollywood. Where did you think the shape of the masks
came from?

The desired 4:3 aspect ratio standard was known right from the start, and
the early TV designers DID realize that the use of round tubes to display
this was a literal case of a “square peg in a round hole”. Rectangular
CRTs for TV use had been developed as early as 1939, with the first
American rectangular tube to enter production in late 1949.

(See Peter Keller’s very excellent “The Cathode Ray Tube: Technology, History,
and Applications” for all the details.)

CRT Deflection Angle

This is the maximum angle the beam makes with respect to the gun axis to fill
the screen.

• All other factors being equal, a 110 degree tube is shorter than a 90 degree tube. This is the principle advantage of higher deflection angle
  CRTs.
  

• High deflection angles means higher deflection power for a given accelerating (2nd anode) voltage.
  

• Higher deflection angle CRTs are more difficult to converge and maintain focus, purity, and uniform brightness across the screen. Reducing geometric
  errors is more challenging. Yoke design is also trickier.
  

CRT Contrast Ratio

Apparently CRTs have made quite an increase lately. Years ago when I
looked into it, CRTs were not much better than about 20:1. Now, folks
are claiming well over 40:1.

One thing to watch, though. The phosphor has two decay curves, a rapid
one followed by a slow one. A change in scene can lower contrast ratio
of a bar chart that appears in a region that was a large white area.

(From: Steve Eckhardt (skeckhardt@mmm.com).)

This comment makes me curious about the claims made by manufacturers of video
projectors. Visually, their images have lots of resolution but mediocre
contrast at large scale. A video monitor looks a lot better in contrast. The
manufacturers, however, claim contrast ratios of 100:1 or better.

Are the numbers simply marketing hyperbole or have I missed something
interesting?

There are several methods for arriving at the advertised numbers for contrast.
One is to simply advertise the number for the imager and ignore the
degradation due to the rest of the system. Another is to m