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February 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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Explaining the workings
of the Trinitron
color cathode ray tube (CRT) with black and white pictures is a little like explaining
the third dimension to a Flatlander.
How do you visualize red, green , and blue in shades of gray? It's like being told
to grasp the concept of tesseract being the 3-D projection of a 4-D cube. Still,
that was the challenge author Forest Belt had when writing this article for a 1972
issue of Popular Electronics magazine, an era where multicolor print was
the realm of high-end glossy-page magazines. Those of us who were around in the
days when Sony's Trinitron hit the market remember well the hype that surrounded
it. Of course my parent's B&W television suffered the same handicap as this
printed page when the commercials attempted to demonstrate the Trinitron's color
contrast and resolution superiority over traditional CRTs with a single electron
gun. Once color computer CRTs were available, they also were differentiated between
those that were and those that were not Trinitron types. BTW, where possible I colorized
the images for your convenience.
The Trinitron - Still a Mystery?
 By Forest H. Belt
Despite a rash of advertising, there are still people who don't know what a Trinitron
is. A few think it's some kind of TV set. Others know it's something special from
Sony, but they're not sure what.
The Trinitron is a picture tube for color television. Sony introduced the first
color receiver using a Trinitron several years ago, a 7-inch model that never got
into production. But more recently, both 9- and 12-inch versions have been imported
in quantity. And just this past winter, a 17-inch model appeared in the U.S. The
"large-screen" Trinitron resembles its predecessors, but incorporates definite changes.
Fig. 1 - Standard triad dot pattern (top) and Trinitron
stripe pattern.
Fig. 2 - At top is shadow mask scheme; below, Trinitron
CRT aperture grille.
Fig. 3 - The gun in a Trinitron emits and controls all three
color beams; in ordinary color-TV picture tube, three separate guns are used to
make beams.
Fig. 4 - In Trinitron, the angle of each beam determines
which phosphor it hits. Beam is larger than stripes; so that it hits two at the
same time.
The Trinitron CRT is touted as being simpler than American color tubes. The advantage
isn't all that obvious to a viewer; but inside the cabinet, the Trinitron operates
with comparatively few circuits. For example: Any color set has critical adjustments
to register the primary red, blue, and green rasters accurately on top of each other.
The term is convergence, and the object is to keep colors from fringing black-and-white
pictures. An ordinary color set, American or Japanese, has seventeen convergence
adjustments. A receiver using the Trinitron has only a half-dozen or so.
Credit for simplification goes to the way the Trinitron picture tube is structured.
It differs from the usual picture tube in several important ways: (1) The Trinitron
phosphor screen is deposited in narrow vertical stripes instead of tiny dots. (2)
The mask behind the Trinitron phosphor is a grille of vertical slots instead of
a shadow mask of round holes. (3) The Trinitron emits three beams, one for each
primary color, but from only one electron gun; the ordinary color CRT has three
separate guns. (4) The beams in a Trinitron are side-by-side whereas in a conventional
color tube they are spaced in a triangular relationship. (5) Special plates converge
Trinitron beams electrostatically; a conventional color CRT converges electromagnetically,
with a yoke of coils around the neck of the tube. (The newer large-screen Trinitron
uses a convergence yoke, controlled by only three adjustments.)
Stripes Instead of Dots. First, consider the phosphor. Compare
the two photos in Fig. 1. The dots arranged triangularly belong to the phosphor
of a conventional color tube. Each triad of dots contains one red dot, one green,
and one blue.
The stripes in the other photo are a Trinitron phosphor. Each stripe is one color.
They line up red, green, blue, red, green, blue, and so on.
Either kind of phosphor needs some sort of mask between it and the source of
the beams. Otherwise, any electron beam could "splatter" onto adjacent dots or stripes,
thus activating the wrong color. In the conventional picture tube, a perforated
metal screen (Fig. 2A) confines the size of each beam. Because the metal sheet
"shades" the edges of the beams, the name shadowmask has become accepted.
The Trinitron electron beams get similar treatment. The masking metal screen
(Fig. 2B) contains vertical slots rather than round holes. Sony calls the
mask an aperture grille. The phosphor stripes are very narrow compared to the beams.
Each beam spreads across two slots in the grille. But the angle at which each beam
strikes the grille slot nevertheless directs it to the correct phosphor stripes.
How this works is illustrated in Figs. 3 and 4. Back in the Trinitron electron
gun, the red and blue beams bend, cross, and aim outward, while the green beam remains
straight. As they pass between the convergence plates, the red and blue beams are
directed back toward the center axis. If convergence is correct, all three beams
cross again precisely at the aperture grille.
You can see the effect on individual beams in Fig. 4. The green beam, coming
through straight, strikes the slotted masking grille exactly perpendicular. The
wide beam spreads over two of the aperture slots - and then some - but the metallic
grille blocks all except what can get through those two slots. The aperture grille
positioning lets the green beam straight through to illuminate two green phosphor
stripes.
The red beam, having been bent outward and then back inward, strikes the aperture
grille from an angle. The slots - the same ones that let two green beam-segments
through - pass only enough red beam to illuminate two red stripes of phosphor. The
angle of the red beam just suits the positioning of the red stripes with respect
to the grille openings, and the red beam does not touch green or blue stripes.
The blue beam approaches from the angle opposite the red. The slotted aperture
passes two beams just wide enough, and at the correct angle, to hit only blue phosphor
stripes.
Of course, all the while, the three beams are being swept from side to side and
from top to bottom to form a raster. As you view the front of the Trinitron picture
tube, the picture looks about like it does on any regular color CRT - unless you
inspect it closely. Then you can see the individual lighted vertical stripes, just
as you can see individual phosphor dots on the screen of a conventional color tube
if you look closely.
Three From One. The sketch in Fig. 3 shows the gun structure
of a Trinitron. Conventional color picture tubes have three separate electron guns,
each with its own heater, cathode, grid 1, grid 2, and focus anode. The three heaters
share common base connections, and all three focus anodes go to one base pin. But
the other elements in all three guns have their separate base connections.
In early Trinitrons - the 9- and 12-inch - only the cathodes are separate. Grid
1 is common to all three cathodes, and thus controls the intensity of all three
beams Simultaneously. You can imagine how this trims grid-circuit requirements.
Only one pin connection serves all three beams. And grid 2 circuitry is cut two-thirds
also, since the G2 pin is common to red, green, and blue beams.
Focus circuitry differs from that in conventional color sets, but not in number
of connections. Ordinary color CRT's require several thousand volts for focus. Not
so, the Trinitron. Only a few hundred volts is needed. Focus supply circuitry can
therefore be simple by comparison.
And Easy Adjusting. Beyond the novelty of generating and controlling
three electron beams with only one gun, the Trinitron's most spectacular advantage
lies in convergence. In 9- and 12-inch Trinitrons, special plates control the crossover
point for the three beams. Ideally, crossover occurs precisely at the aperture slot.
A high-voltage dc field between the convergence plates bends the beams accurately
while they are aimed at the center of the phosphor screen. But remember, the beams
are constantly being swept back and forth. To make sure they cross accurately out
near the edges, a special signal voltage (parabolic in shape) also drives the convergence
plates. Timed to fit each horizontal sweep, the parabolic signal adapts convergence
crossover to the curvature of the phosphor screen and aperture grille.
Compared to a typical color tube, the Trinitron converges with extreme simplicity.
Because the phosphor is striped instead of dotted, slight vertical misconvergence
hardly matters. So, adjustments are reduced to a bare minimum. Once mechanical (static)
convergence is adjusted with four neck magnets, a mere two controls shape the parabolic
signal for dynamic convergence. It takes four magnets plus thirteen controls to
converge other color tubes.
A new 17-inch Trinitron gives up some of the simplicity of small-screen models.
For one thing, the electron gun is more complicated. Each cathode has a control
grid (G1) of its own. That necessitates extra circuits, and alterations in the way
video and color signals are fed to the Trinitron. Otherwise, the gun in the larger
Trinitron hasn't been changed much.
Scanning the wider screen also brought problems that necessitate more elaborate
convergence. The plates inside the tube still get high dc for basic convergence,
but no parabolic signal. Instead the 17-inch Trinitron has a small yoke of convergence
coils around the picture tube neck. Even so, adjustments are much simpler than for
conventional picture tubes. A mere half-dozen controls does the entire job.
Whither Color CRT's? The Trinitron is the first drastic change
in picture-tube technology since color television came on the market. It represents
a major step toward making color receivers easier to adjust and repair. Trinitron
chassis represent the start of a design trend to minimize critical stages, which
in the long run eliminates nuisance maladjustment. Other sections of both color
and monochrome receivers could benefit from similar attention.
Meanwhile, what else can be done for picture tubes? The most sought-after innovation
is a flat screen, with little thickness. The Trinitron design boasts short neck
length as well as small neck diameter. But the front-to-back dimension of Trinitron
receivers subtracts little from other designs. Whoever comes up with a picture tube
under 6 inches from screen to socket will have taken a giant step toward the ideal
of a hang-on-the-wall color picture tube.
Posted May 25, 2023
(updated from original post
on 5/29/2018)
Color and Monochrome (B&W) Television
Articles
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