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The Trinitron
The Cathode Ray Tube (CRT) has been slowly changing since its con-
ception about 50 years ago. Since then the emitter, accelerator and
focus structures at the "gun" end have been added to the vacuum tube
to shape and control the amount of electrons from the gun.
At the target end of the CRT, the luminescent screen is made of a phos-
phor mixture. Phosphor glows white when struck by electrons. Phos-
phor brightness is directly proportional to the amount of electrons that
strike the phosphor. The CRT sport brightness was controllable with a
gun and phosphor screen.
The electron beam produced a spot of light that was stationary on the
phosphor screen. Placing an electromagnetic field near the electron
beam after it left the gun created movement. The spot intensity and
location were now controllable and the CRT became known as the pic-
ture tube.
To produce a color picture on the CRT screen; three independent gun
structures are used. The electron guns produce different amounts of
electrons targeted to their corresponding Red, Green and Blue phos-
phors. Red, Green and Blue are the primary colors for light.
In 1968 the Sony Trinitron picture tube was a departure from the tradi-
tional three-gun color picture tube. Three major changes to the old color
tube created a distinctive Trinitron picture tube:
1. Instead of three small electron guns, focus was improved using one
large electron gun structure that all three beams pass through.
2. Electrostatic convergence plates were added to bend the outer elec-
tron beams so they would land on the corresponding red and blue
color phosphor.
3. A continuous vertical slotted aperture grill at the screen end that:
•
Reduces the effects of terrestrial magnetism.
•
Prevents adjacent and stray electrons from striking the wrong phos-
phor.
•
Allows more electrons to pass, increasing brightness without short-
ening life.
•
Results in a flat screen. This reduces annoying room light reflections
(glare).
®
Picture Tube
1
The remainder of this document is divided into four sections explaining
the construction of Trinitron tube as an aid to the service technician:
•
The Trinitron Electron Gun Operation
•
The Trinitron Screen
•
Picture Tube Defect Symptoms
•
Picture Tube Handling and Vacuum Disposal
The Trinitron Electron Gun Operation
The Sony Trinitron electron gun consists of three cathode assemblies,
five grid structures and convergence plates:
Three cathode assemblies
When heated, electrons are given off from a Barium Carbonate (BaCO
surface deposited onto a cap. The cap serves as a holder for the BaCO
white mixture. The cap is fastened to a sleeve that houses a heating
element (filament). This assembly is called a cathode.
There are three cathodes in the beginning part of the gun assembly by
the pins of the CRT. They all supply electrons in controlled amounts.
The center cathode on the Trinitron tube produces the amount of elec-
trons that correspond to the green color information. These electrons
will eventually land on the green phosphor if things go well on the jour-
ney. The outer cathodes are angled slightly to send electrons through
the gun structure. Their final targets are the red and blue phosphor at
the screen.
Next a voltage is connected to the cathode (sleeve) and a more positive
voltage to the second grid (two) in the gun structure. This difference in
potential will pull the electrons from the cathode's Barium Carbonate
(BaCO
) surface into the gun structure. The difference in potential volt-
3
age between the cathode and grid two will determine the amount of elec-
trons emitted. More electrons landing on the phosphor (screen) will in-
crease the color spot intensity.
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3
3,
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