Sony CTV-25R1 Manual page 7

Five Grid Structures
The electron gun consists of the cathode and several metal rings called
grids. The grid name came from the controlling grids in a vacuum tube
where the interelectrode elements were originally shaped like a screen
mesh. There is no structural resemblance between the picture and
vacuum tube grids. In the picture tube, the grid rings are applied differ-
ent electrical potentials to focus (shape) and accelerate (speed) the elec-
trons from the cathodes. Each part of the gun has a voltage applied to it
for a specific purpose.
Trinitron Electron Gun Parts
Name
Filament / Heater Brings the cathode to
(Inside the cathode emission temperature.
assembly).
Cathode assembly Houses the electron
emitting chemical and the
heater.
Grid 1 / Control Grid Reference potential for
cathode emission.
Grid 2 / Screen Grid Brightness limit.
Grid 3 / Accelerating Accelerate stream.
Grid 4 / Focus Sets focus point.
Grid 5 / Accelerating Post Accelerating.
*FBT = Flyback Transformer in consumer TVs. Transformer is assembled with
rectifiers in a doubler or tripler configuration to develop 25-35kV of DC. The high
voltage is used in the picture tube to accelerate electrons. Larger picture tubes
require a higher high voltage from the FBT to move the electrons a longer distance.
Control Grid One
Electrons at the cathode are attracted to a positive potential. The grid
one ring is the next electrode structure in the electron gun. In consumer
TVs, grid one is fixed at a 0Vdc potential for reference. If a positive
potential were placed on the cathode, electrons would not be attracted
to grid one. At a +200Vdc potential, electrons would not leave the cath-
ode. If no electrons enter the gun, the TV screen is dark. The picture
tube is said to be in "cutoff".
Purpose Applied Voltage
Test voltage = 6 Vdc @
0.64 Amp. Actual voltage
= FBT pulse, 6V rms.
R, G or B signal voltage.
+200V= Cutoff. No
electrons output.
0V = All electrons output.
Ground via current
limiting resistor.
400Vdc (approx.)
HV from FBT*.
300Vdc (approx.)
Focus Control range
= 200Vdc to 1kVdc.
HV from FBT*.
3
On the other hand, placing the cathode at the same potential as grid one
(0Vdc) is the same as if grid one were invisible. The maximum amount of
electrons is attracted toward the positive grid two structure, resulting in
maximum screen brightness.
By varying the voltage at the cathode from 0 Vdc to +200Vdc (cutoff), the
amount of electrons available to the gun structure to produce screen
brightness can be controlled.
Screen Grid Two
The higher voltage at the second grid ring accelerates the electron beam.
This voltage is connected to the TV circuitry's "screen control". The screen
control is adjusted to limit the maximum beam acceleration (brightness).
This limit avoids over driving the tube which shortens the life of the cath-
odes and phosphors.
Acceleration Grid Three
The very highest voltage in a TV (from the flyback transformer) is applied
to the third grid ring. This creates a large magnetic field to further accel-
erate the three electron streams from the cathodes.
Focus Grid Four and Acceleration Grid Five
The lower voltage at focus grid ring four slows down the electron stream
so they bunch up, thickening the beams. By varying the focus voltage
applied to grid four, the beam thickness is controlled. A thicker beam
means the electron stream will focus at a point closer to the gun (before
the screen).
When a very bright spot is called for, more electrons are sent from the
cathodes. As a result the beam is at its thickest at the G4 focus ring. In
a small electron gun, the G4 focus ring is closer to the thick beam than
the single Trinitron gun where the focus ring is much larger.
Grid four's magnetic field is the strongest at the metal grid ring. More
peripheral electrons are attracted to the focus grid ring of the smaller
electron gun. Some of these peripheral electrons are lost from the stream
as G4 grid current, limiting beam thickness. The limited beam thickness
results in a shift in focus point. This causes reduced focus during bright-
ness peaks in the smaller electron gun. Therefore, at high brightness
levels it is advantageous to have a large G4 focus structure. A wider
dynamic focus range is achievable with the large single Trinitron gun.