Sony CTV-25R1, Manual

Page: 11 / 108

7
The Trinitron Screen
In front of the electron gun are the:
•
Deflection Yoke
•
Aperture Grill (AG)
•
Phosphor Stripes
•
Getter Assembly
Deflection Yoke
The yoke consists of two coils of wire mounted on the glass bell of the
picture tube in front of the (internal) convergence plates. One coil gener-
ates a magnetic field to move the electron beams in the X-axis and the
other coil moves the beams in the Y-axis. Guided by the deflection yoke,
three electron beams first sweep across the aperture grill along the X-
axis from left to right (from the front as you would watch TV). At the end
of the horizontal sweep, the beam retraces back to the left side of the
screen. Meanwhile the yoke’s magnetic field moves the three beams
down (Y-axis) one line before the beams sweep horizontally across the
aperture grill again. This process then repeats. Finally, at the bottom
right corner of the picture, the beams are returned to the top left corner of
the screen.
The deflection yoke has difficulty providing a magnetic field to sweep the
beam so it matches the screen shape. The yoke’s magnetic field is stron-
ger at the corners of the picture then at the top/bottom and sides (X & Y-
axis).
Improvements in deflection yoke construction have compensated for the
reduced top/bottom deflection (Y-axis). Along the X-axis, the weaker
magnetic field causes the picture to look like an hourglass. This is be-
cause there is insufficient picture scan, which produces a dark area at
the left and right sides of the picture tube.
Increasing the current through the horizontal windings of the yoke com-
pensates for this hourglass picture. The yoke current is then gradually
increased line by line until the middle of the picture for maximum width ,
the curve is reduced as the beam continues to scan downward. The
result is a straight picture. This type of yoke distortion to the picture is
called pincushion distortion. The correction circuit that changes the yoke
current is called the pincushion stage.
Aperture Grill Construction
The aperture grill (AG) is an aluminum panel located behind the picture
tube screen with vertical slits cut out. The aperture grill is welded to a
steel frame that holds it completely flat in the vertical direction and curved
in the horizontal direction. Consequently, the resultant picture tube face
shape is like the front of a cylinder. This flatter surface reflects less room
light and, therefore, produces fewer glares from the ambient light. This
is another feature that sets the Trinitron apart from other picture tubes
that are spherical in shape.
Although the grill is held flat, it still can move slightly, especially in larger
tubes. In larger tubes, there are two horizontal wires that run across
equidistant across the grill, preventing the slots from vibrating or shifting.
These two wires found in the grill are called anti-vibration damper wires.
Aperture Grill Purpose
In diagram 5, the slits in the aper-
ture grill allow the electron beam to
pass through and land on the phos-
phor. The electron beam meant to
land on the green phosphor is
shown: