Sony CTV-25R1 Manual page 9

Grid ring five is applied a very high voltage to accelerate the beam so it
comes to a fine point some distance away (at the center of the screen).
The ratio of voltages at G4 and G5 determines where the focus point is
positioned. This focus point forms an arc when the electron beam is
swept from left to right by the deflection yoke.
Unfortunately, the picture tube screen does not match this focus arc, so
the beam will only be in focus at the center of the screen. To correct this
physics problem, the G4 focus grid voltage is modulated with a parabolic
waveform (shape is like a bowl) at the horizontal rate. The parabolic
waveform moves the focus points forward so they match the screen.
In a TV, high voltage drops during bright scenes because of heavy cur-
rent demands. When the high voltage applied to grid five drops, the G4-
G5 focus voltage ratio changes. This voltage change causes the focus
point to change during the brightest spots. One method to maintain the
focus voltage ratio is to take both the focus voltage for G4 and high
voltage for G5 from the same flyback secondary transformer winding. If
G5's high voltage drops, so does the G4 focus voltage. The focus volt-
age ratio and picture focus are maintained during bright scene changes.
5
The focus is customarily adjusted for sharpness when snow (no station)
is present. The rapid changes from black to white when snow is dis-
played on the screen are the most taxing on the high voltage system.
Setting the focus under these dynamic conditions will insure a well fo-
cused picture within the normal viewing range.
Convergence Plates
The Trinitron gun uses one gun, which three electron beams pass through.
These three beams diverge as they pass out of the gun. Electrostatic
convergence plates bend the outer electron beams back so they land
adjacent to the center electron beam on the corresponding red and blue
phosphors.
Four convergence plates are used to bend the outer electron beams.
The two center plates are connected to the flyback generated high volt-
age. The two outer plates are connected to a voltage a few hundred
volts less than the high voltage. A variable resistor (CV) external to the
picture tube determines the exact voltage.
As the outer two electron beams pass through the convergence plates,
they are bent (attracted) inward toward the higher voltage plate. Adjust-
ing the CV control changes the voltage to the outer convergence plates.
The deflection angle of the outer beams can be changed so they con-
verge and pass through the same aperture grill slot by the screen as the
center green beam. After the beams pass the aperture grill, they diverge
to land on their corresponding red, green and blue phosphors to produce
a white dot.
An incorrect adjustment of this CV control causes the outer beams to
pass through other slots in the aperture grill. The outer beams will pro-
duce a red and blue dot near the green one instead of a single white dot.
There is no CV control in newer Sony TV sets. The CV control end of the
picture tube's high voltage resistor is grounded so there is still a differ-
ence in convergence plate potiential. Plastic rings with tabs called "V-
Stat" control permit you to magnetically perform the same static conver-
gence as the CV control. These plastic rings are located at the back of
the yoke and contain a few small pieces of metal molded into the plastic.
This metal alters the yoke's magnetic field for beam convergance.