Toshiba TW40F80 User Manual
Page 70
70
Amplitude Correction
To vary horizontal amplitude, it is necessary to vary a
sawtooth wave current flowing into the deflection coil. These
are two methods to vary the current; a method which varies
L
H
by connecting a variable inductance L in series with the
deflection yoke, and a method which varies power supply
voltage (across S-character capacitor) for the deflection yoke.
As the DPC circuits is used in the this chassis, the later
method which varies the deflection yoke power supply volt-
age by modifying the bus data is used.
4-1-2. Linearity Correction (LIN)
(1)
S-curve Correction (S Capacitor)
Pictures are expanded at left and right ends of the screen
even if a sawtooth current with good linearity flows in
the deflection coil when deflection angle of a picture
tube increases. This is because projected image sizes
on the screen are different at screen center area and
the circumference area as shown in Fig. 9-7. To sup-
press this expansion at the screen circumference, it is
necessary to set the deflection angle
q
1
to a large value
(rapidly deflecting the electron beam) at the screen
center area, and to set the deflection angle
q
2
to a small
value (scanning the electron beam slowly) at the cir-
cumference area as shown in Fig. 9-7.
In the horizontal output circuit shown in Fig. 9-8, ca-
pacitor C
S
connected in series with the deflection coil
L
H
is to block DC current. By properly selecting the
value of C
S
and by generating a parabolic voltage de-
veloped by integrating the deflection coild current
across the S capacitor, and by varying the deflection
yoke voltage with the voltage, the scanning speed is
decreased at beginning and end of the scanning, and
increased at center area of the screen. The S curve cor-
rection is carried out in this way, thereby obtaining
pictures with good linearity.
Fig. 9-7
Fig. 9-8
θ
2
θ
1
t2
t1
t2 = t1
θ
2
θ
1
<
θ
2
θ
1
t2
t1
t2 > t1
θ
2
θ
1
=
(a) S-character correction (b)
TR
D Co
Cs
L
H
Deflection coil
(a) H output circuit
(b) Sawtooth wave current
(c) Voltage across LH
Fast deflection
Slow deflection
(d) Synthesized current
Vcc