Toshiba MM20E45, Техническое учебное пособие

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9-6
1-3 Change of Capacitor
First, FET QH36 (2SK947) is used to change S-character
capacitor. This is necessary because the theory of S-curve
correction utilizes resonance of this capacitor and deflection
yoke.
To get good correction, the capacitor value should be selected
so that deflection current of S-curve can become similar
figures, in such way that resonant frequency is set high when
hor scanning frequency is high, and is set low when low
reversely.
When resonant frequency is too high against scanning
frequency, as shown in Fig. 7 (a), element of S-character
superimposed on deflection current becomes too large, to
cause over correction and to result in shrinkage picture on
screen edge. Contrarily, when resonant frequency is too low,
element of S-character becomes too small, to cause less
correction and to result in stretching on edge (Fig. 7 (b)).
This model, which is multi-scanning of 15kHz to 40kHz,
when scanning frequency is low, turns QH36 on to increase
capacitor value. This causes resonant frequency get down, to
result in good linearity. This change is done around 28kHz,
and the changing signal is supplied from output of frequency
discriminating circuit as mentioned above.
The second is switchover of resonating capacitor. The value
of flyback pulse generated at collector of hor output transistor
varies with the resonant frequency of retracing period. And
this frequency depends on the inductance of deflection yoke,
the primary inductance of transformer TH02 corresponding
to FBT and the resonant capacitor.
The value of this flyback pulse is in proportion to voltage of
power source, and is in reverse proportion to scanning
frequency.
Since this model is multi-scanning, it changes power voltage
corresponding to input signal frequency. To keep size constant
in spite of frequency change, power voltage is raised with
frequency high, and is decreased with frequency low.
Therefore, the flyback pulse becomes similar value even if
the frequency changes.
But the sizes are different remarkably between in TV mode,
and in RGB mode. In RGB mode, the under scan method that
picture screen is smaller than picture tube screen, is employed.
But in TV mode, the over scan method that the screen is
larger than the tube, is usually employed. As a result, in TV
mode, flyback pulse becomes large. And in TV mode, since
it is not necessary to make flyback pulse width narrow, pulse
width is made wide to decrease peak voltage, by turning
QH29 on when frequency is low.
This switchover, the same as S-character capacitor, utilizes
output signal of frequency discriminating circuit, and is
operated on around 28kHz.
In high frequency mode, width of flyback pulse is made
narrow (approx. 4 µs) because blanking period itself of input
signal is short. In high frequency mode, it is made wide
(approx. 10%).
Fig. 7
(a)
(b)