NEC JC-1531 VMA-2, Service Manual

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5-1-5. Parabola Waveform Generating Circuit
The parabola-waveform generating circuit is mainly composed of two blocks (See Fig 5-1-l 0 and Fig
Each block includes a multiplier which has input terminal. In the first block, a parabolic waveform is
generated from the saw tooth wave. The saw tooth wave is generated at pin of and is A C coupled
by to each terminals of
The multiplier of squares the A C coupled saw tooth waveform and produces a parabolic waveform.
Trapezoid distortion is corrected by the voltage applied to pin of through This voltage is
determined by the vertical size and varied by VR404. The amplitude of the parabola waveform has to be
proportional to the
voltage of H deflection. In the
second
block, the transaction of making the amplitude of
the parabola waveform and the amplitude is user control adjustable. +B voltage of H deflection is divided by
R409 and VR403.
The parabola wave and the H deflB are applied to each terminals of
the user control adds to
subtracts from the divided H deflection voltage. The positive or negative user control voltage is obtained
transforming the control voltage from the microprocessor. The +B H deflection voltage varies as the H
frequency varies the necessary range of control voltage is also proportional to the +B H deflection voltage. So
in order to keep the variable quantity of the side pin cushion correction not so different at any frequency, the
range of control voltage is different for each four bands of horizontal frequency.
With using above method, the last parabolic wave is output from pin
of
FIRST BLOCK (PARABOLIC WAVEFORM GENERATION DETERMINED BY V SIZE)
,
V CONTROL
SUBTRACTION CONTRAST SAW TOOTH A C
BLCK
MULTIPLIER 1
CIRCUIT
SECONDBLOCK PARABOLIC WAVEFORM AMPLITUDE GENERATION PROPORTIONAL TO
THE MICROPROCESSOR ADJUB H DEFLECTION VOLTAGE.
(Fig 5-l-10) The BLOCK DIAGRAM of PARABOLA waveform generating circuit
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