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VERTICAL AND HORIZONTAL DEFLECTION CIR-
CUIT
1. SYNC INTERFACE CIRCUIT
In MODE 1 (CGA MODE), the polarity of horizon-
tal and vertical sync are both positive but in
MODE 2(EGA MODE) the polarity of horizontal
sync is positive and the vertical sync is negative.
If the HN sync signal is applied in MODE 1,
then the output voltage level from pin nO.8 of
IC102 (EX-OR gate) is OV and the output voltage
level from pin no. 3 of IC102 is positive.
If the HN sync signal is applied in MODE 2,then
the output voltage level from pin no. 8 of IC102
is 3.2V and the output polarity from pin nO.3 of
IC102 is negative.
According to the polarity of vertical sync,
MODE 1 and MODE 2 are selected automat-
ically.
Vertical Deflection Circuit
Vertical circuit is composed of IC102 (74HCTlS86),
IC301
(LA7830).
IC401
(lA7851),
IC402
(MC14053BCP) and its relative circuits.
The vertical sync signal from IC102 which polarity
IS positive or negative is applied to pin no.19 of
IC401.
Pin no.19 of 1C401 is connected to the vertical
oscillation circuit.
The frequency of oscillator is controlled by the volt-
age of pin 18 which can varied by the V-HOLD
VR(VR306).
The output circuit of 1C301 Is controlled by V-SIZE
VR(VR 307) to vary the vertical size of the raster,
that is related both MODE 1 and MODE 2.
But
V-SIZE of MODE 1 Is only controlled by CGA V-SIZE
VR(VR402).
Horizontal OSCillator, AFC and Drive Circuit
The horizontal sync signal with positive polarity is
applied to pin 1 of 1C401.The output from the fly-
back transformer (T403) is connectored pin 4 of
1C401 as part of the automatic frequency circuit
(AFC). H-CENTER VR(VR407) determines the posi-
tion of picture in MODE 1 and MODE 2 but CGA
H-CENTER VR(VR403) controls the position of CGA
picture only. The horizontal oscillation frequency is,
-6-
obtained from pin 12 of 1C401 and it is fed to the
next horizontal drive circuit. The pulse-switching
mode of the drive and output stage is a reverse
polarity type and that's done when the drive transistor
0401 is on and the output transistor 0402 is off.
HORIZONTAL OUTPUT AND HV RECTIFIER
Horizontal drive voltage which developed at pin 12
of the deflection processor (IC401) is amplified
through the horizontal drive stage (0401) and
coupled to the base of the horizontal output cicuit
via the horizontal drive transformer(T401). Refering
to Fig.3, the horizontal circuit generates the horizon-
tal scan and high voltage to be applied to the picture
tube. The function of the horizontal output stage
(0402) is to serve as a switch for the horizontal
output circuit. Refter to Fig.4.
During the horizontal scanning period, 0 operates
(S1 is closed, S2 is opened) and the current is
applied in one direction through the horizontal coils
of
the deflection yoke (ly) and the capacitor (MODE
1 :Cl, MODE 2:C1,C2). During retrace time, 0 be-
come to operate(S1 is opened, S2 is closed) and
the current is applied In the opposite direction
through the damper diode (D),
the horizontal coils of the deflection yoke (ly) and
the capacitor Cl, C2.
The high voltage required to be applied to the anode
of the picture tube is generated by boosting the
pulse from the collector of 0402 through T302 during
the flyback(retrace) period and applying this boosted
pulse to a serious of rectifiers. Refering to Fig.5, high
voltage regulation is accomplished internally in T 403.
HIGH VOLTAGE SHUT-DOWN SYSTEM
The shut-down circuit prevents the high voltage from
rising above a presence level. Under norminal operat-
ing conditions, this circuit is inactive. Operation of
the protector circuit depends upon an AFC pulse,
which apears at pin 3 of the horizontal output
traformer (T403). It monitors an AFC pulses sub-
jected to rectification by D406. Since it is in propor-
tion to the voltage of that AFC pulse,
if
the incoming
high voltage increases and exceeds its limit, the AFC
pulse voltage aiso inceases. As a result, there is a
larger voltage produced to R422 so that its potential
will eventually be higher than the voltage(
+
8.2V} of
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