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THEORY OF OPERATION
GENERAL
This is a high-resolution monochrome display monitor,
using a paper-white phosphored CRT with etched
faceplate.
It uses one video signal input and separated sync
signals.
It can operate in all of the VGA modes.
DESCRIPTION
1. Main Board
1-1. Power Supply
This is a constant-frequency switch-mode power
supply using L4960 monolithic integrated circuit.
The output voltage regulation is achieved by means
of a variable duty cycle. The output voltage of this
power supply is fed back into tbe pin 2 of L4960.
This voltage is compared to reference voltage in-
ternally. The difference of these voltages control
the duty cycle of output current. 0401 to 0404,
and C404 to C405 from input DC supplier.
C408, C409, R404 are the components
for fre-
quency compensation, R403, C406 are for oscilla-
tion of sawtooth wave. C407 is for soft start of this
circuit. 0405 is a flywheel diode for continuous
flowing current. Output voltage is varied byadjust-
ing VR401 potentiometer.
1-2. Horizontal processor IC302
IC302 performs following functions.
-
Horizontal oscillator
-
Phase-locked loop for frequency and phase
comparison.
-
Phase-locked loop to compensate various delay
times of the horizontal output transistor.
The sync signal from 0303 is fed into pin 3 of
IC302 and flyback pulse into pin 4. These two
signals are compared and the circuit is locked to
a correct frequency and phase.
5
1-3. Horizontal output stage 0305
0305 is a horizontal output switch which is closed
during the scan period and opened during the
flyback. It sinks a driver signal from IC302 through
0304. The flyback period is determined by the
resonance of C316 and deflection yoke. C316
makes the necessary S-correction.
Because of the losses in the deflection yoke, a
linearity correction is required. This is achieved by
L302.
The horizontal screen size can be varied by ad-
justing L303. The high voltage of 12.5KV for CRT
anode is also generated in the flyback transformer
by use of the flyback pulse.
1-4. Vertical processor IC 201
IC201 performs following functions.
-
Vertical oscillator
-
Ramp generator
-
Linearity control
-
Output amplifier
There are three vertical size controls : VR202,
VR203, and VR204. VR202 is for mode 3, VR203
for mode 2, and VR204 for mode 1. Each mode
is automatically selected by IC301.
1-5. Dynamic Focus
0501 is used to provide dynamic focus voltage
which is added to G4 voltage of the CRT. This
voltage is needed to change G4 voltage according
to the angle of the beam.
2. CRT Board
IC101 (M51392P) contains wide bandwidth video
amplifier, gain control, pedestal clamp and bright-
ness control circuit.0103 and 0104 from a cascade
video amplifier circuit of which its output is applied
to the cathode pin of the CRT.
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