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should normally not be. For the half if the line that
includes blanking the sync is allowed to pass.
3.4.
BASE DRIVE
When turned on, transistor 0410 turns off the
horizontal switch 0409. At the same time, current is
stored on transformer T40l.
When 0410 opens up
this stored power supplies turn-on current for 0409.
Resistor R442 sets the average power level to the
base drive circuit.
Snubbing circuits surround
transformer T40l. They control the discharge current
waveform into the base.
3.5. BASE DRIVE RESISTOR
The base drive resistor determines the amount of
base drive.
If the transistor is over driven the V
set
looks very good, but the current fall time is poor. If
the base current is too small the current fall time is
very fast. The problem is that the transistor will have
many volts across C-
E
when closed.
The best condition is found by placing the transistor
in the heaviest load condition.
Adjust the base
resistor for the least power consumption.
diagram 3.
3.6. CHOPPER
The horizontal section does not have a horizontal
power supply voltage like most monitors. A chopper
is used to take the 150 volt video supply and create
the necessary power for the horizontal section. The
duty cycle of the chopper is controlled by a pulse
width modulator (PWM) IC40l. The PWM watches
the current in the horizontal yoke and adjusts the
duty cycle to obtain the desired width.
13
Yoke current can be measured in several different
ways.
The yoke current is directly related to the
voltage across the S capacitor, the average voltage
from the chopper and the peak flyback voltage.
The flyback pulse is upside down for the PWM. A
capacitor
and diode is used to invert the flyback
pulse. A resistor divider drops the 1200 volt flyback
pulse to 2.5 volts for the PWM.
3.7. HORIZONTAL SWITCH/DAMPER
DIODE
On the right hand side of the screen, transistor
0409 conducts current through the deflection yoke.
This current comes from the "S" correction capacitor,
C433, which has a charge equal to the supply voltage.
Diode D415 allows current for the left hand side of
the screen to flow back through the deflection yoke
to C433.
3.8. FLYBACK CAPACITOR
The flyback capacitor connects the hot side of the
yoke to B
+ .
This component determines the size
and length of the flyback pulse. Choose the flyback
capacitor to adjust the
second anode voltage.
Capacitors C431 must be precision high voltage high
current components.
3.9.
"SO CAPACITOR
Capacitor C415 corrects outside versus center
linearity in the horizontal scan. The voltage on The
S cap has a parabola plus the DC horizontal supply.
Reducing the value of S cap increases this parabola
thus reducing the size of the outside characters and
increasing the size of the center characters.
3.10.
HORIZONTAL LINEARITY
In the yoke current path there is a saturable coil.
Just like a size coil, any inductance in series with the
yoke will reduce the size of the picture.
This
saturable coil will change inductance depending on
the amplitude and direction of current flow. At the
start of a trace the linearity coil has an inductance
of 20 percent of that of the yoke. By the center of
the trace, the linearity inductance has decreased to
about 4 percent of the yoke where it remains for the
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