Heathkit H9 Manual page 51

page 49
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HORIZONTAL
SWEEP CIRCUITS
The horizontal sweep system is controlled by
triggered oscillator, IC3o1. The free-running fre-
quency of this oscillator, which is set by resistor R325
and capacitor C312, is slightly less than the 15494 Hz
horizontal sweep rate. Horizontal sync pulses are
coupled through C314 to the *Vcc input (pin 6) of
IC301, The sync pulse changes an internal reference
voltage that causes the oscillator to reset prematurely
and oscillate at the desired horizontal rate'
The output (pin 1) of IC301 drives transistor Q307
which, in turn, drives the horizontal output transis-
tor, Q1. The duty cycle (the on-to-off ratio) of Q1 is
determined by R328 and Rgzg. The collector of Qt
drives the primary of the horizontal output trans-
former T2. The high voltage secondary is rectified
internally to provide 12 kV for the anode of the CRT.
When the horizontal oscillator causes Q1 to turn off,
the rapid collapse of the flux in transformer T2 gen-
erates a 2oo-volt pulse across the primary winding.
Capacitor C321 charges immediately to the maximum
of this pulse, C321 then discharges quickly through
linearity coil L301, the horizontal deflection yoke,
and capacitor C322 until the yoke current is
maximum. The current through the yoke at this point
is considered negative (flowing from C321 down
through the linearity coil, the yoke, and C322). This
negative current causes the magnetic field of the yoke
to quickly pull the electron beam to the extreme left
side of the CRT screen (retrace). The yoke field begins
to collapse as it tries to keep the current flowing in the
same direction.
Diode Dr, which is part of Qr, now conducts and
effectively removes C321 (shorts it out) from the cir-
cuit. As the yoke field continues to collapse, the
charge on C322 continues to increase' This action
produces a near linear change (decrease) in current,
which causes a linear change in the yoke field. As the
yoke field decreases, the electron beam moves from
the left toward the center of the CRT screen.
Before the yoke current reaches zero at the center of
the screen, transistor Q1 is turned on by the horizon-
tal oscillator. Transistor Q1 also keeps C321 out of the
circuit. Current will cease to flow when the yoke field
collapses completely. At this point, the electron beam
will be at the center of the CRT screen' The charge
stored in C322 now begins to discharge through the
horizontal deflection yoke, linearity coil L301, and
transistor Q1 in a positive direction. The current
through the yoke is considered positive (flowing from
C352 up through the yoke, the linearity coil, and
transistor Q1). Diode Ds is now reverse biased and out
of the circuit. The change in polarity of the yoke field,
as the yoke current increases in a positive direction,
moves the electron beam from the center toward the
right side of the CRT screen. Capacitor C322 con-
tinues to discharge until the beam is at the right side
of the screen and Qr turns off again. This process
repeats for each horizontal scan line.
HIGH VOLTAGE SUPPLY
During retrace, the 200-volt pulse across the primary
of T2 is coupled through resistor R335 to voltage
doubler D305, D306 and D307, C324,C326, andCgzz .
This provides a 300-volt supply for grid 2 of the CRT'
The voltage from the blanking winding of Tz is
rectified by diode D304 to produce a -150-volt
supply for the focus control.
VIDEO AMPLIFIER
The video information
that comes from the character
generator is a string of 1's and 0's. This data is coupled
to the base of Qgo9, which drives the emitter of video
output transistor Q308. The collector of Qgoa drives
the cathode of the CRT. The brightness control, R338
which sets the voltage on grid t of the CRT, controls
the brightness of the displayed characters.