Heathkit H9 - Page 51

page
49
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ry
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.