Service manual
IMl015
of
Q120
and Q12l
varying
inversely
with
temperature.
As
the
temperature
in-
creases,
the
thermistor
resistance
decreases;
thereby
lowering
the
effective
source
voltage
applied
to
the
main
timing
network
consisting
of
R187,
R185
and C140.
This
action
slows
down
the
charging
current
into
C140
and
holds
the
off
time
of
Q12l
constant.
The
other
timing
network
for
Q120
and Q12l
consists
of
R18l and C138. The
time
constants
chosen
are
such
that
the
output
square
wave
at
Q12l
is
positive
for
38
~seconds
and
grounded
for
25
~seconds.
This
establishes
the
proper
duty
cycle
for
the
output
stages.
The
output
at
Q12l
is
DC
coupled
to
pre-driver
inverter
Q122
which
produces
sharp
rise
and
fall
times
for
coupling
to
the
driver
transistor
Q129.
Q129
is
driven
alternately
into
saturation
and
cutoff
by
the
square
wave
ac
coupled
from Q122.
Its
output
is
transformer
coupled
to
the
horizontal
out-
put
stage
Q3.
Phasing
of
TlOl
is
chosen
such
that
Q3
turns
off
when
Q129
turns
on.
This
allows
Q3
to
turn
off
quickly,
thus
minimizing
power
dissipation.
During
conduction
of
the
driver
transistor,
energy
is
stored
in
the
coupling
transformer.
The
voltage
at
the
secondary
is
then
negative
and
keeps
Q3
cut
off.
As
soon
as
the
primary
current
of
TlOl
is
interrupted
due
to
the
base
signal
driving
Q129
into
cut
off,
the
secondary
voltage
changes
polarity.
Q3
~tarts
conducting,
and
base
current
flows.
This
gradually
decreases
at
a
rate
determined
by
the
transformer
inductance
and
circuit
resistance.
The
horizontal
output
stage
has
three
main
functions:
to
supply
the
yoke
with
the
correct
horizontal
scanning
currents;
develop
18
kV
for
the
CRT
anode and
DC
voltage
for
the
CRT
bias,
focus
and
accelerating
grids.
Q3
acts
as
a
switch
which
is
turned
on
or
off
by
the
rectangular
waveform on
the
base.
When
Q3
is
turned
on,
the
supply
voltage
plus
the
charge
on
C158
causes
yoke
current
to
increase
in
a
linear
manner and moves
the
beam from
near
the
center
of
the
screen
to
the
right
side.
At
this
time,
the
transistor
is
turned
off
by a
negative
voltage
in
its
base
which
causes
the
output
circuit
to
oscillate.
A
high
reactive
voltage
in
the
form
of
a
half
cycle
positive
voltage
pulse
is
developed
by
the
yoke's
inductance
and
the
primary
of
T3. The
peak
magnetic
energy
which was
stored
in
the
yoke
during
scan
time
is
then
transferred
to
C156
and
the
yoke'S
distributed
capacity.
During
this
cycle,
the
beam
is
returned
to
the
center
of
the
screen.
The
charged
capacitances
now
discharge
into
the
yoke and
induce
a
current
in
a
direction
opposite
to
the
current
of
the
previous
part
of
the
cycle.
The
magnetic
field
thus
created
around
the
yoke moves
the
scanning
beam
to
the
left
of
the
screen.
After
slightly
more
than
half
a
cycle,
the
voltage
across
C156
biases
the
damper
diode
CR12l
into
conduction
and
prevents
the
flyback
pulse
from
further
oscillating.
The
magnetic
energy
that
was
stored
in
the
yoke from
the
discharge
of
the
distributed
capacity
is
released
to
provide
sweep
for
the
left
half
of
scan
and
to
charge
C158
through
the
rectifying
action
of
the
damper
diode.
The
beam
js
the:l
at
the
center
of
the
screen.
The
cycle
will
repeat
as
soon
as
the
bias
voltag~
of
Q3
becomes
positive.
3-4