Service manual
IMIOIS
The
vertical
oscillator
Ql16
is
a
thyristor
functioning
as
a programmaole
unijunction
and
operates
as
a
relaxation
oscillator.
The
free
running
fre-
quency
is
set
by
the
DC
voltage
at
it's
gate
and
anode.
This
voltage
is
determined
by
the
resistive
voltage
divider
network
of
RIS3, RlS4 and RISS.
This
voltage
can
be
varied
by
the
vertical
hold
control
RIS4. The
oscillator
is
synchronized
by a
negative
vertical
sync
pulse
applied
to
the
gate
of
Ql16
from
QllS
through
C123.
The
sawtooth
forming
network
consists
of
C126, Cl27 and RlS7.
These
capacitors
charge
exponentially
at
the
vertical
rate
during
the
vertical
scan
time.
The
vertical
height
control
adjusts
the
amplitude
of
the
sawtooth
waveform by
controlling
the
charging
rate
of
C126 and C127.
To
maintain
a
linear
charging
rate,
the
output
of
Ql17
is
fed
back
through
R160
and
R161
to
the
junction
of
C126
and C12Z. The
charging
path
is
from
ground
through
C126
and C127,
past
the
anode
of
Ql16' and
through
the
vertical
height
control
CRlS8)
to
B+.
The
ver-
tical
oscillator
is
at
cutoff
during
the
time
that
these
capacitors
are
charging.
When
the
anode
voltage
exceeds
Ql16
gate
voltage,
it
turns
on and
rapidly
dis-
charges
C126
and
C127
through
LI02. The
tuned
circuit
consisting
of
LI02,
C126
and
C127
provide
a
stable
control
of
the
dropout
time
to
maintain
interlace.
The
sawtooth
signal
at
Ql16 anode
is
direct
coupled
to.
the
base
of
Qll7.
This
transistor
is
a
darlington
pair
emitter
follower
driver
for
the
vertical
output
amplifier.
It
presents
a
high
imput impedance
in
shunt
with
RIS7
to
prevent
loading
of
the
wave
shaping
network
across
which
the
sawtooth
waveform
is
shaped.
It
also
provides
a low
output
impedance and
high
current
gain
to
drive
the
base
of
the
vertical
amplifier
Ql.
The
positive
going
sawtooth
waveform
at
Ql17
emitter
is
fed
back
through
the
resistive
voltage
divider
of
R160
and R161.
This
divider
along
with
C127
inte-
grates
the
sawtooth
waveform and
introduces
a
parabolic
component
to
control
linearity.
The amount
of
feed
back
is
controlled
by
the
vertical
linearity
control
R160.
Height
control
RlS8
varies
the
amplitude
of
the
sawtooth
voltage
developed
by
controlling
the
effective
B+
applied
to
RIS7 and
therefore
controls
the
vertical
raster
size
on
the
CRT.
The
vertical
output
stage
Ql
uses
a
NPN
power
type
transistor
operating
as
a
class
AB
amplifier.
The
output
is
transformer
coupled
to
provide
a
proper
impedance match
with
the
yoke.
CRI08,
R164
and
C128
form a clamp
circuit
which
limits
the
collector
voltage
at
Ql
to
safe
levels
during
retrace.
R139
prevents
oscillations
by
providing
damping
across
the
vertical
yoke
coils.
3.S
HORIZONTAL DEFLECTION
Transistors
Q120
and
Q121
and
their
components form an
astable
multivibrator
operating
at
the
horizontal
rate.
Zener
diode
VRI03
and
R177
provide
a
stable
6.2
volts
source
to
this
circuit
from
the
18
volt
supply.
The
network
consist-
ing
of
R189,
R190
and
thermistor
RTIOI
is
used
to
stabilize
the
frequency
of
the
multivibrator
with
temperature
variation.
The
frequency
of
the
multi-
vibrator
normally
would
increase
with
temperature
due
to
base-emitter
voltage
3-3