Service manual
IMI035
Section
4
THEORY
OF
OPERATION
4.1
VIDEO AMPLIFIER
The
video
amplifier
consists
of
QlOl and
its
associated
circuitry.
The incoming
video
signal
is
applied
to
the
monitor
through
JlOl-8
and RlOl
to
the
base
of
QlOl.
Transistor
QlOl
has
a nominal
gain
of
15,
and
operates
as
a
class
B
amplifier.
QlOl
remains
cutoff
until
a
DC
coupled,
positive-going
signal
arrives
at
its
base
and
turns
it
on.
Rl03
provides
series
feedback
which makes
the
terminal
to
terminal
voltage
gain
relatively
independent
of
transistor
parameters
and
temperature
variations.
RI02 and CIOI
provide
emitter
peaking
to
extend
the
bandwidth
to
12~1Hz.
.
The
negative
going
signal
at
the
collector
of
QIOI
is
direct
coupled
to
the
CRT
cathode.
The
class
B
biasing
of
QIOI
allows
a
large
video
output
signal
to
modulate
the
CRT's
cathode
and
results
in
a
maximum
available
contrast
ratio.
The
overall
brightness
at
the
screen
of
the
CRT
is
also
determined
by
the
neg-
ative
potential
at
its
grid
which
is
varied
by
the
brightness
control:
4.2
VERTICAL DEFLECTION
QI02
is
a
thyristor
used
as
programmable
unijunction
and
together
with
its
ex-
ternal
circuitry
forms a
relaxation
oscillator
operating
at
a
vertical
rate.
The
sawtooth
forming network
consists
of
AIOI, Cl03 and
Cl04.
These
capacitors
charge
exponentially
until
the
voltage
at
the
anode
of
Ql02
exceeds
its
gate
voltage
at
which
time
QI02 becomes
essentially
a
closed
switch,
allowing
a
rapid
discharge
through
LIOI. The
rate
of
charge
or
frequency
is
adjustable
by
AIOI. The
oscillator
is
synchronized
by a
negative
pulse
coupled
to
its
gate
from
the
vertical
drive
pulse
applied
externally
at
JIOI-9.
A
divider
network
internal
to
AIOI
sets
the
free
running
frequency
by
establish-
ing
a
reference
voltage
at
the
gate.
This
programs
the
firing
of
QI02 and
amounts
to
resistive
selection
of
the
intrinsic
standoff
ratio.
The
frequency
is
controlled
by
passive
components
only.
CRIOI
provides
temperature
com-
pensation
for
Ql02
while
controlling
the
gate
impedance
to
allow
easy
turn
on
and
off
of
QI02. LIOI forms a
tuned
circuit
with
Cl03 and Cl04
during
conduction
of
QI02 which
provides
a
stable
control
on
the
drop-out
time
of
QI02
to
assist
in
maintaining
interlace.
Ql03
collector
to
base
forward
diode
clamping
action
prevents
the
voltage
from
swinging
too
far
negative
during
this
flywheel
action.
The
sawtooth
at
the
anode
of
Ql02
is
direct
coupled
to
the
base
of
Ql03.
This
stage
functions
as
a
darlington
pair
emitter
follower
driver
for
the
output
stage
QI04.
It
presents
an
extremely
high
impedance
in
shunt
with
AlOI and
prevents
the
Beta
dependent
input
impedance
of
QI04 from
affecting
the
frequency
of
the
4-1