Service manual
IMlOls
SECTION
3
CIRCUIT
THEORY
3.1
GENERAL
INFORMATION
This
section
describes
the
circuit
theory
of
the
TD
series
monitor.
This
section
is
to
be
used
with
the
waveforms and
schematics
found
in
section
5
and 6
of
this
manual.
3.2
VIDEO AMPLIFIER
The
video
amplifier
consist
of
transistors
QlOl
through
Ql03,
integrated
cir-
cuit
UlOl and
transistor
Q10S
through
Qlll.
A
composite
video
signal
is
applied
to
the
PWA
through
Jl02-3
and
is
ac
coupled
to
the
differential
amplifier.
The
differential
amplifier
consists
of
QlOl and Ql03
with
Ql02
as
the
constant
current
source
for
the
pair.
The
video
gain
of
this
stage
is
essentially
unity.
Hum
is
rejected
when
S2
is
in
the
ON
position
because
of
the
inherent
common
mode
rejection
of
the
dif-
ferential
pair.
This
stage
presents
an
input
impedance
of
10K
to
the
incoming
video
signal.
The
composite
video
is
ac
coupled
to
the
electronic
attenuator
UlOl and
direct
coupled
to
sync
amplifier
Ql12.
Ul0l
is
an
integrated
circuit
and
its
gain
is
controlled
by
the
contrast
control
R3.
Its
advantage
over
the
conventional
method
is
that
the
video
signal
is
not
routed
through
the
contrast
control
and
the
stray
capacity
associated
with
these
long
leads
does
not
cause
a
roll-
off
in
high
frequency
response.
The
video
signal
is
ac
coupled
to
the
base
of
QlOs. QlOs
with
Ql06 forms a
compound
series
feedback
stage.
This
configuration
provides
a
high
input
impedance and a low
output
impedance
to
drive
emitter
follower
Ql08.
It
also
has
a
voltage
gain
of
9.
The
output
signal
from
Q106
is
coupled
to
the
base
of
Ql08, and
its
base
is
biased
by
the
keyed clamp
transistor
Ql07.
The
function
of
the
keyed clamp
stage
is
to
clamp
the
blanking
level
of
the
composite
video
signal
to
a
fixed
reference
voltage
which
is
constant
regard-
less
of
scene
content.
It
functions
as
a
DC
restorer
and
forces
the
input
voltage
during
blanking
at
Ql08
to
be
1.5
volts.
The
base
of
Ql07
is
driven
by
composite
negative
sync
and
caused
Ql07
to
saturate
at
the
trailing
edge;
thus
clamping
occurs
during
the
back
porch
of
the
composite
video.
Ql08
is
another
emitter
follower
which
isolates
the
keyed
clamp from
the
out-
put
stages
Ql09,
QllO and
Qlll.
Transistor
QllO and
its
components
compris
p
the
video
output
driver
with
a
gain
of
IS
to
18. The
bias
voltage
for
QllO
is
supplied
by
DC
coupling
from Ql08 which
in
turn
is
biased
by
the
keyed clamp.
QllO
operates
essentially
as
a
class
B
amplifier
and
is
referenced
to
blank-
ing
level
and
allows
a
greater
video
swing
in
the
output
stage.
R13s
adds
series
feedback
which
stabilizes
the
voltage
gain
and
operating
point
against
transistor
and
temperature
variation.
Clls
and C13s
increase
the
gain
of
the
3-1