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PX-7
is
included
in
the
color difference
signal
where
the
voltage
is
lower than the achromatic
level.
If
this
signal
is
then
modulated
by
the
color
signal,
there
will
be
a considerable increase
in
color subcarrier
leak at this section, resulting in
a
delay
when
passed
through the
carrier
color
signal
filter,
and
the generation of
a
spike
during
external/internal
switching
(see
Photo. 13-17)
at
the video
output
terminal.
Characters are
thus
colored green
in this
case.
To
improve
this situation,
the
R-Y
and
B-Y
signals
are limited
by
D106
and
D132
to the
achromatic
level
when
OVLYF
is
H
(external
video
display
section).
VR109
(white
adjustment
control)
is
the control
used
to adjust
the
achromatic
level.
The
Q249
and
Q250
base
waveforms
during
computer and
synthesis
modes
are
shown
in
Photo. 13-15
and
Photo.
13-16.
Photo. 13-18
shows
the
improvement
on
the switching
spike
achieved
by
the
overlay eliminator
circuit.
Q249
and
Q250
are
level shift transistors
(pnp
type) for
the
B-Y
and
R-Y
signals,
and
are involved
in
temperature compensation
for
the junction
voltage
between
the base
and
emitter
due
to
application
of emitter outputs
from
the matrix
circuit
Q202
and
Q203
(npn type
transistors).
In
the
bias
circuit,
too,
temperature compensation
involves
the use of
similar
pnp
and
npn
transistors.
Since regulated
power
voltage
is
required
by
the
carrier
color
signal
modulator
circuit
IC107 and
the
bias
circuit,
the voltage
is
regulated a
second
time
by
the
triple
terminal regulator
IC116.
•
Carrier color
signal
filter/mixing amplifier/burst
ATT
The
carrier
color
signal
output
from
the
carrier
color
signal
modulator
is
passed
via
a mixing
buffer
(Q246) and
carrier
color
signal
filter
(*)
to be
added
to
the
Y
signal
(luminance
signal)
from
the
VDP
in
accordance with
the
R248/R254
ratio,
thereby forming the composite video
signal.
After
being amplified to
about
lVp-p by Q244/Q245,
this signal
is
subject to
a level
shift
of
about
5V
at
D105.
The
signal
is
then
passed to the pin
8
input
of
the analog switching IC (IC108). Since the
LI
04/
C247
series
resonance
circuit
connected
to
the
input
and
output of
this
switch resonates
at
the
color
subcarrier
frequency, only the
composite
video
signal
burst
is
attenuated
by
R274 when
a
BGP
pulse
(burst
interval
L)
is
applied
to
pin
6
(the
DC
component
being bypassed
by LI
04
without
any
level
shift).
The
burst
level in
the
composite
video
signal
is
thus
matched
with
standard
signal
value
during
computer mode.
(
*The
carrier
color
signal
filter
is
a
band
limiting
bandpass
filter
used
to
minimize dot
interference.)
112
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