Radio Shack Quick Printer II Technical Reference Manual

At
pin
28 of the VDG,
the
Y signal
contains
composite
video
and sync
information.
An
example
of this
type
of video
signal
is shown
in
Figure 7. This
signal
together
with
three color
signals,
CHB (pin
9),
0A
(pin
11),
and 0B
(pin
10)
is
connected to
the MC1372
color
mixer chip
(U12).
. SYNC PULSE
.
BLANKING
LEVEL
BLACK LEVEL
-
GREEN
BACKGROUND
FIGURE?.
VDG
VIDEO
OUTPUT
The MC1372 is
a
complete Color
TV
video
modulator
chip.
However, for the Color
Computer it is
used
only as a
color
mixer chip.
Figure 8
shows a
block diagram
of the MC1372
chip. Pins 1 and
2
of the MC1372
form an
oscillator
circuit,
but
this
is
not
needed in
the Color
Computer.
The
reference
clock frequency
of
exactly
3.579545
MHz
is
produced by
the
MC6883
chip (U10).
This
signal is
then
rounded
off
by R17
and C53
before being
connected
to pin
2
of the MC1372
(U12).
Pin 1 is
left
unconnected.
Also,
the duty
cycle
adjust
input
(pin
3)
is
left
unconnected
since a 50%
duty
cycle
signal
is
produced by the MC6883.
The
RF oscillator
portion
of the MC1372
is
not
required, so
diode CR8 and
resistor R20
are
used to
kill the
oscillation.
Also
the orientation
of CR8
controls
the
generation
of normal
or
inverted video.
Using
the color
signals
from
the VDG,
U12
produces
the
chroma
output at pin
8. This
signal
consists
of the
reference
color
burst and
the video
color
information.
The MC1372
requires this signal
to
be
AC
coupled
to
pin
10.
This
is
accomplished
by capacitor C42.
Resistor R18
determines
the
Luminance
to
Chrominance ratio.
The
complete
color video
signal is
available at
pin 12
of the
MC1372.
However, at
this
point
the
signal has
no
drive
capability
and must
be
amplified
before
it can
drive the
modulator. This
amplification is
accomplished
by Q1
and
the
associated
circuitry.
The video
amplifier
is
powered
by
8
volts
DC.
This
power supply is
produced
off
of the
+12V
supply
by
zener diode CR16,
(a
3.9 volt
zener
diode). R69
is
used to
stabilize the
zener voltage
while C12
and C25
filter the
voltage. This 8V
supply
is then
connected
to
the
collector
of
the
transistor
(Q1),
and
through a
3.9K
pullup to
the
video
output pin
of
the
MC1372
(pin 12).
The video
amplifier is a
common
collector
amplifier
designed
to
provide current
gain. R 19
and
R8form a
voltage
divider for
the
video input signal.
R9
and R21
are used to
modify
the
voltage
divider to
compensate
for
variations
in
the
output
voltage
of the MC1372.
R46
is a
bootstrap
resistor
used to
provide as much
current gain
from
the
amplifier
as
possible.
R47 is
a biasing
resistor
for
the
amplifier.
The video output
from
the emitter
of Q1 is
connected
directly
to pin 1
of the UM1285-8
modulator (U5).
This signal
should
be adjusted
with R21
so that
the
blanking
level is
exactly
2.35V. A
sample
of this
video
signal is
shown in
Figure
9.
OSCILLATOR
1
CLOCK
O-
OUTPUT
RF
COLOR B
LUMINANCE
MODULATOR
INPUT INPUT
OUTPUT
5
Q9
Q
12
CHROMINANCE
OSCILLATOR
AND
CLOCK
DRIVER
CHROMA
MODULATOR
B
CHROMA
MODULATOR
A
30
DUTY
CYCLE
ADJUST
RF
MODULATOR
A
RF
OSCILLATOR
6 O 13q7
6
COLOR
A
COLOR
RFTANK
INPUT
REFERENCE
INPUT
14
6
'
CHROMINANCE
INPUT
8
CHROMA
"°
MODULATOR
OUTPUT
FIGURE
8. MC1372
BLOCK
DIAGRAM
28

Radio Shack Quick Printer II Specifications

General

Brand	Radio Shack
Model	Quick Printer II
Category	Desktop
Language	English