Radio Shack Quick Printer II Technical Reference Manual

At the
output
of the
transformer
assembly,
four signals
are
connected to
the
main
board;
ground,
the
input to
the +5
volt
regulator
circuit, and
the
two sides
of the 33.5
volt
secondary.
The
two lines
from
the
33.5 volt
secondary
are
connected
to
the
bridge
rectifier CR
1
1
.
At
the
output of
bridge CR11,
are
three
nearly
identical
regulator circuits.
Each one
uses a
three
terminal
regulator
chip to
generate the
DC
supply
voltages.
C7
is
the filter
capacitor
for the
+12 volt supply.
This is
connected to U17
(a
78M12
regulator),
and
the
+
side
of the bridge.
CR 14
provides
protection for
the
regulator
chip.
C14
is
the
output
capacitor
for the
+12 volt
regulator. The
negative side
of
the
bridge
is
connected to
filter
capacitor C9.
From C9,
the
-12
volt is
identical to
the
+12
volt
regulator circuit
with the
exception
of
C17.
This
capacitor is
needed
because two
regulator
circuits
are sharing
the same
filter
capacitor.
The
-5
volts
regulator
circuit
is
current
limited by
resistor R67.
Also, due to
the very
low
current, the
protection
diode
and the
large
output
capacitor
have been
eliminated.
Compared to the
other
regulator
circuits,
the +5
volt supply is
a
complex
assortment
of resistors,
capacitors,
and transistors.
However,
this circuit
will
provide a
reliable
1.35 amp
maximum
current. The
circuit begins
with filter
capacitor
C10.
Connected to C10 is
the pass
transistor
Q2,
and its bias
resistor
(R59).
Q2 controls
the flow
of
current into
the +5
volt line,
but the
actual job
of regulation is
performed by U13
(a 723C
adjustable
voltage
regulator). The
regulator
output
(pin
10)
is
buffered by
Q3. Then
Q3 is
used to
drive the
base
of
pass
transistor
Q2.
Figure
14 shows a
Block
diagram
of the
723C
regulator
chip.
The
regulator
is
powered
from the
+12 volt
supply.
In
operation, a
7.15
volt
zener
reference
is
available
at
pin 6.
Resistors R60,
R61,
and R62
then
divide
this
reference
voltage
down to
5 volts,
which is
connected
to pin 5
of the
regulator.
Pin 5 is
the
non-inverting
input to
internal
Op-amp
Zb
(as
shown
in Figure
14).
The
inverting input
is
connected
through
a
feed-back
resistor (R63)
to the
+5 volt output.
Op-amp
Zb is
then
turned on
or
off in
response to
changes on
the
5
volt
line.
C11
is the
frequency
compensation
capacitor
for Zb.
The output
from
Op-amp
Zb
controls
transistor Qa,
which
drives the
regulator
output. Qa
is also
powered from
the
+12
volt supply
through
resistor R58.
There
is
also a
second
control source
on Qa,
which
is
transistor
Qb. This
transistor
allows a
current
limit to be set
by
resistors R65
and R64.
If
the current
limit is
exceeded,
Qb
will turn on
and
turn
off
Qa.
Returning
briefly to
pass
transistor
Q2,
the
next
component
is
R66,
which
limits the
output
current.
Overvoltage
protection
is
provided by CR17,
a
6.2V
zener diode. C16
is
the
output
filter
capacitor
for the
+5 volt
supply.
v+
v
<=
OP
AMP
POWER
jV]
-(e)
(ih-fl
&-
Vref
COMP
FIGURE 14. 723
REGULATOR
BLOCK
DIAGRAM
34

Radio Shack Quick Printer II Specifications

General

Brand	Radio Shack
Model	Quick Printer II
Category	Desktop
Language	English