Power Supplies; Vdc Power - Radio Shack TRS-80 Service Manual

2.8
POWER
SUPPLIES
The
standard
power
input
to
the
printer
is
1
15VAC, 60
Hz.
For the
230VAC, 50 Hz
option, a different drive
motor assembly
is
used.
The power
input
is
applied through
a circuit
breaker
(S1)
at
the
rear of
the
printer
and
then through
a line
filter
to
the
multi-
tap transformer
(T1).
From
the transformer the
main power goes
to
the
logic board.
The
secondary windings
of
the transformer develop the following
voltages:
VOLTAGE
12VAC
35VAC
27.5VAC
WINDING
11-12
13-14-15
16-17
The
1
2VAC
and
35VAC
voltages are used
as inputs
to
the
+5VDC
and
-1
2VDC
power
supplies
on
the electronics board,
where
they are
rectified, filtered
and
regulated.
The 27.5VAC
is
used
to
generate the
+35VDC
unregulated
output.
2.8.1
+5V
REGULATOR
(Figure 2-7)
The
1
2VAC
output
from
the
secondary winding
of
the transformer
is
rectified
by the full-wave diode bridge
located
on
the
electronics board
and
filtered
by
C2
located
in
the
cavity.
The
unregulated,
filtered
1
5
VDC
output of
the bridge
rectifier
is
fused and
supplied
to
the
power
driver circuits.
The
15VDC
is
also applied
to
the
+5V
regulator circuits
which
provide
a
regulated
5VDC
output.
12 J002
P002
T'11 vAc
m
__rg
TO
CIRCUITS
+15
VDC
+5
VDC
~l
TO
TAPE READER,
CONTROL
PANEL,
LEFT MARGIN
INDICATOR
Figure 2-7.
+5
VDC POWER
SUPPLY
2-7