Radio Shack 26-1154A Service Manual page 8

1.3
PHYSICAL DESCRIPTION
A printer mechanism, printed circuit board, cover assembly
set, and
holder for roll paper comprise the major assemblies of the printer. See
Figure 1-1.
The three cover set provides structural support and consists of
a
base cover,
a
body cover, and
a
top cover. The base cover supports the
printed circuit board and the printer mechanism.
The body cover fits over and
around the base cover and contains
a
flat tray
for the ribbon. It also houses
the ribbon drive motor and gear mechanism.
The top cover snaps into the body
cover and prevents contact with the print mechanism and electronics.
Printed
paper exits through
a slot in
the rear of the cover which has a serrated edge
to provide
a tear bar for
removing printed pages.
The printed circuit board
(pcb)
contains all the printer electronics
including an "on board" DC power supply with an input transformer, filter
capacitors, and fuse.
The pcb fits underneath the printer mechanism
and
is
approximately
the same size as the
bottom cover although
the electronic
circuitry requires only about one-third of
this area. This allows
the
printed
circuit input edge card connector, which protrudes from
the bottom rear of the
printer, to be
an
integral part of the pcb
for increased reliability.
It also
allows the POWER ON/OFF and ONLINE/OFFLINE switches, which protrude from the
bottom front to
be
mounted directly on
the pcb.
The printer mechanism consists of
a
left and right end plate which
secure the carriage assembly shafts and carriage rack, the platen, and the
paper feed roller assemblies. A solenoid attached to
the left end plate
provides the drive for paper movement.
The solenoid
is
mechanically linked
by
a
pawl which engages
a
ratchet on the paper feed roller.
When
the solenoid
is
energized the paper feed roller
is
rotated incrementally resulting
in a
forward
(upward)
movement of paper one-sixth
inch
(4
mm) or
one line.
The carriage assembly
is
driven back and forth on the carriage
shafts by the drive motor pinion gear which engages the rack.
The print head
is
transported along the platen by the motion of the carriage assembly.
A
flexible cable connects
the
drive motor and print head solenoids
to the
pcb.
1-2