Radio Shack Quick Printer II Technical Reference Manual

have a video
signal, check
U7,
pin
33. There should
be
a
3.579 MHz clock signal
at this
point. If you have
a clock signal
but no
video at pin
33,
then
U7
is
bad. (You
might also check
pin 17
just to make sure the
chip has
+5
volts.)
If you have
a video signal
at pin
28
of
U7
but still
have no
TV
picture, the
next point
to check is
U12, pin 12.
Again there
should be
a video signal
at this point.
If you
do not have
a
video
signal on pin
12 then
check
the inputs
to the chip.
(Pin
2
=
3.579
MHz clock; pin
11
=
+5
volts;
pin
9
=
input
video;
etc.) Also check
for
+8
volts on
the top side of
R16.
If
all
the inputs are present
then
U12 is
probably
bad.
The
last place
to check
the video
signal is
on the
modulator
input (pin
1
of
U5). If you have
the video
signal
at the point
and
at the
proper
DC level, then
U5 is probably
bad. If the
video
signal
is not present,
then
Q1 or
one of the
associated
biasing
resistors
is probably
the faulty
part.
(Make
sure you
check for
+8 volts on
the collector
of
Q1
.)
If
the video
signal
is
at the wrong
DC level, try
adjusting
R21
.
If the
signal will
not adjust
to the proper
level
then
most likely
one of
the
biasing resistors for
Q1 is bad.
U12 may also cause
this
pro-
blem
if the output
from pin
12 is
out of spec.)
If
the video
problem is
associated
-with
only one
mode of
operation
(graphics,
semigraphics,
or
alphanumerics)
then
one of
three
chips is
causing the
problem;
U6, U7, or
U10. If
some
of the
dots are
missing from
alphanumeric
characters
then
U7
is
faulty.
If some
characters
are being
displayed
as the
wrong
character,
then the
correct
data is
not being
supplied
to the
data
inputs
to
U7. U6 latches
this data
so it
may be
causing
the problem
or
there
may
be a short
on the data
lines.
If
some
of the
graphics
modes
do not
work,
then check
pins
27,
29, 30,
and 35 of
U7.
These
pins
should
change from
high
to low
as
you change from
one mode
to the next.
These
signals
are supplied
from
U4, which
could
be causing
the problem
if
signals
are changing
properly.
If these
pins all
seem to
change,
then
the
problem
is
caused by
U10
or
U7. (Since
U10 is in
a
socket,
try replacing
it first.)
JOYSTICK INTERFACE
Before
you tear into
the computer
to
fix
a joystick related
problem, try
a second
set of
joystick controllers
to make sure
the
problem is
in the computer.
The only possible fire switch
problem,
that will
not also
show up
as
a
keyboard
problem,
is if
L2 or L3 is broken
or
the etch
is cut or shorted
.
To troubleshoot
the
joystick interface
you
should
have the
Diagnostic
joystick test
running.
If the
problem is missing
blocks, then
one of the bits
going
to the
D/A
converter
is not
changing properly.
Set both
joysticks
to
the
bottom right
corner.
Use an
oscilloscope
to check
the D/A
pins from
U4
(pins
4
through
9).
If
all of
the pins are
changing then
U4
is
good. Also check the
output
pins of
U2,
they should
all be
changing.
Finally, look
at
the
output
of the
D/A converter
(U14, pin
8).
At this
point
you should
see an even
stairstep
function.
If one
or more of
the steps
is twice
as large
as the
other steps
then one of
the
resistors is
bad
.
If the
problem
is
that the joysticks
do
not work in
one or
more
directions,
connect
an oscilloscope
to
U9,
pin
9. With
the
Diagnostic
joystick
test running
you
should see four
DC
voltage
segments.
One
dimension on
each
joystick
should
affect one
of
the segments. Try
moving
the
joysticks
around to
check
this. You
can look
at the
DC voltages
directly
from
the
pots at the
input pins
of
U9.
If
the inputs
of
U9
are
changing
(including
select
pins
6 and
7)
but
the output
is not,
then
U9 is probably
faulty.
If
U9 seems
to be working
properly
then
check
U14.
Set both
joysticks
to the
center.
Checking
pin
8
of
U14 you should
see
an
even
stairstep
function
going
up to
approximately
2.5 volts.
The
output,
pin
14,
should
be switching
each time
the stair-
step
on pin
8
peaks. If
the
output is
not switching,
then
U14
is
not working.
CASSETTE
INTERFACE
If
you are
having cassette
problems,
run the
joystick
test first
to confirm
that the
D/A is
working.
If the
computer
passes the
joystick test
then
there are
only
three
other
components in
the
output
circuit
that could
be failing;
R41
, R42, or
C82.
The cassette
input circuit
is
a simple
sinewave-to-squarewave
converter.
To
test it
you will need
a long tape
of
consistent
data. By
running
this tape,
your
should
be able
to
compare the
input
sinewave
to the
output
squarewave
and
see
if the con-
verter
is
working.
If this circuit
is
working,
the
only
other part
involved
in
the cassette
interface
is
U4.
If
the problem
is
motor
control,
relay
K1 is
most likely
bad.
To
test this,
check
to
see that
the collector
of
Q4
is switching
as
you
use the
MOTOR
ON
and
MOTOR
OFF
command.
If
it is
not
switching,
Q4 or
the PIA
output
pin
is faulty.
KEYBOARD
INTERFACE
The keyboard
interface
is a very
simple
electronic circuit.
The only
digital part
ised
is
U8. Usually
a keyboard failure
will
be
caused by a mechanical
failure
of the
keyboard itself,
or
a short
or open in the calbe.
Mechanical
failures
will
usually
be only
one isolated
key failing.
Cable failures
will cause an
entire
keyboard row or
column
not to work.
RS-232 INTERFACE
The
RS-232 Interface
utilizes
three level
converter
circuits.
Isolating
the problem
in these three
circuits
will be
a simple
matter of
comparing
the input to the
output.
Connect
a DIN-type plug
to the
I/O Serial
jack
to
short
together
pins
1,
2,
and 4.
Now
type
in
the
following
test
program:
45

Radio Shack Quick Printer II Specifications

General

Brand	Radio Shack
Model	Quick Printer II
Category	Desktop
Language	English