Model 7475A
width
is
increased until
the
comparator output inverts.
This
pulse width information is used by
the
microproc-
essor
to
regulate power to
the
motor drive circuits.
6-44.
MOTOR CURRENT SENSOR CIRCUIT
6-45.
Motor drive current flows through a
0.1
n resistor,
R7.
The
resulting voltage developed across
this
resistor
is
applied to
the
inverting
input
of
the
comparator ampli-
fier UlO (U9).
The
other
input
to
the
amplifier
is
fixed
at
approximately +0.315 V. Consequently,
if
motor current
reaches about 3.15
A,
the
comparator output
changes
from high to low, indicating excessive motor drive current.
6-46.
PEN
DOWN CIRCUIT
6-47.
When
the
microprocessor receives a
pen
down
instruction, a 15.6 kHz pulse signal
is
issued to
the
pen
down driver
UL
Because more power
is
required to acti-
vate
the
pen
solenoid
than
is
needed to hold
it
after
activation,
the
pulse
is
widened for a
short
period
of
time
to provide
this
extra
power.
The
duration of
the
pulse-
width-modulated signal
is
controlled
by
the
gate
array
U6
as
a function
of
the
18/26 Vdc unregulated supply
voltage.
An
air
damper slows
the
descent of
the
pen to
protect
the
pen tip.
6-48.
POWER
SUPPLIES
6-49.
Four
major voltages
are
generated
by
the
7475A
circuitry. Low
current
linear
supplies provide
the+
12 V
and
-5
V required for
the
servo IC's.
The
+5
V supply
provides
the
power for
the
remaining logic circuitry.
Regulation for
the+5
V
linear
supply
is
provided
by
U21
(U20).
The
fourth supply
is
the
unregulated 18/26 V
supplied to
the
main
drive motors
and
the
pen
carousel
drive motor.
The
18/26 V supply
can
be unregulated
because
of
the
voltage
sensing
and
servo
gain
adjust
cir-
cuitry described
in
earlier
paragraphs.
In
addition to
these supplies, a
-12
V source is provided on
the
RS-232-
C {Option 001) version.
This
source
is
used
in
conjunc-
tion
with
the
+ 12 V supply to operate
the
I/O
(input/
output) line drivers.
6-50. RECOMMENDED TEST EQUIPMENT
6-51.
Test
equipment
needed
to
maintain
the
7475A
is
listed
in
Table
6-2.
Models other
than
those listed
may
Section
VI
be used
if
their
specifications equal
or
exceed those
of
the
models listed.
6-52. TROUBLESHOOTING
6-53.
If
the
plotter does
not
operate properly,
make
sure
the
input/output
connector is seated correctly.
If
this
is
not
the
problem, remove
the
top
case
as
instructed
in
Top
Case
Removal,
and
make sure all cables
are
secure
in
their connectors.
6-54.
EXCHANGE
PRINTED
CIRCUIT
ASSEMBLY
6-55.
The
printed circuit assembly
is
available
on
an
exchange basis;
that
is, a factory-repaired
and
tested
assembly
may
be ordered
and
the
defective assembly
returned
for credit. Refer
to
Section
IV
for
ordering
instructions
and
the
exchange
part
number
for your par-
ticular assembly. Because
of
the
complexity of
the
cir-
cuits,
it
is
recommended
that
you replace
and
return
a
defective printed circuit assembly to Hewlett-Packard
rather
than
attempt
repair.
6-56.
DRIVE MOTORS AND
PEN
SOLENOID
6-57.
The
drive motor/encoder assemblies
and
the
pen
solenoid are
not
repairable. Motor
and
encoder cables
are replaceable.
The
motor cable (twisted pair)
is
PIN
07470-60016.
The
encoder (flat) cable
is
PIN
8128-4027.
I
WARNING
I
Maintenance
described
herein
is
per-
formed with power applied to
the
plot-
ter
and
the
top
case
removed.
Such
maintenance
should be performed only
by service-trained personnel who
are
aware
of
the
hazards
involved
(for
example, fire
and
electrical shock). Where
maintenance
can
be performed without
power applied,
the
line cord should be
disconnected.
Table
6-2.
Recommended Test Equipment
TYPE
RECOMMENDED MODEL
Computer/
controller
HP
Model 85 Personal Computer with
the
appropriate interface:
HP
82937A HP-IB Parallel Interface
HP
82939A RS-232-C Interface
and
HP
8120-3258
RS-232-C
Interface cable
Rom Drawer
HP82936A
I/ORom
HP
00085-15003
Digital Multimeter
HP3465A
Oscilloscope
HP
1740A
Optical Comparator
B&L
81-34-35
6-7
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