Motor Current Sensor Circuit; Pen Down Circuit; Power Supplies; Recommended Test Equipment - HP 7475A Service Manual

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