HP 7475a Service Manual

Model 7475A
c. Remove the top case as instructed in Top Case
Removal.
d. Connect the power cord. Hold the ENTER push-
button down while setting the LINE switch to ON.
e. Press the I pushbutton.
f. Disconnect (from the printed circuit assembly)
the motor drive cable (twisted pair) indicated by
the results of step b.
g. Use an oscilloscope to look at the drive signal at
the connector. This should be a series of narrow
pulses. See Figure 6-6. If not, the problem is in the
motor drive circuit, servo IC, gate array, or the
microprocessor.
h. If the signal in step g. is correct, monitor the sig-
nal at pin 4 of the encoder cable (with the cable
connected to the PCA) while rotating the motor
manually. This should produce pulses varying
with the speed of motor rotation. If not, the encoder
is defective. Replace the motor/encoder assembly.
If pulses are present, the problem is probably in
the servo IC.
7475-A-16-1
NOTE
Plotter will not operate in any other
mode until power has been turned OFF
and ON.
Note; 20ms/Div., 10V/Div.
Figure 6-6. Drive Signal Pulses
6-64. PART REMOVAL AND REPLACEMENT
NOTE
All screws in the 7475A are metric sizes
except the No. 3 self-tapping screws
securing the pen carousel housing and
the pen drop shield.
6-65. TOP CASE REMOVAL
a. Set the LINE switch of OFF (0) and disconnect
the power cord and interface cable.
Section VI
b. Remove the screws at the rear of the plotter. See
Figure 6-7.
REMOVE THREE
SCREWS
Figure 6-7. Top Case Removal
c. Lift the rear of the top case, then the front will
release from the base.
NOTE
When replacing the top case, make
sure the PAPER LOAD/HOLD lever
extends through the case and the tabs
inside the front align properly above
and below the base plate.
6-66. PEN CAROUSEL HOUSING REMOVAL
a. Remove the top case as described in the preceed-
ing paragraph.
b. Disconnect the pen carousel cable from JB on the
main printed circuit assembly as shown in Figure
6-8.
Figure 6-8. Pen Carousel Cable
6-9

HP 7475a Specifications

General

Brand	HP
Model	7475a
Category	Plotter
Language	English