HP 7925D Service Manual page 58

Alignme~t and Adjustment
WARNING
The following adjustments are per-
formed with power supplied to the
disc drive, and protective covers re-
moved. Such maintenance should be
performed only by service-trained
personnel who are aware of the
hazards involved (for example, fire
and electrical shock).
b. Remove the terminal block cover on the power supply.
c. Using an HP 970 Digital Voltmeter (or equivalent
battery-operated voltmeter, for isolation from AC
ground paths), measure the voltage across terminals 1
and 4 on TB1 of the power supply (refer to figure
4-23). Also note the power supply strapping by com-
paring the strapping on TBl with the strapping
shown in figure 4-23. These measurements will be
used to determine the seek time adjustment range.
d. On the DSU, set the FUNCTION switch to position
1
(ALTERNATE SEEK).
e. On the DSU, select cylinder address 0 on the top ten
switches (all 10 switches set to the left).
f. With the lower bank of switches, select cylinder ad-
dress 822 (switches 512, 256, 32, 16,4, and 2 set to the
right).
g. On the DSU, press the START pushbutton and allow
the disc drive to alternately seek between cylinders 0
and 822.
h. On the DSU, rotate the DELAY potentiometer until
the seek time from cylinder 0 to cylinder 822 (forward
seek operation) can be differentiated from the seek
time from cylinder 822 to cylinder 0 (reverse seek
operation).
Note: The two seek times will probably be
different.
i. For both forward and reverse seek operations, observe
the digital displays to ensure that the seek time is in
the range specified in the following table and that the
deviation between forward and reverse seek times is
3.0 milliseconds or less. Use the values measured in
step c to determine the proper seek time range.
3-6
7925
STRAPPING LINE
OF THE VOLTAGE SEEK
POWER (Vac, as TIME
SUPPLY measured RANGE
(Vac)
in step c) (milliseconds)
100 90
to
95 46.0
to
49.0
96
to
100 45.5 to 48.5
101
to
105 45.0
to
48.0
120 108
to
110 46.0
to
49.0
111
to
115 45.0
to
48.0
116
to
120 44.5
to
47.5
121
to
126 44.0
to
47.0
220 198 to 200 46.5 to 49.5
201 to 210 45.5
to
48.5
211
to
220 45.0
to
48.0
221
to
230 44.5
to
47.5
240 216
to
220 46.0
to
49.0
221
to
230 45.0
to
48.0
231
to
240 44.5
to
47.5
241
to
252 44.0
to
47.0
Note: The seek time adjustment is set for
the best overall operation of the disc
drive and this time setting will vary
for each disc drive. The best opera-
tion of the disc drive does not neces-
sarily mean the shortest seek time.
Ifnecessary, adjust VC GAIN poten-
tiometer A3R33 on servo PCA-A3
until the values are within the
specified range.
j. On the DSU, press the STOP pushbutton.
k. On the disc drive, set the RUN/STOP switch to STOP.
1. Replace the terminal block cover on the power supply.
3-12. HEADALIGNMENTPROCEDURES
The head alignment procedures include the circumferential
alignment check, circumferential alignment, servo head
alignment, data head alignment, and data head alignment
check. Also, a warmup procedure is included which must be
performed before the alignment procedures or the align-
ment checks can be performed. (See figure 3-5 for head
alignment locations.)
The servo head alignment procedure should be performed
only when necessary. After the servo head has been
aligned, the data heads must be checked for alignment,
and aligned as necessary. Do not attempt to align the
data heads unless the servo head has been replaced, one
or more data heads have been replaced, the data head
algnment check, or circumferential alignment check
reveals that a head is out of the allowable tolerance.