Hp 7925H Air Circulation And Filtration System - HP 7925D Service Manual

Appendix A
POWER AND MOTOR
REGULATOR PCA·A9
I"'~~~=--~.!.....:.....~_ ABSO LUTE
FILTER
CENTRIFUGAL
BLOWER
PREFILTER
ENCLOSURE
PREFILTER
7925
7301-98
Figure A-9. HP 7925H Air Circulation and Filtration System
the heat sink on power and motor regulator PCA-A9 and
the remaining path is distributed over the components
mounted on the PCA. The flow of air from the heat sink
exits through the ducting and vent openings provided at
the rear of the enclosure. The fan on the card cage assem-
bly directs cooling air to the peA's housed in the
assembly.
The remaining half of the developed air flow passes
through the filtration element in the absolute filter where
99 percent of all contaminants 0.3 micron or larger are
trapped. After the air is thoroughly filtered, it is ducted
into the pack chamber. When a disc pack is installed, all
critical areas will be purged of any foreign matter. Also,
the high positive pressure developed within the pack
chamber tends to reject any foreign matter that may be
airborne.
Figure A-IO shows the critical elements involved in the
read/write process, i.e., the read/write gap, the flying
height of the heads, and the thickness of the oxide coating
on the disc surfaces. The flying height of the heads is an
average value .due to the surface irregularities of both the
heads and discs. Figure A-IO also shows various types of
contaminants and their size relationships.
If
a particle
was hard enough and of the right size, it could scratch
A·3D
either the oxide coating or the head surface. Even if it was
not hard enough to scratch, it may be large enough to
increase the head-to-disc spacing, thereby causing data
errors.
Therefore, to prevent potential damage due to head-to-disc
contact and possible data losses, it becomes extremely
important to maintain the cleanliness of the air supply
within the disc drive. To ensure that clean air will be
present, the disc.drive must be operated in the specified
environment and the cleanliness of the prefilter and effi-
ciency of the absolute filter must be checked on a regular
basis. Refer to Section II, Maintenance in the main man-
ual, for the absolute filter output air pressure measure-
ment procedure. Further, the absolute filter must be
changed whenever the air flow through it becomes re-
stricted and the output air pressure drops below the value
specified in paragraph 2-13 in the main manual. Refer to
Section V, Removal and Replacement in the main manual,
for the prefilter and absolute filter replacement
procedures.
A-66. POWER DISTRIBUTION SYSTEM
The power distribution system primarily consists of the
power panel assembly, power supply assembly, voltage