MAI Basic Four 2000 Series, Service Manual

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BFISD 8079
9.3 FUNCTIONAL INSCRIPTION
The following paragraphs describe the theory of the various electronic subsys-
tems that comprise the 50 Megabyte 5.25" Winchester Drive. This information
provides maintenance personnel with a comprehensive understanding of the func-
tions of the drive.
A brief discussion of disk recording principles is followed by a functional
description of the Winchester Drive unit, explaining how the drive interfaces
with the Winchester Drive Controller.
The interconnection of the major logic blocks is shown in figure 9-5.
9.3.1 Basic Disk Principles
The recording medium for the 50 megabyte drive is a stack of four 5 1/4-inch
disks with six recording surfaces (see figure 9-6). Each disk is coated with a
layer of magnetic oxide. The coating is burnished to a flatness that allows
the read/write heads to "fly" in proximity to the surface without actual phys-
ical contact.
Data is recorded in serial fashion on concentric rings on each disk surface by
holding the head in a fixed position over the rotating disk. (These rings are
referred to as "tracks." The disk drive unit positions the heads precisely
over the tracks. Corresponding track positions, both upper and lower on each
disk, are referred to as one "cylinder" of data.)
A center-tapped coil is mounted on the core of the read/write head to perform
the read/write function. The recording flux direction is controlled by ener-
gizing the center tap connection, causing current to flow through the bifiler
winding from the center tap to either one end or the other. When reading, the
ends of the coil are switched to the input of the read amplifier. Data is
erased by writing new data on the track. The read/write coil is mounted onto
the core of the ferrite slider.
As the disk rotates, it creates an "air bearing" around its surface. This air
bearing moves under the slider and exerts an upward force on the shoe. A down-
ward pressure is placed on the slider by the spring flexure of the head. Sup-
ported by carefully balanced aerodynamic forces, the head shoe then flies at a
point where the downward force of the load spring is equal in force to the air
pressure under the head. The slider's flying height is influenced, in part, by
the air-bleed channel in the middle of the slider. This channel allows some
air to bleed from under the slider, decreasing the air pressure. This, in
turn, allows the head to fly closer to the disk surface.
The slideris mounted to the head arm assembly via a spring gimbal-mount. This
allows the flying attitude of the slider to vary slightly so that it can follow
minor surface variations without contacting the disk surface. As the disk
starts, or stops, the head takes off from, or lands in, a silicone-lubricated
landing area. When the disk is not spinning, the head rests on, and actually
contacts, the landing zone on the disk.
All heads are mounted on a carriage such that the heads and carriage move as
one unit. The carriage moves the heads radially over the disks' surfaces. All
the heads are positioned at the same cylinder at any given time.
9-14