The brake solenoid is powered by +24V. When the unit is turned off, the solenoid is deenergized and a pad on the end of
the solenoid arm, through a mechanical spring powered mechanism, puts pressure on the aluminum hub of the drive
motor and prevents it from rotating. When power is applied to the unit, the solenoid energizes, which releases the hub
and allows the motor to rotate. The solenoid serves two functions. The first, and most important function, is to rapidly
stop the rotation of the disk platters when power is turned off. This prevents unnecessary rotational contact between the
heads and the disk surfaces after the air-bearing is lost and the disks are slowing down. The second function of the
solenoid is to help prevent rotation of the spindle when the unit is turned off.
The Commutator PCB provides the commutated dc voltage to the coils of the brushless dc motor. Three light-emitting
diodes (LEDs) are mounted on a small PCB beneath the aluminum hub of the motor. These LEDs are illuminated
whenever the power to the unit is on. A vane on the aluminum hub is over the LEDs and rotates with the hub. Slots on
the vane alternately uncover the LEDs and allow light to pass through. Photo-sensitive transistors on the commutator
board are used to sense this light and generate signals X, Y, and Z. The relative on/off relationship of the three signals is
used by the Motor Control PCB to control the speed of the motor by controlling the frequency of the pulses to the coils of
Signals CE1* and CE2* are used to select one of the two pre-amplifiers on the small PCB that forms part of the seal to
the sealed cavity. Signals HSLT1* and HSLT2* select one of the four heads being received by the preamplifiers. Read
or write selection to the disk is also done through the commutator board by the signal WRITE SELECT.
An LM135 temperature sensitive Zener diode is used to measure the temperature of the air over the commutator board.
Three circuits are provided to generate over-temperature signals at three different temperatures. Only one is used to
light the OVER TEMP indicator on the indicator panel. The remaining two are reserved for special applications.
A Bad Track PROM generates signals BSOO-BS03. These signals are used by the electronics PCB to map logical track
addresses into physical track addresses, based upon the number of defective tracks on the disk discovered during
Power Supply. The AC Power Supply supplies all the dc voltages required by the disk unit. The supply
meets a variety of military standards, including MIL-STD-461 Notice 3, CE01 and CE02, and MIL-STD-704A.
The power supply (using an EMI filter) operates from an input voltage of 115V ac, 360-440 Hz. Three dc voltages, +5V, -
12V, and +24V are generated by the supply. The outputs are protected with current limiting and automatic overvoltage