The servo head signal is received on the electronics PCB as the differential signal SERVO R and SERVO B. This
differential signal is amplified and filtered and becomes signal SS. A feedback circuit between pins 4 and 11 of the
differential amplifier, consisting of a FET transistor resistor, provides Automatic Gain Control (AGC).
The Position Signal Generator uses signal SS to generate signal POSX and then POS. Signal POS is sent to the Motor
Control PCB, which uses it to keep the servo heads alined over the correct servo track.
POSX and its inverted signal POSXN are compared with a reference voltage by dual differential amplifiers. If either
signal exceeds 400 millivolts, the OFF TRACK signal will become true. When OFF TRACK signal is true, the Write
Enable (WEN) signal is automatically disabled.
In addition to controlling the position of the heads over a track, the electronics PCB also selects which of the five heads
are to be used and whether the operation is to be a read or write operation. Signals CHIP ENABLE 1* and CHIP
ENABLE 2* are used to select one of the two preamplifier chips on the preamplifier PCB in the HDA. Signals HSLT1*
and HSLT2* are used to select one of the four heads connected to each preamplifier chip. Since there are only five
read/write heads in the HDA, only the second chip has one head connected to it.
Motor Control PCB. The Motor Control PCB controls the rotation of the disk drive motor and, in
conjunction with the electronics PCB, controls the position of the read/write heads by controlling the movement of the
Two relays, K1 and K2, are deenergized when the disk unit is turned on. When they are deenergized, the current limiting
resistor in the HDA is in series with the +24V to the drive motor, limiting the current to 5.0 amperes during motor startup,
and the WRITE POWER signal to the disk is disabled. The rotary arm actuator is also held in the fully retracted position.
When the motor reaches approximately 2500 rpm, the HEAD LOAD ENABLE* signal, generated by the electronics PCB,
energizes the two relays (K1 and K2). The current limiting resistor is switched out of the motor circuit, the WRITE
POWER signal is enabled, and the rotary arm actuator is connected to the POWER AMP.
The signals generated by the LEDs and photo transistors on the Commutator PCB, X, Y, and Z, along with two other
motor control signals, MOTOR ENABLE and IMS (Increase Motor Speed) are decoded by the Motor Control PROM. The
six outputs from the PROM are combined by six separate transistor drivers to provide a sequence of 3-phase motor
power signals, OA, OB, and OC, which rotate the motor in the desired direction with nearly constant torque.
The speed of the motor is controlled by a digital servo loop consisting of a pulse generator, multistage counter, and flip-
flop. The signal X from the Commutator PCB triggers a one-shot every time it occurs, generating a negative going pulse.
This pulse is used to load a counter. The counter is clocked by a 400 kHz clock on the electronics board. If the counter
reaches a count of 2048 before the next X pulse restarts it, the signal SPEED LOW is generated. The next X pulse will
set a flip-flop, generating signal IMS. The IMS signal will remain true until the SPEED LOW signal goes false. When the
motor reaches its desired speed of 3600 rpm, it coasts until speed correction is again necessary.