Teac FD-55 Series, Maintenance Manual

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Outputs of the direction latch and the internal step pulse generator
are input to the shift register and phase drive selector, and they
are converted to the appropriate timing signals for uni-polar one-phase
drive (partially 2-phase drive) of the 4-phase stepping motor. These
phase drive signals are supplied to the stepping motor driver, U 8
(output pins 10 ^ 13). In order to improve the torque margin in the
seek operation, partially 2-phase drive period is provided by the
phase drive selector only ±n the initial stage when the drive phase
is changed. ; - v - c
The output from the internal step pulse generator is also supplied to
the over-drive timer constructed with a re-triggerable counter^ During
the active period of this timer (75msec,approx.), +12V power is supplied
to the stepping motor coils through the over-drive circuit (Q3) to
produce enough torque required for the head seek and the settling
operations. After the completion of the settling, only +5V power is
supplied to the stepping motor through the diode, C R3, which minimize
the power loss by supplying only the required torque for the holding
of the stop position. By the above consideration, heat radiation from
the motor is depressed to the minimum level and the stepping motor
power consumption of 0.25W, approx, at the stop condition is achieved.
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