Hitachi UD 1 Service Manual
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Page 1
SPECIFICATIONS, Track system Motor 4-track 2-channel 2-moîor system . Capstan drive section Uni-torque motor (DD motor) o Reel disc drive section DC motor (with mechanical gov- erno r) OUTLINE OF DEVELOPMENT ln cassette decks using direct drive motors, the capstan and the motor are directly connected, so principal perform- ance items such as wow & flutter, etc. depend directly on the performance of the motor. Multi-pole DC servo-motors and AC motors have been conventionally used for cassette decks; they have various advantages and d isadvantages. Hitachi has conducted research into direct drive motors with the advantages of both types of motor and succeeded in developing a flat, brushless DC servo direct drive motor. This motor is called a uni-torque motor because its torque is constant; vibration is minimized and rotation is smooth. A uni-torque motor, as developed for direct drive record players, has been adapted for use in cassette decks. ln addìtion, single-chip lC. which contains the speed detector circuit, speed control cìrcuit and motor drive circuit has been developed to minimize the number of parts. Fig. 3 is a block diagram of this lC. (1) Servo control A magnetic induction detection cìrcuit is to detect the motor speed. The principle used is that when the magnetic flux changes when there is a conductor in the magnetic f ield, an electromotive force proportional Speed detect ion (Magnetic inductionl Tape speed Wow & flutter Fast-forward time Rewind time Heads 4.7Scm/s 0.03% wRMs 90 sec (C-60) 90 sec (C-60) 3-head system (REC/PLAY combination head, erase head) * This specif ication may be changed for improvement without notice. DESCRIPTION OF UNI-TOROUE MOTOR to the rate of change of the magnetic flux occurs in the conductor. Fis. 1 As is shown in Fig. 1, there are 9 conductors for each pole; 72 conductors under the whole circumference. Assuming these conductors arg numbered 1-72, volL- age is generated in the 1st, 1Oth, 'lgth. 28th, 37th, 46th, 55th and 64th conductors which are positioned where polarity changes, and the direction of the voltage is the opposite for every other conductor, so directions of 8 voltages are same and added together. When the magnet turns, the voltage is induced in the ( next conductor in sequence. A signal which is inverted 72 times rotations of the magnet is obtained. the number of Since this signal l2t4 561A9 l5 l7 l9 t6 t8 Referencevo ltage Limiteramplilier Bolor magnet -2-
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Drive coil (-B I Hall elements consists of voltages from 8 positions on the circum- ference, it is a highly precise signal with a constant output even if there is any unevenness or instabiliîy in the magnet. The conductors are arranged as a highly precise prlnt- ed pattern. (2) Amplification, wave-shaping of the speed detection signal The speed detection signal has low level and an AM component remains in it, so errors will occur if it is used as it is. Accordingly, it is over-saturated and amplified by an amplifier with high gain and a shaping circuit forms it into a square wave signal. By making it a square wave signal, any AM component is llmited and errors become negligible. English Fis. 3 72 Sped detection pailern Pulse generator, f-V converter The voltage of the capacitor is inversely proportional to the speed because it is repeatedly charged and dis- charged at îhe speed deteclion frequency. The pulse generator generates a trigger signal synchro- nized with the speed detection signal. This trigger signal operates the capacitor charging/ discharging circuit and îhe voltage of the capacitor is converted to a DC voltage by a low-pass f ilter. Reference voltage, comparison amplification The DC voltage generated by the f-V converter and the reference voltage are compared and a difference volt- age is output. Since the reference voltage is îhe speed reference, its stability is very important, so a stabi- lization circuit is provided. (3) Differential amp - lnput stage _?_ (4)