Akai GX-370D Service Manual

VII. TRANSPORT MECHANISM

1. TRANSISTOR, RELAY, AND PLUNGER SOLENOID OPERATION CHART
(Refer to Schematic Diagram 1 through 6)

MY NOTE – applies to early system board KH1009A, 'A type', serial No.s up to 70809-2000

(Refer to Schematic Diagram 7 through 12)

MY NOTE – applies to later system board KH1039A, 'B type', serial No.s 71011-0001 onward

2. VOLTAGE SUPPLY TO TORQUE MOTOR AND TENSION AT VARIOUS MODES

VIII. SERVO MOTOR OPERATING PRINCIPLES

1. The detector head core is comprised of a permanent magnet, and the gear installed on the motor, by means of the capstan motor revolutions, changes the magnetic flux of the detector head core. Accordingly, the detector head coil works in the same way and generates AC Voltage (becomes the detector signal described below). When this detector signal voltage (ei) becomes detector signal frequency (fo), this adjusts the capstan motor revolutions proportionately. (See Figs. 21 & 22)

2. When the detector signal voltage generated from the detector head is about 3 mV (at 7-1/2 ips), because the level is low, the perpendicular (up and down) waveform is amplified by the AC Amplifier until the waveform is clipped. (See Fig. 21)

3. Discriminator Coil L-1 (V12031SC-01) and C-210 (0.051/50) at 7-1/2 ips (and C-211 (0.27/100) at 3-3/4 ips) constitute the resonance circuit, and this resonance frequency becomes fr. Because the detector signal frequency generated at the detector coil differs according to capstan motor revolutions, the capacity of the discriminator resonance condenser changes, and the resonance frequency changes at the different tape speeds of 7-1/2 and 3-3/4 ips.

4. When the discriminator input frequency and the resonance frequency fr are simultaneous, the DC signal to be supplied to the next stage DC Amplifier is not generated. Consequently, when the capstan motor rotates at normal speed, a higher than resonance frequency fr, detector signal. Signal frequency fo is established.

5. As shown in Chart 4, when electric current is not flowing to the capstan motor servo coil, the capstan motor revolutions are far faster than normal revolutions. Consequently, in order to maintain normal revolutions, an electromagnetic field is generated at the servo coil by means of collector current flowing to TR-209 (2SD234), and this serves as an electromagnetic brake. This electromagnetic brake and the load torque balances the capstan motor torque and normal revolutions are maintained.

TAPE SPEED	Capstan motor supply voltage	Capstan motor supply voltage at FWD or REV starting time	Voltage and frequency generated at the detector coil	Controlled capstan motor speed	Uncontrolled capstan motor speed
7-1/2 ips	110V	150V	3 mV ±0.5 mV r.m.s/1040 Hz	520 r.p.m.	1420 r.p.m.
3-3/4 ips	85V	110V	1.5 mV ±0.25 mV r.m.s./520 Hz	260 r.p.m.	1380 r.p.m.
Chart 4

6. In case of the capstan motor revolutions having reached the speed of n2, the detector signal produced at the detector coil surpasses fo and advances toward the higher f2 frequency. (See Figs. 23 & 24). Accordingly, the flow of current to the servo coil increases, and because the electromagnetic braking supplied to the capstan motor is increased, the capstan motor revolutions become slower, and normal revolutions are regained.

7. In case of the capstan motor revolutions having slowed to n1, the detector signal produced at the detector coil drops to lower than fo, and descends toward the low f1 frequency (See Figs. 23 & 24). Accordingly, the flow of current to the servo coil decreases, and because the electromagnetic braking supplied to the capstan motor is decreased, the capstan motor revolutions speed up and normal revolutions are regained.

8. To obtain the proper number of revolutions, adjustment of the flow of brake current to the servo coil is necessary.

1) At 3-3/4 ips tape speed
As shown in Fig. 23, resonance frequency fr (between the arrow mark) is changed by adjusting the dust core of discriminator coil L-1 (V12031SC-01). Accordingly, fo is also changed between f1 and f2, and the flow of braking current (io) to the servo coil is also changed between i1 and i2. Consequently, correct tape speed can be attained by using a tape speed measuring tape and a Frequency Counter and adjusting the dust core of coil L-1.

2) At 7-1/2 ips tape speed
As shown in Fig. 24, Qo, of the resonance circuit (within the arrow mark with fr as center frequency), is changed by adjusting discriminator semi-fixed resistor VR-202 (2k B). Therefore, fo, is also changed between f1 and f2 and, the flow of current (io) to the servo coil is also changed between i1 and i2. Consequently, correct tape speed can be attained by using a tape speed measuring tape and a Frequency Counter and adjusting semi-fixed resistor VR-202.

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