Sony TC-766-2 Service Manual page 53

TC-766-2
8. Mechanical Tension Regulator
(Refer to Figs. 8, 9)
(Tension value at the tension arm part.)
Rc 6 changes
In tape transport, the tension arm mechanically
regulates the tape tension.
In this section the tension
Necessary torque to maintain
'constant value, '
f alue) constant tape tension.
. A<TB
(TA) at the entry point of the tension arm and
.
UA
the back tension
(TB) is as shown
in Fig. 9.
TB = K-TA
oo
Thus from the end of a 7-inch reel tape to the
K= ete
Py aes
start of a 10-inch
reel tape
the mechanical
K= (R)
tape tension
tension will be about 1.3 (experimental value)
Te
107
times the variation that occurs.
ree! diameter (R)}
3. If such
mechanical tension
is used, a tension
.
Fig. 9
relative to the radius of the tape amount is
obtained and the tape tension is regulated.
With only this change, however, back tension
will not be sufficient.
That is why this tape recorder uses both a tension
regulator circuit and a mechanical tension regulator
to produce the proper constant back tension.
9. Reel Motor Drive Circuit
(Refer to Figs. 10, 11)
The take-up reel motor operates under constant
torque and the supply reel motor is controlled by
a tension regulator circuit. Reel motors are driven Ov
in the following way.
1. As the base current of the drive transistor is
changed, the impedance between the collector
and the emitter varies and the voltage supplied
to the motor varies to change the motor torque.
By utilizing this relationship, the motor torque
can be controlled. (Refer to Fig. 10)
2. The characteristics of this drive transistor are
indicated in Fig. 11. The variation of collector
current Ic relative to collector-emitter voltage
Vcg is not sinusoidal, but produces a square
wave
that causes unnecessary
vibration in the
Jie
motor.
for
3. Therefore, a bridge rectifier circuit is used to
supply full-wave-rectified
current
to the base
VCE
of the drive transistor and raise base current Ig
Fig. 11
making collector current Ic closer to a sine
wave to drive the motor.