Sony TC-765 - Page 47

5. Reel Motor Starting Circuit (Refer to Fig. 5)
When switching from STOP to forward or from
PAUSE to forward the Reel Motor (M601, 602)
cannot start up right away. (Especially true when
using a 10-inch reel.)
For this reason, the torque of the Reel Motor
(M601, 602) is greater at starting than during running.
1. With the IC701 output »?, Q712 (for solenoid
drive) goes ON and C740 causes Q737 to go
OFF for an instant. (The operation is the same
as the solenoid initial-energizing.)
2. When Q737 goes OFF, Q733 goes ON and the
signal passes through R794 and D742 to add to
Q736. This permits Q736 saturation and about
50V is applied to the supply reel motor (M601)
for starting it. (Path (i))
3. For the take-up reel motor (M602), as Q737
turns OFF, Q738 turns ON and energizes relay
(RY702) which supplies 70V from the power
transformer for motor starting. (Path (?))
4. When discharge at the time constant determined
by R803 and C740 is finished, Q73-7 returns to
the ON state and Q738 turns OFF releasing the
relay (RY702).
5. Q733 gradually goes OFF at the time constant
determined by R792 and C735. Afterwards,
the supply reel motor (M601) changes to
tension-regulator operation and the take-up reel
motor (M602) becomes _ constant-torque
operation.
6. D753 grounds Q739 base bias supplied through
Q733, R793 and D743 to prevent short-
circuiting of the 70V and SOV of the power
transformer through Q739 and the diode bridge
during starting of the take-up reel motor. The
reason for Q733 having a time constant prevents
B+
IC output
pe
Q712
For pinch roller
contact pressure,
solenoid drive.
| TC-765
tension arm vibration caused by voltage varia-
tion when the starting operation changes to the
running operation.
6. Tension Regulator Circuit
If motor torque is uniformly constant, the back
tension torque of tape transport becomes stronger as
tape winding diameter becomes smaller. This causes
variations in the tape speed, tape-to-head contact,
level and frequency characteristics.
Hence, in order to keep tape tension constant,
even when tape winding diameter changes, a tension
regulator circuit (including a mechanical tension
regulator) is employed to control the torque of the
supply reel motor.
1. Since the tape speed is constant in the forward
mode, as the amount of tape on the supply reel
decreases, the revolution speed of the supply
reel (frequency of FG) increases. In other
words, FG generates the signal of frequency in
inverse proportion to the radius of the remain-
ing tape.
2. By rectifying the signal, dc voltage in inverse
proportion to the radius of the remaining tape
is obtained. ,
3. If the supply reel motor torque is constant,
tape tension at the supply reel exit will be in
inverse proportion to the tape amount. In
other words, to maintain constant tape tension
a motor torque which is proportional to the
tape amount is the best.
4. Hence, by using the dc voltage obtained in 2.,
constant tape tension is achieved.
(Please refer to the next page for tension set-up and
circuit operation.)
saturation