F.fwdand Rewind Modes - Teac C-1 Service Manual

20
TAPE TRANSPORT SECTION
4-4 F. FWD AND REWIND MODES
A high level signal is supplied from the contro] PCB to
terminal 7 in the F. FWD mode. Thus, Q655 turns on and
then Q656 turns on and current flows in the right reel
motor via route (1). In the REWIND mode, a high level
signal is applied to terminal 6 and Q657 and Q658 are
turned on and the left reel motor is driven. When the
F. FWD button and the REW button are depressed at the
same time, the SYS-CON outputs both F. FWD and REW
signals. When Q657 is turned on with the REW signal, the
collector current of Q657 drops the level of terminal 7
through D656. Thus Q655 cannot turn on. That is, the
REW mode is preferential in this case.
In either F. FWD or REWIND mode, the take-up motor
load is light at the begining of tape winding and gets
heavier towards the end. Therefore, if the motor is driven
by a constant current, the tape winding speed will slow
down towards the end of the winding. To prevent this,
a current supply circuit comprising Q653 and Q654 is
provided. As the motor load becomes heavier and the
motor speed slows down, the motor impedance (load
resistance of the circuit) lowers and the potential at
point (A) drops. The potential also drops at points (B),
(C) and (D). Point (D) has the equivalent potential to the
drop at point (C) after it has been divided by R658 and
R659. Thus, the potential drop is larger at point (B)
than at (D). Asa result, Q654 is biased more strongly and
the collector current is increased. Increase in Q654 collec-
tor current means an increase in C653 base current. Thus,
the collector current of C653 increases and raises the load
current. When the collector current of Q653 increases, the
potential at point (C) and (A) increases. When the potential
at point (A) increases, the potential at point (B) also in-
creases and the bias given to Q654 tends to become smaller.
However, since the potential at point (B) also increases
together with that at point (C), the bias given to Q654 will
balance when the collector current of Q653 increases to
cov) REEL MOTOR CONTROL PCB
+10V
Ov
HIGH at F.FWD
HIGH at REW
some extent. In the above process, the output voltage of
the circuit increases gradually and raises the load current as
the motor load becomes heavier.. The potential at the
point (A) varies in the range of about 3.0V to 6.5V from
the start to the end of tape winding.
In the F. FWD mode, for example, the drive current flows
in the right reel motor via the route (1) and the motor
rotates forward and takes up the tape. The left reel motor
is pulled by the tape and is rotated inversely generating
back electromotive force in the direction of the route (2).
This back electromotive force is absorbed by the loop in
route (3) and acts as an electromagnetic brake. The reverse
rotation speed of the left reel motor increases towards the
end of the tape and so the back electromotive force is
increased and the loop current is raised. The larger the loop
current, the stronger the electromagnetic brake force. The
back electromotive force absorbing circuit and the motor
current supply circuit described before suppress extreme
change in the tape take-up speed in the fast forward mode.
C653 inserted into the base circuit of Q657 is used to delay
the start of the reel motors in the rewind mode for the
following reasons:
In the PLAY mode, the head solenoid operates and pulls
the erase head and the R/P head into the cassette so as to
contact the tape. If the tape is driven at high speed with
the heads in contact with the tape it is bad both for the
tape and the heads.
Particularly in the AUTO REWIND mode, when the
REWIND mode is selected immediately after the tape end
is reached during the PLAY mode, the tape tension is very
strong at the start of the rewind operation and thus the
head solenoid turns off. In this case, the rewind operation
must be delayed until the head is clear of the tape.
About 40 milliseconds delay elapses during which terminal
6 of the reel motor control PCB goes high, C653 is charged
and Q657 is turned on, and then Q658 turns on and the
motor is driven.
( )VOLTAGE at STOP MODE
Fig. 25 F. Fwd. and Rewind Circuit