Solenoid Drive Circuit - Tascam 122 Service Manual

122
3-2-2 Output control circu it
When power is turned on, the initial reset cir ­
cuit puts U601 in the stop mode. This circuit
does not operate when power is turned off.
When power is cut off and +5.3 V is decaying,
U601 may generate a wrong signal momentarily
due to deviation from the logic threshold level.
The circuit comprising Q606, 0605, R629, and
R630 prevents the mis-operation of the deck
if U601 generates an incorrect signal.
When +5.3 V supply voltage falls to approxi ­
mately 5 V after power is turned off, zener
diode 0605 turns off and, therefore, Q606 turns
off together because its base current is cut off.
As Q606 is off, the emitters of transistors Q601
- Q605 are disconnected from the 0 V (GND)
line. As a result, Q601 - Q605 will not turn on
even if U601 generates an incorrect signal while
+5.3 V supply voltage is falling. This protects
the deck from erroneous operation.
Further, if power is turned off when the deck is
operating in any mode, the deck enters the stop
mode as soon as Q606 turns off, preventing the
tape from becomming slack.
On the other hand, when power is turned on,
0605 and 0606 do not go on until the +5.3 V
line rises to approximately 5 V. Q601 - Q605
do not operate before Q606is turned on. This
means the deck does not start to operate before
the DC supply voltage is sufficiently high.
3-3. SOLENOID DRIVE CIRCUIT
3-3-1 Functions of solenoids
There are three solenoids in the deck; fast sole­
noid (L691), head base solenoid (L692), and
pinch roller solenoid (L693). These solenoids
have specific functions and operate in certain
modes as described below.
1) Fast solenoid (L691)
This operates in the fast-forward and rewind
modes to perform the following mechanical
functions.
(' Releasing brakes applied to the left and right
reel disks.
• Disengaging the reel disk drive gear from the
right-hand (takeup) reel disk and locking the
intermediate pulley assembly in the fast ­
forward position.
2) Head base solenoid (L692)
This operates in the reproduce, pause, record/
reproduce, and record/pause modes to per­
form the following mechanical functions.
• Raising the head base to bring the erase and
record/reproduce heads into close contact
with the tape.
10
• Lifting the pinch roller to the pause posi ­
tion. (The pinch roller does not press against
the capstan shaft in this position.)
• Releasing brakes applied to the left and right
reel disks.
3) Pinch roller solenoid (L693)
This operates in the reproduce and record/
reproduce modes to bring the pinch roller
(raised to the pause position by the head base
solenoid) into contact with the capstan to
cause the tape to run at constant speed.
3-3-2 Fast solenoid drive circuit
See Fig. 3-4.
When the deck is in the stop mode, the circuit
shown in Fig. 3-4 is as fol ' l ows:
*
Pin 14 of U601: L level
*
Q605, Q515, Q516, Q518: off
*
Emitter of Q515: +13 V is applied via 0506.
During fast (fast-forward or rewind) mode, a
fast mode signal (H level) is output from pin 14
af U601. This signal causes fast solenoid L691
to operate in the following sequence.
a. As pin 14 of U601 is at H level, base current
flows through transistor Q605 and turns it on .
b. As Q605 is on, base current flows through
transistor Q515 and turns it on.
c. As Q515 is on, current flows via paths
CD
and
®.
(The supply voltage that causes current
to flow via paths
CD
and
®
is low and there ­
fore the cu rrent on path
CD
can not energize
the fast solenoid.)
d. The current on path
®
(which charges capa­
citor C516) causes transistor Q516 to tu rn on .
e. As Q516 is on, base current flows through
transistor Q518 and turns it on.
f. As Q518 is on, current flows via paths @ and
@. At this time, the supply voltage that
causes current to flow is +39 V and there ­
fore a large amount of current flows through
the fast solenoid (via path @) to energize it.
g. The current on path @ charges C516 and it
stops flowing when C516 is fully charged.
Since the base current of Q516 is cut off, it
turns off. The time interval between the turn ­
ing on of Q515 and the turning off of Q516
(i.e., the time taken to charge C516) is ap­
proxrmately 100 msec which is determined by
the time constant of capacitor C516 and
resistor R536.
h. As Q516 is off, Q518 turns off because its
base current is cut off. , I n this way the current
on path @ is cut off .