12 V Power Source; Power-Mute Signal; Auto Shut.off Circuit - Nakamichi 481 Service Manual

2.3.2. +12 V Power Source

Refer to Fig. 2.3.2 circuit diagram. Only +24 V DC power
supply is used in the N-481. The circuit acts to produce a
+12 V power source from the +24 V DC power supply.
Mechanism control is done by using thus produced +12 V.

2.3.3. Power-mute Signal

Refer to Fig. 2.3.3 circuit diagram and Fig. 2.3.4 timing
chart. Power-mute = L signal is produced pulse-likely
when Power Switch is turned ON or OFF. This L pulse
mutes the amp, circuit and also acts to shut off the shut off
circuit initially.
(1) Power Switch ON
Q433 is turned ON at every positive half cycle of the out
put from the secondary winding of the power transformer.
When Q433 is turned ON, C416 is discharged, as a result,
the voltage of C416 can not exceed the VBE of Q432, and
Q432 is in the cutoff state.
Therefore, the Power-mute =L pulse is produced for a
certain period of time when +24 V is built up after Power
Switch is turned ON.
The Power-mute= Lsignal makes Q4l6 to turn ON, as a re
-sult, Mute signal becomes H and the amp. circuit is
muted.
2.3.4, Auto Shut-off Circuit
Refer to Fig. 2.3.5 circuit diagram and Fig. 2.3.6 timing
chart.
(1) Shut-off Sensor
Light from lamp PL407 is projected through holes in a disc
rotating synchronously with the take-up reel, and the
intermittent flashes coming through the disc are converted
into electrical signals by a phototransistor Q450. These
signals are amplified into square waves, and transmitted
to the shut-off detecting circuit in the subsequent stage.
When the tape-end comes, the take-up reel and the disc
stops rotating, and no pulse is output from the sensor.
Meanwhile, the Power-mute = L pulse is applied to the
shut-off circuit and shut-off is activated.
(2) Power Switch OFF
The output from the secondary winding of the power
transformer ceases quickly, and Q433 is turned OFF.
Consequently, the base current flows to Q432 through
R481, Q432 is turned ON, and the Power-mute signal
becomes L.
The Power-mute L signal makes Q416 to turn ON, as a
=
result, Mute signal becomes H and the amp. circuit is
muted. At the same time, shut-off circuit is shut off by the
Power-mute = L signal.
(2) Shut-off Detecting Circuit and Peripheral Circuits
Shut-off conditions are as follows:
o reached tape-end during PLAY (PLAYBACK or
RECORD), FF, or REW mode
o mode is changed as follows:
from FF to REW mode, or vice versa from FF to
PLAY mode
from RECORD mode to FF or REW mode
When the mode is changed, shut-off is momentarily
activated and the mode is changed to STOP mode in
a short period of time, and after this STOP mode is
over, a new mode is set.
0 Power-mute = L pulse is generated when Power
Switch is turned ON or OFF
0 memory rewind function is activated.
11