Nakamichi 481 Service Manual - page 13

(a)  Reached tape-end during PLAY (PLAYBACK or 

RECORD), FF, or REW mode 

Explanation is made for PLAY mode as an example. For FF 

or REW mode, the shut-off function is the same as for PLAY 

mode.  

As Play Button is locked ON mechanically, Kplay = L. 

Accordingly, R488 (100 kohm) is grounded through Play 

Button and the voltage at the point A becomes approx. +23 

V. Since the voltage at the point A is not lower than the 

emitter voltage of Q424, Q424 is turned OFF and Q426 is 

also turned OFF. (Q424 and Q426 will be turned ON when 

the voltage at the point A is further lowered as described in 

subsequent (b).) 

Q425, Q427, Q430, R470 and C412 consist of a shut-off 

detecting circuit. During PLAY mode, the voltage at the point 

A is approx. +23 V. therefore, Q425 is turned ON and C412 

(2.2 pF) is charged toward +24 V through R470. 

Meanwhile, pulses from the shut-off sensor are applied to 

the base of 

Q427 through R489 and C418, and, at every H 

cycle of the sensor output pulse, Q427 is turned ON and 

C412 is discharged through Q427. When the tape-end is 

detected, pulses from the shut-off sensor are not trans- 

mitted and Q427 is turned OFF, resulting in C412 being 

charged continuously. 

When the voltage of C412 exceeds the sum of the emitter 

voltage (approx. 5.5 V) and the VBE of Q430, Q430 is 

turned ON and the base current flows to 0429. Con 

-sequently, Q429 is turned ON, Q428 is cut off, +24 VS is 

shut-off, PLAY mode is changed to STOP mode, and play 

lamp goes out. 

Q430, Q429, Q428, R476, R474, R456 and C420 consist of 

a Schmitt circuit which provides hysteresis characteristics 

for ON/OFF of Q430. Accordingly, Q430 will be 

turned ON or OFF without chattering for the input waveform 

with a large time constant developed across C412. If Pause 

Button is pressed during PLAY mode, tape stops and no 

pulse is transmitted from the shut-off sensor, but Q427 is 

kept ON since Q423 is turned ON during PLAY.PAUSE 

mode, therefore, no charge is made at C412 and shut-off is 

not activated. 

When shut-off is made at the tape-end during PLAY mode, 

PLAY mode is changed to STOP mode. 

If Stop Button is further pressed, Play Button will be re 

leased and the voltage at the point A returns to +24 V as 

R488 is released from grounding, as a result, Q425 is 

turned OFF and C412 is discharged quickly through D423 

and R467 (10 kohm). Accordingly, Q430 is turned OFF, 

Q429 is turned OFF, Q428 is turned ON, and +24 VS is 

again applied preparing for the next control button opera- 

tion.