JVC RC-838L Service Manual page 3

Technical Information
Hall IC (Integrated Circuit)
This recorder is adopted with the full automatic stop
mechanism. When the tape is finished in any mode: record,
playback, fast forward or rewind, the tape transporting
mechanism stops and the corresponding control buttons
return to their normal positions and the power is shut off.
The tape stop is detected by the Hall IC which is located in
the magnetic field of ring magnet: the magnet is connected
to the shaft of the tape counter and it is rotating while the
tape is running.
Ring
Magnet
HALL ELEMENT
The Hall IC consists of the Hall element and amplifier as
shown in Fig. 2.
The Hall element is one of magneto-electronic converter
and possesses the Hall effect.
Vcc
Qi
Output
Q2
GND
Amplifier Output Stage
Hall Element
Output Voltage
|
The output voltage of IC is
proportional to the magnetic
field strength.
Flux Density B Fig. 3
The Hall IC' acts as an electronic switch which makes or
brakes a circuit according to the magnetic strength.
As the electronic switch does not have any contact, it
provides the longer serviceable life and the higher reliability
than mechanical switches e.g. reed switches.
B Ic
t
a
Z |
fe ||
a
Vi = Ry lle x B/t] Fig. 4
HALL EFFECT
When a conductor or semi-
conductor carrying a current
is placed in a magnetic field
as shown in Fig. 4, the output
voltage is expressed by
ae
RC-838L/LB
where Ry is a parameter called the Hall coefficient,
Ic is the control current,
B is the magnetic field strength,
t is the thickness of the element.
CIRCUITRY OF TAPE STOP DETECTOR
470pABV +
Fig. 5
1. While the tape is running
1) The voltage wave mode as shown in Fig. 6 appears on
the output terminal of the Hall IC.
Fig. 6 Fig. 7
2) The wave (Fig. 6) is amplified by the transistor X901
and the bias voltage as shown in Fig. 7 is applied to the
X902.
The X902 becomes ON state, and the voltage across the
collector and emitter of X902 decreases to cut off the
X903.
3) The collector current of X903 does not flow so that the
solenoid does not function.
2. When the tape stops
2) The DC voltage is choked by the capacitor C902.
3) The bias voltage does not change so that the X902
maintains the OFF state.
4) The voltage across the collector and emitter of X902
drives the X903 to become the ON state and the
collector current flows.
The solenoid functions to stop the mechanism.
3. When the PAUSE switch is pressed
When the PAUSE switch is turned on, the bias voltage is
applied to the X902 and the X902 becomes the ON state.
The following responses are the same as "While the tape is
running"'.
No. 1380