Panasonic RQ-2720 Service Manual page 5

Speaker Monitor Muting During Recording
To prevent howling during recording, it is designed so that monitoring
cannot be done from the built-in speaker.
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When the unit is set to REC mode, Q2 is switched on as bias voltage
is added to the base of Q2. As a result, the input of IC2 is earthed
so that signal does not flow to the speaker. This is called "speaker
monitor muting".
With the earphone inserted into J2, the switch in the jack is changed
over, and Q2 to turn off causing the base bias of Q2 goes out, and
then the monitoring sound can be heard through the earphone.
The sound volume can be adjusted by VR3. This is called "variable
semi-sound monitor" in record mode.
Tape Speed Control
1.
Constant speed control
The motor used for this unit employs a bridge servo (DC electronic
governor) system to keep the tape speed constant.
The above diagram can be simplified as shown in fig. 18.
(a) When the motor speed is increased:
The electric potential at @) drops because the reverse
electromotive force increases.
— potential at © drops— potential at © drops— potential at
© drops.
—collector potential at Q104 drops — voltage applied to motor
drops — motor speed decreases.
(b) When the motor speed is decreased:
The electric potential at @) rises because the reverse
electromotive force decreases.
Henceforth, the operation is the opposite of (a).
Thus, the motor speed is kept constant.
Tape speed control in playback mode
Tape speed control of about + 20% is possible during playback
operation. When switch S5 in VR101 (tape speed contro! volume)
is set to ON, the Q102 base bias varies as VR101 changes.
This finally causes the voltage applied to the motor to be regulated,
thus controlling the tape speed. During record mode, this function
does not work because of S1-2 changeover.
Safety Timer Function
In FF or REW mode, if the STOP button is not pressed after the tape
is completely reeled up, the mechanical load (motor load) will greatly
increase, resulting in excessive consumption of the battery power.
In particular, since this unit is designed to operate at low voltage (3 V),
excessive loading will greatly affect its performance.
The FF or REW
mechanism is provided with a safety timer function so that the
consumption of battery power can be minimized even if the operational
mode is not released.
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2.
3.
When the unit is set to FF or REW, S6 turns ON (Sn. ) and then
the safety timer circuit starts operating. 7
C102 starts to be charged through R122.
At that time, each transistor operation is as follows.
Q106 Q107 Q108
ON OFF OFF
Transistor No. Q105
Condition OFF
Q2
Fig. 16
— Tape speed control circuit —
+8
VR101: Playback tape speed control
VRIO2' Tape speed adjustment VR.
51-2: Record/playback switch (Prayoa
SS: Playback tape speed control
cK poten)
'ON/OFF switch
R103: Timer adjustment VR
6: Timer switch (shown in OFF position)