Sony TC-730 - Circuit Description; Record Amp Circuit; Muting Circuit

CIRCUIT DESCRIPTION
The following describes operation of main circuit:
1. RECORD AMP CIRCUIT
First stage of the record amplifier comprises
NPN and PNP type transistors connected in parallel.
This circuit improves linearity characteristics of
record amplifier for MIC jack input signals.
Approximately 50 dB linearity can be obtained.,
Therefore, can record signals with greater input level
(approximately —10 dB, 0.25 V to MIC jack) with
low distortion. Linearity for conventional circuit
which has only one transistor is approximately 30 dB.
2. MUTING CIRCUIT
During direction change of tape transport (about
three seconds), from the time when tape stops moving
until tape motion reverses and recoveres to normal
tape speed, B + voltage of bias oscillator is cut off in
record mode and the muting circuits (Q306 and Q406)
of playback amplifier are activated in playback mode.
When turning POWER switch ON, the multivibrator
(Q504 and Q505) is kept stable as follows:
POWER switch is turned ON.
Initial current through C505 turns 0505 ON.
Decreased collector voltage of 0505 turns Q504 OFF.
After C505 has been charged, Q505 base bias is
applied through R507.
0505 is kept ON.
Decreased collector voltage Decreased collector voltage
of Q505 keeps 0306 and of 0505 keeps O506 OFF.
Q406 OFF.
Playback amplifier normal- O503 is kept ON and sup-
ly works. plies B+ voltage to bias
oscillator.
When changing the unit from FWD mode to REV
mode or vice versa, this circuit acts as follows:
FWD— REV REV — FWD
Negative trigger pulse Positive trigger pulse
through D614 turns through D613 turns
Q505 OFF. Q504 ON.
Increased collector voltage
of 0505 turns.0504 ON.
Collector voltage of Q504 decreases.
Current flows through R506 and through R507 and
C505.
C505 discharges for about three seconds. During the
discharge, Q505 is turned OFF by decreased base bias.
Increased collector
voltage of O505
turns Q506 ON.
Increased collector
voltage of Q505
turns Q306 and
Q406 ON.
Q306 and Q406 Q503 turns OFF
ground audio and cut off B+
signals. voltage to bias
oscillator.
After the discharge, current stops flowing through
R507 and C505.
Q505 is turned ON by increased base bias.
0504 is turned OFF.
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