Signal Switching Circuit; Nrsystemcircuit - Teac C-1 Service Manual

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AMPLIFIER SECTION
3. SIGNAL SWITCHING CIRCUIT
The audio signal is routed by the SOURCE and NR
SYSTEM switches and controlled by the FET switchs.
Figure 3 shows the signal switching schematically.
3-1 NR SYSTEM CIRCUIT
(1) dbx (EXT) position
Q805 and Q811 go on and the dbx LED lights up if the C-1
is connected to a dbx unit (RX-8). LINE IN and MIC
signals are selected by the INPUT switch and amplified by
U301 after being level controlled by the RECORD gain
control. The output from U301, called the LINE INPUT,
is split and sent to the dbx encoder and to the monitor
circuit (Q809). The encoded signal is received back from
the dbx unit at the RCV terminals and is fed via Q811 and
U302 to the record head. The signal from the playback
head is sent to the dbx decoder after amplification. The
decoded signal is returned to the RCV terminal, then via
Q805 and on to the monitor circuit via Q807. If the NR
SYSTEM switch is in the dbx (EXT) position but without
a dbx unit being connected, recording and playback are not
possible, though the LINE INPUT signal can still be moni-
tored.
(2) OUT Position
Q803 and Q813 are turned on. The LINE INPUT signal is
split, directly to the monitor circuit (Q809) and the other,
bypassing the Dolby Encoder via Q813, is amplified by
U302 and fed to the record head. The signal from the play-
back head is amplified and then fed to the monitor circuit,
bypassing the Dolby Decoder via Q803.
(3) DOLBY Position
In the DOLBY position Q801 and Q815 are on and the
Dolby LED lights up. The LINE INPUT signal is split,
one path going to the. monitor circuit (Q809) while the
other is routed through the Dolby Encoder by Q815 and
thence via U302 to the record head. The signal from the
playback head is decoded by the Dolby Decoder and then
goes to the monitor circuit.
3-2 MONITOR CIRCUIT
(1) SOURCE position
Q809 and Q817 are turned on. The LINE INPUT signal is
split into two routes after passing through Q809. One signal
passes through the OUTPUT volume control, and is sent to
the OUTPUT terminal after being amplified.
The other signal is sent via Q817 to the PPM drive unit and
drives the pick level meter needle.
The peak level meter indicates the recording input level
adjusted by the RECORD volume control. The OUTPUT
volume control adjusts the source monitor output level.
(2) TAPE CAL position
Q807 and Q817 are turned on. The playback signal is split,
one path going to the OUTPUT terminal and the phone
amplifier via the OUTPUT volume control amplifier while
the other is routed to the meter circuit via Q817. Thus the
peak level meter indicate the level recorded on the tape. the
OUTPUT volume control changes the monitor output level
without changing the meter indication.
(3) TAPE OUTPUT position
Q807 and Q819 are turned on. The tape is monitored as in
the TAPE CAL position. However, since the meter circuit
input is taken after the OUTPUT volume control in the
TAPE OUTPUT position, the meter indicates the value
controlled by the OUTPUT volume control.