Dual C 824 Serviceanleitung Seite 4

Description of functions
Analogue section
Playback
The voltage induced in the sound head (approximately 300 12V during
playback of the DIN reference level) is first of all amplified by 0 101
and Q 102 and correspondingly equalized by the negative feedback
network R 106, R 109, R 110 and C 108. The playback frequency
response can be corrected in the treble frequency range by modifying
the sound head resonance with the aid of capacitors C 101 and C 102
(connection or disconnection). Switchover of the equalization from
120 ps to 70 ps is realized by Q 103 becoming forward biased. The
playback level is set at LINE OUT to 560 mV with VR 101 when
playing back the Dolby reference level (200 nWb/m).
The playback signal is routed through the trap circuit L 101, VR 101
and the recording/playback switch S 1-1 to the input of the Dolby cir-
cuit IC 105, Pin 5. If the Dolby selector switch S 3-1 is switched off,
the signal within the Dolby circuit is amplified linearly by approximately
20 db whilst it is amplified dependent on the frequency and level when
the Dolby switch is active. The output signal is routed from pin 7
(IC 105) to the monitor switch S 4-1 via the recording/playback switch
S 1-5 and R 151. In playback position, the "tape" signal is also offered
to the monitor switch via the recording/playback switch S 1-9 set to
"Source" (source monitoring). With this switch arrangement, the play-
back signal is routed to the output sockets MON. DIN and LINE OUT
irrespective of the position of the monitor switch. The playback signal
is also routed through R 125 to the headphone amplifier IC 103, Pin 3.
The output signal is routed to the display amplifier 0 104 with control
VR 102.
The output signal is muted with transistor 0 106 in order to ensure
that no switching noises reach the output sockets. The HIGH signal for
muting is generated by the device control (Plug A, Pin 7) in the case of
the following functions: stop, pause, fast forward and rewind.
Music finder
The music finder signal SCAN (active LOW) is routed from the device
control viaylug A, Pin 9 to transistor 0 301 which comes reverse biased
and enables the input of the music finder amplifier IC 102, Pin 3. The
AF signal ispicked up at Pin 6 of the Dolby circuit IC 105 and amplified
by IC 102. The signal is routed from the output (Pin 1) via transistor
302, which operates es a limiter, to Pin 6 of IC 102. IC 102 operates
es a trigger element. A static signal is applied to output Pin 7, in case
MUSIC PIECE isactive HIGH. If the pause SPACE is recognized between
two pieces of music, the output changes from HIGH to LOW. This signal
is routed to the device control via Plug A, Pin 9.
Recorsing
Recordings can be made from three different sources. The line input is
switched off when the DIN input is used. The signal coming from the
LINE or DIN input is switched off when a mono or stereo microphone
is connected.
The signal at the Mic input is amplified by the low noise IC 104 whilst
the signal from the DIN input is amplified by Q 601. The signal from
the LINE input is routed directly to the recording level control VR 103.
From here it is routed through the recording/playback switch S 1-3 to
the input Pin 5 of the Dolby circuit IC 105. With S2-1, the MPX filier
is connected via Q 105 in order to suppress residual pilot tone when
recording radio broadcasts.
Within the Dolby processor, the signal is branched to two different out-
puts, Pin 3 and Pin 7 of IC 105. When recording, the signal from Pin 3
uninfluenced by the Dolby process is routed through the recording/play-
back switch S 1-5 to the monitor switch 51-5 (source monitoring). The
signal from Pin 7 is routed directly to the recording amplifier 0 108 and
Q 109 via Fi 154. The recording current isset with VR 104 and the level
and equalization are matched to differing tape types with transistors
Q 110 (Fe), Q 111 (Cr), Q 112 (FeCr) and Q 113 (Met). The recording
signal is routed to the recording head via the trap circuit L 103. The bias
ia added with VR 105. By connecting the resistors R 352 to R 355 with
Q 312 (Fe) to 0 309 (Met) via the tape type selector switches S 5 to
S 7, the oscillator is influenced in such a way that an Optimum operating
point is obtained for the various tape types. The oscillator is switched
on with the recording/playback switch S 1-11. The erasure voltage is set
with VR 107. In record mode, the DIN output is switched off with the
recording/playback switch S 1-9.
Monitor
Contrary to playback, the signal at Pin 3 (IC 105), which is uninfluenced
by the Dolby process, is routed via S 1-5 to the monitor switch S 4-1
and, when the switch is set to SOURCE (source monitoring), is routed
to the outputs LINE OUT and MONITOR. When the monitor switch is
set to TAPE, the information recorded on the tape can be directly
monitored from the tape (off tape monitoring) via the playback branch
and the intermediate amplifier (IC 101). As cross talk of the bias within
the recording/playback head is unavoidable in this operating mode, the
bias must be suppressed by filter L 101.
The gain of the intermediate amplifier IC 101 is approximately 26 db in
order to prevent level losses between TAPE and source monitoring.
Muting
The recording amplifier is muted for all functions except RECORD.
This control signal MUTING RA (record amp) is active HIGH and is
routed from the device control via Plug A, Pin 8 to transistor 0 107
which applies 0 V to the input of the recording amplifier. With this
control signal, a music gap lasting approximately four seconds is also
created during recording when AUTO SPACE is operated.
Level indicator
During both recording and playback, the outputsignal is routed through
the headphone amplifier IC 103 to the display amplifier 0 104. Fre-
quency response correction is carried out (C 124) in order to take into
consideration the reduced modulation at high frequencies. Boosting at
10 kHz with respect to 400 Hz amounts to approximately 10 db. The
+ 3 db mark is set with VR 102. The value isdisplayed by two VU me-
ters with damped return (C 126).
Autotape, tape type selection
The new cassettes on the market have a tape type marking in the form
of recesses on the rears of their housings which are scanned by contact
sensors in the tape deck. When the pushbutton AUTOTAPE is latched
in position, any previously operated FeCr or Met tape type selection
button is mechanically reset and the contact sensors S 508 (FeCr) and
S 509 (Cr) are activated. These switches scan the rear of the cassette for
identity recesses and, via the NO Fl gate (IC 301), control the tape type
indication, the equalization and the bias. This control signal is active
HIGH. Switch open = recoginition of the recess.
•
LE:LI
m ii
Cr FeCr
t
--- Met - 1
Autotape 9aitch Matrix
Cr
FeCr
S509 S508
L,
L,
L,
2
•
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Back Side
Function
Met
FeCr
Cr
Fe
4