Dual C 844 Serviceanleitung Seite 5

Description of functions
Analog section
Playback
The voltage induced in the playback head (approximately 300 IN du-
ring playback of the DIN reference level) is first of all amplified by
501 and Q 502 and correspondingly equalized with the negative
feedback network R 509, R 508 and C 508. The frequency response at
4 kHz can be slightly modified by closing or opening the strap J 1. The
high frequency range can be corrected by replacing the capacitor C 509
(plug-in type). In the case of the 9.5 cm/s tape speed, the playback fre-
quency response is corrected within the 20 kHz range with VR 501 and
L 501. VR 501 should not be adjusted. Adjustment is carried out at the
factory. The basic position of VR 501 is the middle position.
Changeover of the equalization from 120 s to 701./s is realized by the
tape type selector switch S 21 - 3 which causes 0 505 to become for-
ward-biased. At the 9.5 cm/s speed, transistor Q 506 switches over to
35 1.4s whilst 0601 renders transistor 0 505 reverse-biased. During play-
back of the Dolby reference level (200 nWb/m), the playback level at
LINE OUT is set to 550 mV with the VR 502.
The blayback signal is routed through the stopper circuit T 501,Q 504
and VR 502 to the input of the Dolby circuit IC 103, Pin 5. If the
Dolby selector switch S 22 is switched off (0 114 forward-biased), the
signal is amplified in a linear manner by approximately 26 dB within
the Dolby circuit whilst amplification is frequency and level-dependent
when the Dolby circuit is switched on. The output signal is routed from
pin 7 (IC 103) to the input pin 2 (IC 104) of the Dolby circuit via VR
104. Also in this case, when the Dolby circuit is switched off (Q 116
forward-biased), the signal is amplified by approximately 26 dB in a
linear manner (referred to the input level Pin 5, IC 103). The output
signal is routed from Pin 7 (IC 104) to the relay RY 1 which realizes
the source-tape monitor function. During playback, RY 1 and the mo-
nitor switch S 24 are inoperative and, irrespective of the position of the
monitor switch, the tape signal is routed through the amplifier IC 302
to the outputs DIN, LINE and MON. The playback signal is also routed
through R 531 from the LINE
OUTPUT to the headphone amplifier
IC 601 and through R 284 to the VU meter amplifier 0273.
In order to prevent switching noises from reaching the output sockets,
the output signal is muted with Q 271 (see "muting") when the follo-
wing functions are selected: stop, pause, record, standby, fast forward
and rewind.
AFC compensation for IC 104 is realized with the transistor 0 120.
120 is control led by IC 103.
Music finder
The music finder scan signal (active LOW, IC 409, Pin 31) is routed
from the processor to transistor 0414, this becomes reverse-biased and
enables the input of the music finder amplifier IC 403, Pin 2. The AF
signal is picked up by the Dolby circuit IC 103, Pin 3 and is amplified
with IC 403. From the output (Pin 1), the signal is routed through tran-
sistor Q 413, which operates as a limiter, to Pin 6 of IC 403. IC 403
(Pins 5,6, 7) operates as a flipflop.
A static signal is applied at the output Pin 7 and, os the result of this,
PIECE OF MUSIC is active HIGH. If a SPACE is recognized between
the pieces of music, the output changes from HIGH to LOW. This signal
is routed to Pin 38 of the processor IC 409 via R 432.
Recording
Three different sources are available for recording. The LINE input is
switched off when the DIN socket is used. The signal at the Mic Input is
amplified by the low-noise IC 301, whilst the signal from the DIN input
is amplified by Q 272. The Mic signal is routed to the recording level
control VR 101 and the DIN or LINE signal is routed to the recording
level control VR 102, both of which act on the mixing amplifiers Q 101
and 0 102. From here, the signal is routed to the input Pin 5 of the
Dolby circuit IC 101. With the MPX switch S 23, the MPX filter is con-
nected via transistor Q 105 and 0 106 (both forward-biased) in order to
suppress any existing Pilot tone residues when recording radio broad-
casts. The Dolby circuit is switched on by rendering Q 107 reverse-
biased (S 22 set to position (3).
AFC compensation is carried out for IC 101 with transistor Q 111.
0111 is control led by IC 102.
In order to prevent interference of the Dolby circuit when recording,
the high frequency components os from 10 kHz (- 3 dB) to 20 kHz
(- 12 dB) are attenuated by the skewing filter (C 153 and L 101).
When the Dolby selector switch S 22-2 is set to position C, the filter is
connected with the transistor 0 104 (forward-biased). A HIGH active
control signal is applied to the base of Q 104.
In the playback branch, treble attenuation is corrected with the de-
skewing circuit C 252 and L 102 at the output amplifier IC 302. This
circuit is activated with 0 121 (forward-biased) in Dolby C mode.
Within the Dolby processor, the signal is branched to two different
outputs, Pin 3 and Pin 7 of IC 101. The signal from Pin 3 which is not
influenced by the Dolby process is routed to the monitor relay R 41
when the source setting is sellected and used as the source signal. The
signal from Pin 7 is routed through VR 103 to the Input Pin 2 of the
Dolby circuit IC 102. During Dolby B mode, C mode is switched off by
Q 113 (forward-biased). The output signal of Pin 7 (IC 102) is routed
through VR 504 directly to the recording amplifier IC 602. The recor-
ding current is set with VR 504. Depending on the tape type, the level
(sensitivity) is set with the tape type selector switch S 21-2 and the
equalization is set with S 21-1. Additionally, and due to the tape speed
of 9.5 cm/s, the equalization time constant is switched over with Q 509
and Q 511 (both forward-biased). When the tape speed of 4.75 cm/s is
selected, Q 510 is forward-biased and 0509 and 0511 are reverse-bia-
sed. The recording signal is routed to the recording head through the
stopper circuit T 502. The bias is mixed to it with VR 505.
By connection of the resistors R 618 to R 623 with the type type selec-
tor switch S 21-6, 0 605 controls transistor 0 604 which influences the
oscillator OSC 1 in such a way that an optimum operating point is
achieved for the various tape types. The erasure current can be changed
with the strap over diode D 609. With transistor Q 603 (record = HIGH
signal of the base), Q 604 becomes forward-biased and a voltage of ap-
proximately 27 V is applied to the transistor 0610. The record HIGH
signal is also routed through R 606 and D 618 to 0612 which renders
Q 611 forward-biased. Via diode D 617, transistor Q 610 becomes for-
ward-biased due to the switching of Q 611 and the former transistor
activates the oscillator OSC 1. The DIN output is switched off with
relay RY 301 during recording mode.
Fade edit
With the pushbutton S 27 ON, the supply voltage is applied to IC 604
and, via C 603, the flipflop IC 604 is set to its defined initial position.
(Pin 3 LOW, Pin 4 HIGH, Pin 10 LOW). The pushbutton ON must
remain depressed for all following FADE EDIT functions.
Fade out: When the pushbutton S 28 (OUT)
is operated, a HIGH
level is applied to Pin 6 of IC 604. The output Pin 3 changes to HIGH
and thus 0 608 becomes forward-biased, 0 609 becomes reverse-biased
and a positive voltage builds up at the base of 0 610. The time-deter-
mini ng factor is C 604. Depending on the base voltage, 0610 becomes
forward-biased and thus determines the amplitude of the erasure cur-
rent. With the HIGH level from S 28 III.. via D 612 at Pin 13 of IC 604,
the output Pin 10 changes from LOW to HIGH, 0 607 is switched on
with HIGH via IC 603 and the red LED RECORD lights up. The HIGH
signal from Pin 10 is also routed through D 606 to Q 603 which swit-
ches on the oscillator.
Fade in: When the pushbutton 29 (IN) is operated, a HIGH level is
applied to Pin 1 of IC 604, the output Pin 3 changes to LOW, thus
Q 608 becomes reverse-biased and 0 609 connects C 604 via R 639 to
0 V. Via the diode D 620, the HIGH level is routed from the output Pin
4 (IC 604) via the differentiator C 606 to Q 612 which briefly causes
611 to become forward-biased and thus charges up C 605. Q 611 be-
comes reverse-biased and C 605 is discharged via R 641. Thus, a positive
voltage with a decreasing level is applied to the base of 0 610 which be-
comes reverse-biased depending on the base voltage and thus causes the
erasure current to decrease.
When the pushbutton ON (5 27) is released, 0607 (LED RECORD is
extinguished) and Q 603 becomes reverse-biased and the oscillator is
switched off. In order to prevent the inadvertent erasure of music cas-
settes (erasure tab removed) with the fade edit facility, transistor Q 606
is forward-biased (HIGH signal at the base = record safety contact open)
and thus the flipflop IC 604 is prevented from functioning.
Monitor
During playback (0 306 reverse-biased), the relay RY 1 is idle and the
off tape signal from IC 104 Pin 7 is directly switched to the output
amplifier IC 302. In recording mode, the signal from IC 101 Pin 3,
which is uninfluenced by the Dolby process, is routed as the source
signal to relay RY 1. Due to the signal RECORD (HIGH active) at the
base of Q 306, this becomes forward-biased, relay RY 1 is energized
and switches over to the source. In this way, the monitor switch S 24
begins to function. If the deck is switched over to "tape", S 24 applies
a LOW signal to the base of 0 306 and this becomes reverse-biased.
Relay RY 1 is de-energized and switches to "tape". In this way, the in-
formation already recorded on the tape can be monitored. As crosstalk
of the bias within the record/playback head is unavoidable in this ope-
rating mode, the bias must be suppressed by the filter T 501.
Muting, A ut o -S pace
The recording amplifier and the LED peak display are muted during all
functions except RECORD. This control signal MUTING RA (record
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