Dual C 822 Serviceanleitung Seite 6

Circuit description C 822
Analog Section
Playback
The AF voltage of approx. 300 1/V (DIN reference level) produced by
the playback head, is amplified by transistors Q 102 and Q 103. lt then
passes a negative feedback network, consisting of R 153, R 156 and
C 137 for equalization. Capacitors C 131, C 132 and C 133 can be con-
nected in such a manner as to provide correction of treble response during
playback by changing the tape head resonant frequency, whereas swit-
chover from 12011sec to 7011sec equalization time constant is accompli-
shed by 0 103. While applying the Dolby reference level of 200 nWb/m,
the playback level at the Cinch output jacks is set to 580 mV with VR 103.
This adjustment is very important to assure adequate operation of the
Dolby system.
The playback signal is connected to the Dolby input (pin 1 of IC 102)
through the REC/PLAY switch (A/W) S 102. In OFF Position of the
Dolby switch S 301, the signal is amplified by 26 dB within the Dolby
circuit, whereas amplification will be level- and frequency-controlled
when the Dolby system is activted. The Dolby output signal at pin 7
of IC 102 is routed to the headphone amplifier (pin 2 of 1C103) through
REC/PLAY switch S 103 and to the DIN output lack through R EC/PLAY
switch S 105.
To prevent switching noise leakage into the output circuit, the output
signal is shorted by transistor 0 105, whereas the recording amplifier
input will simultaneously be set to OV by 0106. A high signal to control
the muting circuit is supplied by the tape transport control circuit board
during fast forward, rewind and stop functions.
Recording
This cassette deck features three different recording Inputs:
M IC, DIN and LINE. When a source is connected to the DIN input, the
LINE input will be grounded. After connecting a monaural or stereo
microphone to the M IC input, source signals at the DIN and LINE inputs
are cut off. The MIC signal is amplified by low-noise IC 101, the DIN
input signal by 0601, whereas the LINE Input signal is routed directly to
level control VR 101 and from there to pin 5 of Dolby IC 102 through
the REC/PLAY switch S 102.
S 201 inserts a multiplex filter to suppress pilot signal leakage when re-
cording FM stereo broadcasts.
The signal is subdivided within the Dolby processor and routed to dif-
ferent outputs: pin 3 and pin 7 of IC 102. The non-dolbyized signal at
pin 3 is passed to the monitor output (LINE OUT), VU meter and head-
phone amplifier through REC/PLAY switch S 103, whereas the Dolby
signal at pin 7 is routed to the recording amplifier (0 107 and Q 108)
through REC/PLAY switch S 104. Recording current is adjusted by con-
tol VR 102, level and equalization (in accordance with the tape used)
by Q 109 through 0 112. The recording signal is routed to the recording
head after passing a series rejector with VR 105 providing the correct
bias voltage. By connecting R 307 through R 315 by means of tape se-
lector switches S 401 (Fe), S 402 (Cr), S 403 FeCr) and S 404 (Metall,
the RF bias oscillator is set to optimum operating characteristics for
either one of the tape types to be used. In recording mode the monitor
signal will be cut off by REC/PLAY switch S 105.
Loyal Indicator
In recording as well as in playback mode, the output signal is passed to
the headphone amplifier IC 103. C 146 couples the output signal at pin
1 of IC 103 to the meter amplifier (0 104). VR 104 is used for 0 dB
dial calibration of the VU meters. To compensate for the reduced recor-
ding characteristics at higher frequencies, R 172 and C 147 provide ne-
cessary frequency response correction. Response at 10 kHz is 10 dB
higher than at 400 Hz. An analog meter with needle is used for level in-
dication. Its dial scale is subdivided into individual scales for either one
of the tape types which can be used. LEDs which are connected to the
tape selector switches indicate the corresponding sub dial scale.
Tape Transport
Functions selected by Operation of the selector pushbutton switches
such as forward, rewind, pause, fast forward, fast rewind etc. are transfer-
red as active low control signals to the tape transport control IC 403, pro-
ducing logic decisions in form of static high signals at its output. These
selected control signals are routed to the appropriate transistor stages
by means of decoupling diodes after passing the time delay circuit com-
posed of R 430 through R 433, C 416 through C 420 and the AND-Iogic
interlacing IC 404 which supplies a high signal (DLL AUS). For better
understanding of the switching process, FF (fast forward) mode is quoted
as an example: a high signal is routed through diodes D 411 and D 416
to a differentiating (time) circuit (C 435/R 438), turning on transistor
Q 407. This transistor supplies an overvoltage to solenoid FF for a pre-
determined period of time, whereas 0404 applies the 0V livel. This over-
excitation assures instantaneous and reliable activation of the solenoid
switch. After expiration of the preset time period, 0 406 is switched
off and D 419 maintains latching of the solenoid switch until Q 404
cuts off the supply voltage when a low signal is applied to its base by
IC 403 or DLL EIN, thus releasing solenoid switch FF. The same
principle applies to all other solenoid-activated magnetic switches.
In playback mode, FF and RW are excited, whereas the recording mag-
net — normally connected to the 0 V supply — will be excited in re-
cording mode by an overvoltage from 0 410 with 0 413 and D 418
supplying the latching voltage.
The motor of the tape transport mechanism is turned on by switching
transistor 0419 when a high signal is applied to its base by diodes D 424
through D 427. In DLL EIN status, a low signal is applied to the base of
O 419 through D 414 to switch the motor off.
DLL
The DLL system uses an opto-electronic infrared light conductor device
which operates without mechanical contact points. The IR transmitter
LED 505 is controlled by the free-running oscillator IC 402 and transistor
Q 401 (period: 1 ms, frequency: 1 kHz). These pulses warrant adequate
operation which cannot be influenced by equivalent light and reflection
interference. Photo transistor 0 501 converts the light pulses into electri-
cal pulses, which are then processed by the comparator IC 402. When
the light path is interrupted (DLL EIN), a high signal will be present at
the output (pin 7) of IC 402 during the period of light Interruption (dark
period). C 407 is a blocking capacitor. At the same time, a high signal is
present at pin 4. lt will remain at high level even after switch S 108 is
closed (cassette removed), whereas a low signal is available at pin 2 of
IC 401 to control the DLL EIN status. In case of Pause or Record func-
tion or Pause only, 0 417 is turned on by diodes D 422 and D 423. In
addition, the DLL EIN status becomes active, turning on 0 416 and
effecting the stop function by applying a low signal to pin 2 of IC 403.
At the same t ime,the Record and Pause pushbuttons are locked to prevent
erroneous Operation by a high signal (DLL EIN) at pin 4 of IC 401,
whereas the lamp driver IC 401 is activated by a signal through D 402
which is applied to pin 14, causing the STOP LED indicator to light
up. Another low signal from IC 401 pin 2 (DLL EIN) isapplied to Q 419
through D 414, causing this transistor to cut off the tape transport motor.
Record Blocking Circuit
Open contact S 507 activates the Record Blocking circuit. When 0 418
becomes conductive, a low signal is applied to the input (pin 9) of IC 403.
This will deactivate the recording circuit and prevent the record pushbut-
ton from being erroneously operated.
Tape End Switching Circuit
A multi-slot magnetic disc which is attached to the tape counter drive
pu I ley, senses tape motion and transmits a "tape running" signal to IC 501,
incorporating a Hall sensor which responds to changes of the magnetic
flux density. Pulses supplied by IC 501 are routed to pin 2 of IC 405,
effecting automatic stop of the tape advance at tape end. Transistor
0 420 supplies a low signal through diode D 430 to cut off 0421. In this
condition, the tape transport mechanism is operating. A high signal at
pin 1 of IC 405 activates this integrated circuit. If no pulses are applied
to pin 2, IC 405 will transmit a low signal from its pin 6 to pin 2 of
IC 403, which is equivalent to a STOP signal. If a high signal is applied
to 0 421 either from diode D 429 (DLL EIN) or D 428 (Pause), this
transistor becomes conductive, connecting the low signal to pin 1 of
IC 405, thus disabling the tape end switching circuit.
Muting circuit
The muting signal is supplied by transistor 0 411. If this transistor is cut
off, a high signal will be present at its collector and the muting circuit
is operative (see circuit description of analog section). A lowsignal (play,
pause) at D 431 cuts off Q 422, whereas Q 411 becomes conductive
after a short time delay (C 428) and a low signal will be present at the
collector of C 411 to disable the muting circuit. Is will be activated again
by high active signals from diodes D 432 (rewind/fast forward), D 431
(STOP) and D 433 (DLL EIN)
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