Grundig TVR 3735 FR/TOP Servicehandbuch Seite 32

Description
of tape) supply a HIGH level as soon as they detect the beginning or
end of tape. The output voltages of the phototransistors are fed to
IN01-(7 / 6).
Head Servo Control
The head servo control ensures that the rotational speed and the
phase of the headwheel drive are correct. The complete control is
carried out in the ≤ C (IN01). The actual values are derived in the
headwheel motor driver IC from the information given by the Hall
generator or the position coil located in the headwheel motor (three-
phase motor) in connection with the phase voltages. At plug contact
PN02-(3) there are the PG (phase generator) pulses of 25Hz for the
phase control. The FG (frequency generator) pulses of 600Hz for the
speed control are provided at PN02-(6). The PG/FG pulses are fed to
the deck computer IN01-(86 / 87).
On IN01-(78) the ≤ C feeds out a pulse-width-modulated square-wave
signal (DRUM PWM) containing information on the speed and the
phase. This signal is integrated with RCN15 / CCN21 and is fed via the
plug contact PN02-(2) to the headwheel motor driver. It is used as a
control voltage.
Tape Servo Control
The capstan motor is a three-phase motor which is fitted with Hall
generators. These generate signals which are fed to the capstan motor
driver IC (LB1897) in the capstan motor assembly. Depending upon
these signals the IC commutates the individual phases of the capstan
motor.
Switching over of the sense of rotation (CAP F/R) is carried out via
Pin 21 of the IN01. For this the ≤ C feeds out a LOW level for the forward
sense of rotation or a HIGH level for the reverse direction. This control
signal is passed through plug contact PN03-(2) to the capstan motor
driver. For speed control, the FG Hall element (Magneto Recitive
Element) in the capstan motor generates pulses at a frequency of
757Hz at the rated speed. The pulses (PG) from the capstan motor
driver IC for speed control are fed via plug contact PN03-(3) to
IN01-(85). The tachopulses (actual value) are compared in the IN01
with an internally generated reference value. From IN01-(77) the ≤ C
feeds out a pulse-width-modulated square wave voltage (CAPSTAN
PWM). This is integrated by RCN10 / CCN29 and fed via plug contact
PN03-(5) as a control voltage to the capstan motor driver IC.
For the Stop function, IN01-(23) supplies a LOW signal. The capstan
motor will stop.
Tracking / Autotracking
During recording, encoded 25Hz-CTL-pulses are recorded onto the
tape via the sync head. These pulses are required on playback for
tracking control.
On recording, the 25Hz pulses are taken from IN01-(92) and are then
passed through connector PAY03-(1) to the sync head.
On playback, the recorded CTL pulses are scanned by the sync head
and are then fed to IN01-(92).
When a cassette is loaded, the Autotracking function determines the
optimum track position on playing back. For this, a voltage (DC
ENVELOP) is derived from the envelope of the FM packages and fed
in to the main computer IN01 on the analog input (Pin 3). This voltage
is fed out in the head amplifier on IY04-(6).
On the basis of the tracking centre position, the nominal tracking value
is increased or reduced. For each of the two directions, the appropriate
nominal value is determined at which the voltage derived from the FM
envelope (DC ENVELOP) starts to decrease as against the maximum
determined voltage level. The mid-value between the two limit values
is then used as the optimum tracking value. On completion of this
measurement, the Autotracking function is switched off and the deter-
mined phase is controlled by the CTL pulses.
If more than two successive CTL pulses are missing, the Autotracking
function is reactivated assuming, in this case, that a new recording with
a different tracking position is played back.
2 - 8
2.3 Signal Chassis – TV Signal Electronics (TV)
2.3.1 Frontend
The Frontend has the function of amplifying and demodulating the IF
signal fed in from the tuner. The resulting signals are the CCVS signal
and the audio signal.
Coming from the tuner output U101-(1), the IF signal passes through
the surface acoustic wave filter SF01, which determines the IF band-
pass. Via I501-(48 / 49), the signal is fed to a gain controlled wideband
amplifier with synchronous demodulator, and subsequently to a video
amplifier. Another stage in this IC is used to generate the control
voltage for the wideband amplifier and the tuner. This control voltage
is fed from I501-(54) to the tuner U101-(8). The control threshold level
is adjustable by the software. The demodulated IF signal is fed out from
Pin 6 of I501. After the amplifier stage QC504 the signal path divides.
– For video signal processing, the demodulated IF signal is subjected
to the sound trap Z503 / Z504, in which the audio component of the
CCVS signal is attenuated. Subsequently, the signal is fed to the
input selection switch of the video signal processing stage I501-(13).
Additionally, the signal is passed through QC512 to the output
selection switch I504-(5).
– The demodulated IF signal for FM audio signal processing is passed
through QC505 and the IF filter Z502 to I501-(1). Subsequently, the
filtered sound IF is subjected to the limiter, the PLL demodulator and
is passed on to the preamplifier / mute circuit. The demodulated
audio signal passes through the integrated selection switch
(SW+VOL) with volume control and I501-(15) to arrive at the output
stage I601-(1 / 9).
The IF signal for AM audio signal processing (SECAM-L) is fed from
contact 1 of the tuner U101 through the amplifier stage QCL01 to the
surface acoustic wave filter SF02. This filter is provided with an
additional input for the vision and sound carriers in Band I of the
SECAM-L standard which are reversed when compared to the other
Bands. The inputs, Pin 1 (Band I) and Pin 2, of the surface acoustic
wave filter SF02 are released by the switching voltage "SECAM-L'1"
via QCL03 / QCL02. This voltage is at HIGH-level on Band I of the
SECAM-L standard. The filtered IF signal is then taken to IL01-(1/14).
In IL01, the signal is passed to a gain controlled amplifier and an AM
demodulator. Subsequently, it is fed through IL01-(7) to a sound
selection switch (SW+VOL) in I501 via Pin 2. The audio signal is fed out
from IL01-(9) and passed through the amplifier QC501 to the output
selection switch I501-(2).
2.3.2 Video Signal Processing
The input selection switch of the video signal processing stage
(CVBS+SW) receives two CCVS signals for selection, the signal from
the Frontend at I501-(13) and the external CCVS signal, e.g. from the
VCR stage, at I501-(17). Subsequently, the signal path divides. One
path is to the luminance, the other path to the chrominance processing
stage.
– The trap separates the luminance component (CVS) from the CCVS
signal. The following delay line (Y-delay/peaking) compensates for
the time differences between the luminance and the chrominance
signal. The peaking stage improves the sharpness of the edges. The
CVS signal is then taken via Pins 28 / 27 to the "matrix" stages where
the R/G/B signals are generated.
– The chrominance signal is filtered by passing it through the chromi-
nance bandpass and is then fed to the decoder PAL/NT. For
decoding the colour signal, the burst is separated from the
chrominance signal and is used to synchronise the X-4.43 oscillator.
The quartz X502 at Pin 35 determines the 4.43MHz frequency for the
oscillator. This colour carrier frequency is used to demodulate the
colour component signals which are then subjected to the delay line
to leave the I501 as R-Y (Pin 30) and B-Y (Pin 29). The two signals
are fed back into I501-(32 / 31). In the "matrix" stages which follow
the R/G/B signals are generated using the luminance signal.
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