GRUNDIG Service
2 - 11
k
TVR 3710 …, TVR 5100 …,TVR 5500 …
Descriptions
Beam Spot Suppression and CRT Discharge
When switching off a TVR the line output transformers 5550 of which
are not provided with a bleeder resistor, the picture tube must be
discharged separately and afterglow of the screen must be prevented.
For this, the rectified heater supply for the emitter stage T7902 is
stopped. The transistor stage T7901 which follows will conduct and
pulls the reference voltage of the RGB amplifier stage to chassis
potential. As a result, the picture tube is fully driven and discharged.
Because there is no focusing voltage, there is only a diffuse spot to be
seen on the screen.
In line output transformers 5551 with integrated bleeder resistor, this
resistor takes over the function of discharging the picture tube. To
ensure that the electron beam is immediately cut off when the TVR is
switched off, grid 1 of the picture tube is also cut off. For this, C2911
is charged up to +180V during operation. When the TVR is switched off,
so does T7902 and T7903 conducts. Consequently, the positive plate
of C2911 receives chassis potential via T7903 and grid 1 of the picture
tube is driven at reverse polarity.
3. Signal Chassis (PSSP2)
3.1 Signal Chassis – Deck Control /
Deck Electronic (DE)
Function Overview
Deck control is effected by the deck computer IC7410, TVC (Toshiba
Video Controller) together with the microcomputer IC7801 on the
keyboard control unit. Data communication between the two micro-
computers takes place via the bidirectional serial interface DATD1 /
DATD2 / CLKD1.
The deck computer is a microcomputer especially developed for video
recorders. The operating system stored in the integrated mask-
programmed ROM of the
µ
C is defined by the VCR option code in the
EEPROM (see Service Test Programme - Option Code). The comput-
ing speed is determined by quartz Q1400.
The deck computer is responsible for driving and checking the tape
deck including the servo systems for the tape transport and headwheel
control. It also controls the record/playback switching process and the
release of the recording currents.
The deck control consists of the following function groups:
3.1.1 Reset
3.1.2 Threading Motor Control
(cassette compartment / threading mechanism)
3.1.3 Winding Tachopulse Processing
3.1.4 Beginning of Tape / End of Tape Detection
3.1.5 Head Servo Control
3.1.6 Tape Servo Control
3.1.7 Tracking / Autotracking
TMP91C642
3.1.1 Reset
When connecting the video recorder to the mains, the IC7411 gener-
ates the switch-on reset pulse with the reset capacitor C2454 on pin 4.
The resulting "POR" (Power On Reset) on IC7411-(17) is applied once
as a HIGH pulse (approx. 30ms) via T7405 to the deck computer
IC7410-(46) "IPOR". On another path, this "POR" pulse is also fed via
the phase-shifting delay circuit T7805 on the keyboard control unit to
the control computer IC7801-(47) as a negated pulse.
3.1.2 Threading Motor Control (cassette compartment /
threading mechanism)
The cassette compartment and threading mechanism is driven by the
threading motor. For driving the threading motor, the deck computer
IC7410 feeds out the control signals TMO (Pin 53) and THIO (Pin 16).
These signals drive the threading motor via the threading motor driver
IC7402-(5/6, 7/8) and plug contact 1903-(1/3).
The
µ
C detects the position of the tape deck by counting the threading
tachopulses (FTA) in connection with the INIT switch and the identifi-
cation "TAS" for the beginning of the tape, and "TAE" for the end of the
tape. The threading tachopulses (FTA) are supplied to the
µ
C via plug
contact 1905-(11) and IC7411-(5/15) (FTAD). The tape deck is fitted
with a switch for initialisation of the threading tacho (INIT) and another
for erase protection (RECP). The voltages resulting from the switch
settings are coupled via the plug contacts 1905-(14/10), R3471 (INIT)
and R3472 (RECP) and fed to the IC7410-(56).
The cassette compartment is mechanically connected with the thread-
ing tacho generator (vane wheel). When loading a cassette into the
cassette compartment, the cassette must be inserted as far as is
necessary to generate three threading tachopulses. Afterwards, the
deck computer activates the threading motor which then takes over the
loading of the cassette.
When ejecting the cassette, the phototransistor detecting the begin-
ning of the tape is released mechanically a short time before the lift
reaches the end position. A short time later, the threading motor is
automatically switched off by the sequence control computer.
3.1.3 Winding Tachopulse Processing
The optocouplers on the left (WTL) and right (WTR) reels produce 8
pulses per revolution. These pulses are formed into rectangular
signals by IC7411 (pins 7/6 –> pins 13/14) and applied to pins 15
(WTLD) and 14 (WTRD) of IC7410. This IC calculates the instantane-
ous tape position and the total length of the cassette from the ratio of
these two frequencies.
3.1.4 Beginning of Tape / End of Tape Detection
To identify the beginning of tape and end of tape, the VHS cassette has
a clear foil, 13 -19cm long, on each end of the tape. The beginning and
end of tape is identified by two optocouplers. For this purpose, the
transmitting diode which is triggered by T7490 is inserted into a hole
in the middle of the cassette. The phototransistors are located at the
outside, left and right, of the cassette.
The phototransistors "TAS" (beginning of tape) or "TAE" (end of tape)
supply pulses as soon as they detect the beginning or end of tape. The
output voltages of the phototransistors are fed via the plug contacts
1905-(15/7) to the analog inputs of the
µ
C IC7410-(55/57).
3.1.5 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
µ
C (IC7410). The actual values are derived in the
headwheel motor driver (IC7301) on the head amplifier board from the
information given by the Hall generator or the position coil located in the
headwheel motor (threephase motor) in connection with the phase
voltages. At the output IC7301-(6) there are the PG (phase generator)
pulses of 25Hz for the phase control and also the FG (frequency
generator) pulses of 450Hz for the speed control. From the connector
1904-(1) the PG/FG pulses are fed to the deck computer IC7410-(18).
On IC7410-(30) the
µ
C feeds out a pulse-width-modulated square-
wave signal (REEL) containing information on the speed and the
phase. This signal is fed via the plug contact 1904-(6) to the headwheel
motor driver IC7301 located on the head amplifier board and is used
as a control voltage.
3.1.6 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 via the pins 8…13.
Depending upon these signals the IC commutates the individual
phases of the capstan motor.
Switching over of the sense of rotation (CREV) is carried out via pin 26
of the LB1897. For this the
µ
C (IC7410) feeds out from pin 44 a LOW
