k
2 - 8
GRUNDIG Service
Descriptions
GV 26 EURO, GV 6000 EURO, GV 6300 EURO
With V
cc
< V
dis1
, the IC7020 changes to the Burst Mode. The short-
circuiting power is low because the interval between the half-wave
starts is large. The pulse width is reduced along with the decreasing
load. If the load continues to decrease, IC7020 switches the frequency
back to approx. 50kHz (standby operation) from a certain threshold of
the voltage at Pin 7 (depending on the external circuit connected to
Pins 12 / 16). As a result, the switching losses at the transistor and
within the transformer are low.
In recorders with a standby power consumption of <1W, a HIGH level
at solder contact 1509-(18) causes T7050 to be switched through so
that the represented secondary voltage becomes too high. By the
pulse width of the T7040 drive voltage, IC7020 reduces the secondary
voltages and thus the power consumption. The 5VSTBY standby
voltage is still present.
Overvoltage
At an operating voltage V
cc
> 17V at Pin 1 of IC7020 the output stage
is switched off.
Excess Temperature
IC7020 is fitted with an excess-temperature sensor for blocking the
logic if the permissible chip temperatures are exceeded (typ. 155
o
C).
After the temperature has fallen a new start-up is possible by re-
connecting the video recorder to the mains.
TMP91C642
2. Family Board (PMB)
2.1 Family Board – Deck Control /
Deck Electronic (DE)
Function Overview
Deck control is effected by the deck computer IC7400, TVC (Toshiba
Video Controller) together with the microcomputer IC7201 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 – Level 40). The computing
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:
2.1.1 Reset
2.1.2 Threading Motor Control
(cassette compartment / threading mechanism)
2.1.3 Winding Tachopulse Processing
2.1.4 Beginning of Tape / End of Tape Detection
2.1.5 Head Servo Control
2.1.6 Tape Servo Control
2.1.7 Tracking / Autotracking
2.1.8 EEPROM
2.1.1 Reset
When connecting the video recorder to the mains, the IC7460 gener-
ates the switch-on reset pulse with the reset capacitor C2467 on pin 4.
The resulting "POR" (Power On Reset) on IC7460-(17) is applied once
as a HIGH pulse (approx. 30ms) via T7402 to the deck computer
IC7400-(40) "IPOR". On another path, this "POR" pulse is also fed via
the phase-shifting delay circuit T7231 on the keyboard control unit to
the control computer IC7201-(12) as a negated pulse.
2.1.2 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
IC7400 feeds out the control signals TMO (Pin 35) and THIO (Pin 10).
These signals drive the threading motor via the threading motor driver
IC7440-(7/8, 5/6) and plug contact 1944-(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
IC7460-(5/15) (FTAD). The tape deck operates one switch each for
initialisation of the threading tacho (INIT) and for erase protection
(RECP). The voltages resulting from the switch settings are coupled
via R3478 (INIT) and R3480 (RECP) and fed to the IC7400-(53).
In up-position, the cassette compartment is mechanically connected
with the threading tacho generator (butterfly sensor). 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.
2.1.3 Winding Tachopulse Processing
The optocouplers on the left (option) and right (WTL/WTR) reels
produce 8 pulses per revolution. These pulses are formed into rectan-
gular signals by IC7460 (pins 7/6 –> pins 13/14) and applied to pins 8
(WTLD) and 9 (WTRD) of IC7400. This IC calculates the instantaneous
tape position and the total length of the cassette from the ratio of these
two frequencies.
2.1.4 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 D6460 which is triggered by T7458 / T7463 is
inserted into a hole in the middle of the cassette. The phototransistors
are located at the outside, left (T7461) and right (T7462), of the
cassette.
The phototransistors T7462 "TAS" (beginning of tape) or T7461"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 T7467
and T7468 to the analog inputs of the
µ
C IC7400-(52/51).
2.1.5 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 (IC7400). The actual values are derived in the
headwheel motor driver (IC7300) 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 IC7300-(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 to be supplied to the deck computer IC7400-(12).
On IC7400-(24) 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 to the headwheel motor driver IC7300-(13)
and is used as a control voltage.
