Schaltungsbeschreibung / Circuit Description
STR 100 DX
2 - 4
GRUNDIG Service
GB
Circuit Descriptions
1. Power Supply
The operating voltages for this satellite receiver are generated by DC-
to-DC converters (MC34063) which can be used for converting the
voltage up or down.
S Q
R
Q1
Q2
170
I
Oscillator
pk
C
T
Comparator
+
-
1.25V
Reference
Regulator
1
2
3
4
5
6
7
8
Switch
Collector
Switch
Emitter
Timing
Capacitor
Gnd
Comparator
Inverting
Input
V
CC
I
pk
Sense
Drive
Collector
MC34063
The power supply for the satellite receiver is taken over by modified
switched-mode power supply circuits by means of flyback converters
at a switching frequency of about 100kHz.
1.1 IC1720, 14/18Volt supply
The switch control IC1720 generates the 14/18V LNC supply. This IC
contains a control amplifier with reference voltage, a current sense
input for limiting the current in short circuit condition, and an oscillator
with constant frequency (CC1725). Depending on the output voltage,
the conducting phase of transistor Q1 can become shorter or even
complete output pulses can be missing.
The supply voltage for IC1720 is fed in on pin 6. The drain contacts of
the MOSFET transistor CT1725 are connected via the primary winding
1/3 of the flyback converter transformer TR1725 with the input voltage.
Dependent on the reference voltage of 1.25V, IC1720-(5), the IC1720-
(2) drives the switching transistors CT1725. If the voltage on pin 5 is
lower than the reference voltage, CT1725 becomes conductive and
current starts to flow through winding 1/3. The inductance causes the
current to rise linearly. During the off-phase a positive voltage builds up
on TR1725-(6) which charges the electrolytic capacitor C1727 via
D1725 to the required LNB voltage. When the stepped down voltage
on pin 5 exceeds the reference voltage, CT1725 switches off. The
voltage on diode D1725 decreases again because of the constant
power consumption and so does the stepped down refernece voltage
on IC1720-(5) via CR1727 and the resistors connected to ground.
Transistor CT1725 switches on.
CT1726 reduces the power loss of the switching transistor CT1725. If
IC1720 switches off, then CT1726 conducts, consequently, the gate-
source capacity discharges more quickly.
The voltage dropped on resistor CR1721 is fed through CR1723,
CR1725 to IC1720-(7) and reduces the primary current of transformer
TR1725.
The power consumption of the satellite receiver is about 800mA at an
operating voltage of 15V (12W).
1.2 14/18V LNC Voltage Switch-over
The voltage divider on IC1720-(5) is dimensioned so that, with the
transistor CT1746 switched off, the LNC voltage is 14V. To switch over
to the 18.5V supply, the switching voltage U
14/18V
bridges the resistors
CR1724, CR1715 by means of the transistor CT1746.
1.3 14/18V LNC Voltage Off
To switch off the LNC supply, the switching voltage U
LNC OFF
connects
the resistor CR1731 via CT1756 to chassis. The voltage on IC1720-(7)
is pulled to chassis potential via 4.7k
Ω
and pin 2 stops feeding out drive
pulses.
1.4 LNC Overcurrent Protection
The current limiting resistors CR1781, CR1782 are so dimensioned
that the "LNC Power" is switched off via CT1781, CT1782 as soon as
the LNC current exceeds a level of about 500mA. The lead LNC_PG
transmits the overcurrent message to
µ
P CIC1400-(60) and with the
U
LNC OFF
command it switches the "LNC Power" off at a scanning cycle
of 4-5 seconds approximately. Simultaneously, the message "LNC
Power" is indicated on the screen.
1.5 The 22kHz Switching Frequency
The switching voltage U
22kHz
applies the 22kHz switching frequency,
IC1100-(29), via CT1744, CT1743 to the MOSFET switch CT1732.
This switch bridges the diode CD1740 at the cycle of the 22kHz
switching frequency thus superimposing the active LNB voltage with a
22kHz signal at about 600mVpp. This 22kHz output signal can option-
ally also be pulse-modulated with a DiSEqC Protocol.
1.6 The +33V, +12V, +5V Operating Voltages
This power supply unit operates in principle like the14/18V converter
with the only exception that the converter cannot be switched off via pin
7. The voltage to be compared with the reference voltage on IC1700-
(5) is derived via CR1762 and CR1763 from the 5V supply (stabilised
to 5V).
The voltage drop on resistor CR1701 is applied to IC1700-(7) and is
used to limit the primary current of the transformer TR1710.
1.7 Stand-by Operation
In stand-by mode, the +12V and +5V loads are switched off by the
MOSFET switches CT1733 via CT1736, CT1737, with the switching
voltage U
Standby
to reduce the power consumption.
The power consumption of this satellite receiver in Standby < 0.5W.
2. System Control
This system contains the mask-programmed microcomputer (CIC1400)
and the NVM-type EPROM CIC1420 for the programme data like
channel table, station ident, deviation, and so on. The exchange of data
between the
µ
P and IC1100, IC1330, and the SAT Tuner is carried out
on the I
2
C-bus.
2.1 Pin Configuration of Processor CIC1400
Pin 30:
Power-on reset for the processor, generated by IC1402. The pro-
gramme sequence will not start before all supplies of the receiver have
built up.
Pin 28, 29:
The system clock is generated by an oscillator whose frequency is
determined by XT1402 (4MHz). A proportion of this signal is fed
through CC1025 and CR1025 and is used to provide clock for IC1100.
Pin 59:
The IR input is active LOW.
Pin 60:
LNC_PG (Power Good) is used to detect the presence of a short-circuit
on the LNC. The LNC Power is switched off via U
LNC OFF
and IC1760 if
the LNC current is < 350mA.
Pin 61:
Data output to the Remote Cinch socket, e. g. for data link transfer.
Pin 62:
VDR-output to the display on the SAT Mouse.
Pin 63:
Switching voltage U
22kHz
switches the 22kHz frequency ,IC1100-(29), to
the "LNC Power" supply to drive the LNC's. This 22kHz output signal
can also be pulse-modulated with a DiSEqC protocol.
Pin 64:
Switching voltage U
14/18V
to select horizontal or vertical polarity on
voltage-controlled polarotors; "High" = horizontal.
Pin1:
Switching voltage U
LNC OFF
to switch off the LNC supply.
Pin 2:
Switching voltage U
Standby
to switch off the +12V and +5V supplies.
Pin 13:
Data input from the Remote Cinch socket for remote control (e. g. Timer
operation) by a video recorder.
Pin14:
Connection to local "TASTER UP" (programme up) key in the SAT
Mouse.
Pin 19:
Scans whether there is a sync signal fed out from the sync signal
detector circuit IC1330. If the signal is too low or poor the
µ
P switches
to the internal synchronising signal and the screen background be
