Power Supply; Signal Path - Grundig STR 100 microSAT Serviceanleitung

STR 100 microSAT
Circuit Descriptions
GB

1. Power Supply

The power supply for the receiver operates on a voltage of 11V...26V
on battery operation and of 220V...240V when connected to an
external power supply unit.
8
Drive
Collector
I pk
7
Sense
I
pk
Oscillator
6
V
CC
Comparator
+
-
Comparator
5
Inverting
Input
MC34063
The operating voltages are generated by DC-to-DC converters
(MC34063) which can be used for converting the voltage up or down.
The voltage down converters IC1760 und IC1710 provide the +12V
and +5V operating voltages.
The up-down DC converter IC1720 produces the 14V/18V supply for
the LNC as well as the +30V tuning voltage for the SAT tuner via the
storage choke L1725. The switching transistors in CIC1725 are
operated on the principle of pulse width modulation in dependence of
U (L1725 = storage choke, C1727 = charging capacitor). The
14V/18V
output voltage provided at C1727 is applied via CR1727 to the control
input IC1720-(5) (= 1.25V) of the circuit.
The current measuring resistor CR1721 is used for limiting the current.
The resulting voltage drop is evaluated by IC1720-(7) and in the case
of a failure, the LNC-voltage and the tuning voltage are reduced (LNC
short circuit current U 1721 > 250mV). The LNC voltage is monitored
CR
by the microcomputer via the voltage divider CR1749/CR1747. If it is
<5V the microcomputer switches to standby after 3s.
The voltage U switches over a voltage divider at IC1720-(5) via
14/18
CT1735 thus changing the LNC voltage.
In standby mode, the U voltage is at High level switching off all
Standby
switching controls at Pin 7.
The voltage U activates the 22kHz generator CIC1745, NE555 and
22kHz
via the transistors CT1743 and T1740, it modulates the LNC voltage
with a 22kHz frequency and an amplitude of approx. 0.6V
2. Microcomputer
The mask-programmed microcomputer CIC1400 controls the whole
sequence of system operations and the on-screen-display. It is used
also to decode the infra-red remote control commands from the IR-
receiver IC1430-(2).
The data for all programmes and options are stored in the EEPROM,
CIC1420 (NVM 16kBit). The data traffic between the individual function
2
groups is carried via the I C bus. The microprocessor works correctly
on the following basic conditions:
a) Operating voltage +5VD on pin 15.
b) Oscillator frequency 4MHz on pin 26 (approx. 3Vpp with 10:1 test
probe). This frequency is used at the same time as a reference
frequency for the oscillator of the audio signal processor IC1250.
c) Reset pulse:
Every time the SAT receiver is switched on, the reset input on pin 1
of the microprocessor is held at "Low" level until the supply voltage
exceeds 4.6V.
GRUNDIG Service 2 - 3
Switch
1
Collector
Q2
S Q
R
Q1
170
2 Switch
Emitter
C
T
3
Timing
Capacitor
1.25V
Reference
Regulator
4
Gnd
.
pp
Schaltungsbeschreibungen / Circuit Descriptions
d) I 2 C bus:
The bidirecitonal bus consists of the System Data Lead, SDA
IC1400-(22), the System Clock Lead, SCL IC1400-(21). All are
connected via pull-up resistors to +5V.
The only possibility of checking the bus leads during servicing is by
measuring the TTL levels (L < 0.8V; H > 2.4V) with an oscilloscope.
Service Note
If you measure on the data leads "Low" signal or chassis return there
may be a short circuit. To localize the fault, disconnect the data leads
one after another from the individual sub-assemblies (tuner,
memory CIC1420, audio processor IC1250, video matrix CIC1220).

3. Signal Path

The +30V and +5V supply voltages must be present at the tuner
contacts 1 and 4, respectively. The I
frequency selected during on-screen-display. This frequency is
downconverted to an IF of 479.5MHz. The IF-bandwidth can be
programmed to be "wide" (27MHz) or "narrow" (18MHz).
The signal is processed by a following FM-demodulator to a baseband
signal (tuner contact 6), an AFC-signal (contact 7) and a field-strength-
dependent AGC-signal (contact 8, testpoint P7).
The baseband signal, approx. 300mVpp, is still pre-emphasized and
combined with the energy dispersal signal, the DC-level is typically
2.5V.
When the frequency is set correctly, the AFC-voltage is typically 2V.
The LNC voltage is fed in on contact 9.
This signal makes it possible to switch to two different HF-levels via
cable using special multi-switches.
Inversion of the video signal (necessary for systems with an input
frequency lower than the oscillator frequency of the receiver) and
change of the IF-bandwidth is effected via the I
3.1 Video Signal Path
From tuner contact 6 the unclamped baseband signal is passed
through an inverting stage CT1015 and an adjustment control R1017
(level adjustment when changing the tuner) to the input of the video
processor CIC1170-(20).
This IC is responsible for:
- matching the amplitudes from stations with different video deviation
- selecting the video mode, Linear or PAL-Deemphasis
- clamping the video signal.
For amplitude compensation, the microprocessor regulates the volt-
age at the gain input pin 19 via the HUB1 / HUB2 (DEVIATION1 /
DEVIATION2) leads. The values for the different deviations are read
out from the memory on a per-programme basis so that the following
DC-voltages are applied to pin 19:
Level Level
DEV 1 DEV 2
High High
High Low
Low High
The same voltage levels as specified in the chart apply to the negative
frequency deviations too. In this case, the video signal is simply
inverted in the tuner.
The switching voltage "U " at CIC1170-(2) switches over between
Deem
PAL-deemphasis (Low) and Linear (High).
The clamping input on pin 9 is preceded by a 5MHz lowpass F1033.
From the output CIC1170-(14) the clamped CCVS signal is passed on
by the inverter CT1040 to the video matrix CIC1220-(2) and to the
switch-IC CIC1060-(3). Dependent on the setting of the video switch
CIC1060, the transistors CT1451, CT1054 and CT1210 amplify either
2
C bus sets the tuner to the input
2
C bus.
Voltage
Setting CIC1170-(19)
25MHz ca. 4V
22.5MHz ca. 3.8V
16MHz ca. 3V
2 - 3