Grundig STR 631 Service Manual Download Page 29

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STR 631 / STR 632

Schaltungsbeschreibung / Circuit Description

GRUNDIG Service

2 - 5

Port 8

Bits 0 to 7 are used to drive transistors Q 704 to Q711, active HIGH,
to illuminate segments of the 7 segment display on those model
where this is fitted.

On FIP models bits 5, 6 and 7 form a three wire bus for communica-
tion to the FIP controller chip.

On models with 2 single LEDs bits 5 and 6 are used, active HIGH,
to illuminate the green and red LEDs respectively.

Port 9

Bit 0 is a Pulse Width Modulator peripheral used for controlling the
current source on models with a magnetic polarotor.

Bit 1,2 and 3 are 7 segment LED display strobe signals, active LOW,
for the hundreds, tens and units digits respectively.

Bit 4 is an active LOW input from the Mode local key.

Bit 5 is an active LOW signal AUTH used to invoke pay-per-view on
the VideoCrypt circuit.

Bit 6 is a variable width pulse, active LOW, used to control a
mechanical polarotor on those models with this feature.

Bit 7 is an active LOW signal CMD_MUTE used to reset the
VideoCrypt circuit after a channel change.

Port A

Bit 0 is an active HIGH input CLEAR from the VideoCrypt circuit
indicating that a non-scrambled signal is being received.

Bits 1, 2 and 3 are active LOW inputs from the local keys Stand-by,
Dwn and Up.

Bits 4, 5 and 6 are outputs used to control the status of the LNC
regulator circuit. Respectively they are V+, active LOW to add
700mV to the LNC voltage. LNB_OFF, active HIGH to remove
power from the LNC. H/V to select horizontal and vertical polarity on
voltage controlled polarotors, HIGH is horizontal.

Bit 7 is an active LOW input LNB_SENSE used to detect the
presence of a short-circuit on the LNC.

2.1 Non Volatile Memory

IC601 and IC602 (when fitted) are serial E2PROMS and contain all
factory and user programmable data to configure the receiver.

3. Power Supply

A UC3842 integrated circuit (IC200) controls the operation of the
power supply, a flyback switch mode system. Start up is via R201
supplying minimal power to pin 7 of the IC. Once the power supply is
running then the supply voltage to the IC is rectified by D204 from the
control winding 1/5and filtered by C202.
R204 and C203 determine the frequency of the pulse width modulation
driving the power FET Q201. For stabilisation a feedback voltage
rectified by D206 is compared to a reference voltage within the IC. A
small drop in the feedback voltage due to loading will cause the pulse
width of the drive to Q201 to increase. This will turn Q201 on for a longer
period and provide additional energy transfer in order to compensate
for the loading effect. Pin 3 if the IC is a current sense input and will turn
the drive to Q201 off in the event of excessive current drain from a
heavy secondary load. D208 and D209 protect the IC against failure of
Q201. The secondary voltages are determined by the potential divider
of R206 and R207, both are close tolerance resistors. In “stand-by” the
level on R230 is sent low by the microcomputer ;turning off Q223 and
Q222 and removing the switched 5V supply to the receiver. Connector
PL201 is situated near the front of the unit and is used for secondary
voltage test points. -30V_DRV is used on models with a fluorescent
indicator panel to provide the -30V supply for this device. The ground-
ing for the FIP is also taken back to the “noisy” side of the power supply.

4. Tuner

All functions within the tuner are controlled from IC600 via I

C Bus into

the Programmable Counter.

First IF signals from the LNC, in the range 900MHz to 2150MHz, are
presented to either of the LNC inputs. Selection of LNC input is from
a port on the Programmable Counter. Note that some models have
only LNC1.
Because of the possibility of LNC power being on these inputs they are
ac coupled into G whose gain is adjusted automatically by the AGC to
ensure a constant level at the input to the FM Demodulator.

The output of G drives the mixer which feeds the Surface Acoustic
Wave Filter (SAWF). This band limits the nominally 479.5MHz second
IF signal to 27MHz. After filtering the signal is FM demodulated to
provide the baseband video output of the tuner on pin 9. A low pass
filtered version of the video signal is passed to a 5 level analogue to
digital converter inside the Programmable Counter so that it can be
read via I

C for AFC purposes on models which have Search Tuning.

Signal selection is achieved by use of a Phase Locked Loop (PLL). The
Local Oscillator (LO) is of the voltage controlled type. The output of the
LO is divided down by the programmable counter and its phase
compared in the Phase Detector with a crystal controlled reference.
The output voltage from the Phase Detector is used to control the LO
so that the value in the Programmable Counter precisely defines the
LO frequency which goes to the mixer.

Tuner Block Diagram

5. Video Noise Filter

Baseband video from the tuner is passed through a 10MHz low-pass
filer to remove unwanted noise before use by the Video Processor and
Audio Demodulator.

6. LNC Power

The power for the LNC is delivered from a controlled voltage source,
which is protected against overloads.

IC200 is a programmable regulator whose output voltage is deter-
mined by R313, R311, R312, D304. When the H/V signal is low Q302
and Q300 are off and Q304 is on giving an output voltage of approxi-
mately 13V. In this condition power to the regulator is via D300 from the
21V supply rail. When H/V is high Q302 and Q300 are on and Q304 is
off raising the output voltage to approximately 18V. Power to the
regulator is via Q300 from the 26V supply rail. When V+ is low Q305
is turned off and an additional 700mV is added to the LNC supply on
models with this feature.

If the LNC load current exceeds approximately 500mA Q301 turns on
which turns on Q303 and Q306 signalling via LNB_SENSE to IC600
that there is an LNC overload and reducing the supply voltage to limit
the energy delivered. On sensing the overload IC600 takes the
LNB_OFF signal high to remove LNC power and prevent overheating.
IC600 attempts to re-establish power to the LNC approximately every
second until the fault is cleared.
LNB_OFF is also used to remove power from the LNC in stand-by and
when LNC power is not needed, for instance on multi-user systems.

On models with two LNC inputs power is directed to the appropriate
input by darlington transistors Q200 and Q102, for LNC 1 and LNC 2
respectively. These are switched via Q102 and Q103 from a port on the
Programmable Counter.

LNC 1

LNC 2

AGC

SAWF

Demod

Programmable

Counter