Philips LX3950W/01 Service Manual Download Page 75

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11-25

Frequency setting

Frequency setting is necessary when the 4-position slide switch
is changed. In an internal look-up table the 4 required frequen-
cies for the local oscillator are stored. The output towards the
synthesiser chip in the frontend is in I²C form. The synthesizer
is the only device on the bus. During frequency setting the Mute
is active.
Scanning of the channel switch should occur within every 250
milliseconds in any mode.

In normal mode when Tuned is going “High” the channel data
should be transmitted to the synthesizer to protect the receiver
in case of hang-up situations.

Muting

The mute of the receiver is controlled by the IF receiver part but
can be overiden by the microcontroller.
The mute action initiated by the microcontroller is necessary
during channel switching and in the standby mode. Muting has
priority on all other main tasks. The hardware Mute is the result
of an AND function between the “Tuned” and “Stereo” informa-
tion. The Tuned information can be read by the microcontroller (
pin 2). The same pin (pin 2) has to be put in output mode to
activate the microcontroller initiated Mute. This Mute must be
used during the frequency setting and during the standby
mode.

Powersave mode

The main supply can be switched off with the “powersave”
control line to reduce the current consumption. The
microcontroller is always powered by the power supply. All
other electronics can be put into powersave mode.
After approx. 10 minutes of no “Tuned” signal the receiver goes
into powersave mode. Once in powersave mode, the receiver is
operated in “listen-sleep” mode to reduce the average current
consumption.

Power supply

The board is having a low drop voltage regulator 7790, which
has an output voltage of 8 Vdc.
The 5 V supply for the micro controller is extracted from the 10
Vdc input supply via a zenerdiode 6751. The 5 Vdc supply of
the synthesizer is extracted from transistor 7606. The
powersave signal is also provided to the connector for the
amplifier and for AV via the amplifierboard the SMPS is
controlled.

The AV-version of RX Amplifier board

General:

The amplifier module is intended to deliver 2 x 50 Watts into 8
ohm speakers. Speakers are connected through “easy-fit”
connectors. See amplifier blockdiagram.

The crossover of the built-in high pass – filter will be around
30Hz. This eliminates sub-bass info going to the surround
speakers.
The amplifier uses “class D” amplifier-chip TDA8920.

Additionally, the module is featured with a 2-colour indicator,
green indicates the active state while red is standby.
There is also an on-board 10 Vdc supply conditioning, specially
intended for the TX receiver.

DC-references and supply:
+/- 15 Vdc preamp supply: The supply for the preamp is made
by using a d/-15 Vdc from main power /- 29.5
Vdc. Doing so assures a good symmetrical start-up on the
power lines of the preamp, this suppresses possible plops.This
supply is 15V for all operating conditions, except power save
standby.

This signal is coming from zener 6101 via diode 6100.
Transistor 7104 and 7107 are not saturated at that moment.
When the receiver generates a ”mute” (MUTE_RX = 4Vdc on
pin 6 of connector 1106) then 7104 saturates, diode 6100
blocks and the level on pin 6 -of IC 7101 becomes 2V1. The
signal is now formed by zener 6106 via diode 6103. All DC
references stay as they were; the class D keeps on oscillating,
the led stays green but no sound is produced on the outputs.
The control line STBY_OUT (connector 1107-pin 3) to the
power supply stays low.

When the receiver goes into powersavemode (= after 10
minutes no signal from the AV set), STANDBY signal is high;
then the circuitry around 7107 pulls down the mode line of the
amplifier to nearly 0V via the MUTE_AMPn signal. (Note that
the mute line from the receiver is also high under this condi-
tion).
The amplifier is completely in standby mode, the oscillator in
amplifier IC 7101 stops and all internal references are disabled.
The led red will be on, while green is off. At the same moment
transistor 7106 is not saturated anymore and the control line
towards the switched mode power-supply becomes high. This
switches off the main +/- 29.5Vdc supply and the whole unit
comes in a real power save mode. 7112 helps to pull down the

+/- 29.5 Vdc main supply: Will be switched off when the unit
goes into power save-mode. See circuitry around 7107 and
7106.
+ 10Vdc receiver supply: Is always present, receiver stays
always active
Note that all supplies have the same ground reference.

Mute circuitry:

In normal operating circumstances, the voltage on the- “mode”
pin of the amplifier IC7101 will be 4V5. This is the
MUTE_AMPn signal.

mute line of the amplifier-chip 7101 as fast as possible to avoid
plops at this transition.

When the receiver detects a valid signal from the AV set, this
power-save-line becomes continuously low, and after a small
time (2152, 3143, 3147), 7104 is off. At the same moment the
mute line becomes low and the amplifier is enabled again. Also
the control line (connector 1107-pin 3) to the power supply
becomes low, thus starting up the main +/- 29.5 Vdc supply

Amplifier

The amplifier TDA8920TH operates on +/-29.5 Vdc and can
deliver 2 x 50 Watts into 8 ohm loads. Using the very efficient
class D - technology, it minimizes the dissipation. The class - D
chip runs on a ± 300kHz oscillator, therefore a 12dB / octave
output filter is needed (coil 5104 and 5105, and capacitor 2122
and 2125).
Input signals are 180° phase shifted by 7100-C and 7100-D, (
the effect - speakers have to be in phase with left and right
main speakers, otherwise bass reproduction will be poor) Input
signals are filtered in the two-section high pass filter around
7100-B and 7100-A. The – 3 dB point is somewhere around
30Hz.
Input sensitivity of the amplifier for maximum output: typical
1.4V rms

Figure 1-26 Microcontroller functions

Figure 1-27 Block diagram amplifier

Figure 1-28 Mute ampn circuit

AUDIO OUTPUT AMPLIFIER

1106

+10V

MUTE

3117

7110

MUTE WIRED

POWERSAFE

1109