Philips DVDR70/001, Service Manual

Page: 172 / 260

Circuit-, IC descriptions and list of abbreviations EN 172 DVDR70 & DVDR75/0x1 9.
The feedback control for the switched-mode power supply is
done by changing the conductive phase of the switching
transistor so that either more or less energy is transferred from
the rectified mains voltage at [2309] into the transformer. The
regulation information is provided by voltage reference [7315].
This element compares the 5V-output voltage via voltage
divider [3332, 3333, 3334] with an internal 2.5V reference
voltage. The output voltage of [7315] passes via an
optocoupler [7314] for insulation of primary and secondary
parts as a current value into pin 3 on the IC [7313]. The switch-
on time of the transistor [7307] is inversely proportional to the
value of this current.
Overload, power limitation, burst mode:
With increasing load on one or more of the power supply
outputs, the switch-on time for the power transistor [7307]
increases, and thus also the peak value of the delta-shaped
current through this power transistor. The equivalent voltage of
this current profile is passed from resistors [3321] and [3352]
via [3365] to pin 5 of the IC [7313]. If the voltage on pin 2
reaches approx. 0.4V in one switching cycle, the conductive
phase of the switching transistor is ended immediately. The
check is done in each individual switching cycle. This process
ensures that no more than approx. 60W can be taken out from
the mains ( = power limitation ).
If the power supply reaches the power limit, the output voltages
and the supply voltage Vcc on pin 1 of the IC [7313] will be
reduced following further loading. If Vcc is less than approx. 9V
at any point during this process, the output of the IC [7313, pin
6] is blocked. All output voltages and Vcc decrease and a new
start-up cycle begins. If the overload status or short-circuit
remains, the power limitation will be activated immediately and
the voltages will again decrease, followed by another start-up
cycle ( Burst Mode ). The amount of power taken up from the
mains in burst mode is low.
Standby modes:
In the ‘AV-Standby‘ operating mode of the set, the ’ION‘ control
line is primarily used to switch off all output voltages for Basic
Engine and Digital Board (supplies 3V3, 5V, 12V, 5N and 4V6
at Connectors 1932 and 1933) of the power supply. This
reduces the amount of power taken from the mains. In Low
Power Standby mode additionally the ‘STBY’ control line is
used to switch off output voltages 5SW and 8SW. This reduces
power consumption to less than 3W, if additionally the display
is switched off. The power supply will continue operating in
Standby mode with a switching frequency of approx. 25 kHz.
9.3.3 Frontend
This unit is designed to support two basic versions, which are
distinguished by a different assembly variant only (one for
multistandard and the second for Pal-I only) and comprises the
following parts:
•
Tuner UV1316K [1705]
• IF amplifier & video demodulator IC TDA 9818/9817 [7710]
• Sound processor MSP3415G [7600]
Tuner and IF selection
The Tuner [1705] converts the RF-signal coming from the
antenna input to an IF-signal. The tuner is fully controlled via
I
2
C-bus of the CC-
µ
P. [1705] is also equipped with a “passive-
loop-through” between antenna-in and -out to save power in
stand-by of the set, when the complete part is not supplied.
The IF frequency of the video carrier is 38.9 MHz for all
systems except SECAM L' (34,0 MHz).
A quasi-split audio system is used. Separate surface-wave
filters (SAW) are required. [1701], [1703] for video, [1702] for
audio. [1701] is switched into the signal path for DK/I-SECAM
L/L' reception, if the signal “SFS_TS” is “high”. In this case the
switches [7704], [7705] are open and the diode [6703] is
conducting. [1703] is switched into the signal path for BG
reception (“SFS_TS” is “low”). Then the switch [7712] is open
and the diode [6704] is conducting. For DK/I-SECAM L/L'
reception, an additional circuit for suppressing the audio carrier
of the adjacent channel is used. This circuitry is adjusted by coil
[5710] for maximum suppression at 40.4MHz.
IF demodulator
The signal from the tuner and IF-selection circuit is processed
by the demodulator IC TDA 9818/9817 [7710]. The signal
“PSS” to pin 3 switches between demodulation of positive
(SECAM only) or negative modulated video carriers. A QSS-
audio-IF signal SIF1 is generated for demodulation in the
sound processor [7600]. The audio-IF carrier is selected in the
audio SAW filter [1702]. This filter is switched for SECAM L’. If
the signal “SB1” is “high”, the switch [7714] is closed and the
diode [6705] is not conducting. For all other standards the
diode [6705] is conducting and the switch [7714] is open. The
output signal of this SAW filter is firstly processed in the TDA
9818. Audio carriers are converted from the tuner IF level to the
audio IF position and further processed in the audio
demodulator [7600]. The AFC coil [5711] on the TDA 9818/
9817 is adjusted so that when a frequency of 38.90 MHz is
supplied to the IF output of the tuner, the AFC voltage on pin
17 of [7710] is 2.5V. The setting of the picture carrier frequency
for SECAM L in the TDA 9818 is achieved by connecting pin 7
of the IC via a resistor [3710] to ground. The switch [7701] and
the signal “SB1" do this. The HF-AGC is set using the
potentiometer [3724] so that, with a sufficiently large antenna
input signal (74 dB
µ
V), the voltage at the IF output of the tuner
[1705] pin 11 is 500 mVpp. This setting must be carried out
when the audio carrier is switched off. The demodulated video
signal appears on pin 16 of [7710]. The AGC voltage at pin 4 is
used to determine the antenna signal strength after a buffer
[7717] with the signal “AGC” and an analog input port of the
CC-P. The trap [1704] reduces the sound carrier remainders in
the video for BG standards. The trap [1706] works in the same
way for the Pal-I standard only. For all other standards the
switch [7713] is closed via [7706] and “SFS_TS”-line set “high"
to bypass this trap. In these cases the selectivity of the SAW
filter [1701] is sufficient. The coil [5713] for non-BG standards
realizes a frequency response correction. This correction is not
desired for SECAM L' and therefore short-circuited by [7716]
(signal SB1 is “high” and [7702] has on-status). The
demodulated video signal “VFV” is available after the buffer
and limiting stage for noise peaks [7711]. The FM-PLL
demodulator function of TDA 9818 is not necessary and
therefore deactivated by the resistor [3739].
Audio demodulator
The sound demodulation is done by the MSP3415 [7600],
which is also fully controlled via I
2
C-bus by the CC-P
(determination of bandwidth, amplitude, standard, ...).The
audio signals are available at pin 30 and pin 31 of [7600] and
fed as “AFER”- & “AFEL”-line to the audio-I/O for further
processing.