Philips MRV640 Service Manual page 112

EN 112 9. DVDR615, MRV640
[1986]). By generating a high level on pin 16 of the CC the
digital PCB can be reset (inverter [7817] in between).
Most of the other parts are controlled by the µP via I2C-bus
("SDA"- and "SCL"-signal). The FETs [7821] and [7822] are
used for adaptation of the 3V3-level on CC-side to the
components supplied with 5V.
The CC can also reset the display-board-µP by pulling pin 39
to high.
The transistor [7819] acts as a level shifter for the "INT"-signal.
In the European sets a bi-directional interface is established
between the recording unit and the TV device at pin 10 of the
Scart ("P50"-line/Easy Link). The processing is done via pin 14
(output) and pin 38 (input) of the CC and the circuit around
[7813], [7814] and [7815].
9.2.4 EEPROM
The EEPROM M24C16 [7808] is an electrical erasable and
programmable, non-volatile memory. The EEPROM stores
data specific to the device, such as the AFC-reference value of
the Europe IF-part, the clock-correction-factor, etc. It is
accessed by the µP via the I2C-bus.
9.2.5 Sync Separator
To detect whether a video signal is available or not a separate
IC [7825] is used to extract the sync information out of the video
signal that is also routed to the digital board for recording.
While on the input a low-pass-filter ([2823] and [3869]) limits
the bandwidth an additional filter (circuit
around [7818]) on the output avoids distortions. Afterwards the
sync-signal is routed to pin11 of the CC.
9.2.6 Fan Control
To avoid unwanted temperatures inside the set (especially the
Laser on the OPU of the drive is very sensitive) a fan is located
on top of the basic engine. The speed control is dependent on
the ambient temp. A NTC resistor [3134] located on the display
board measures the temperature. An operational amplifier
[7902-B] generates a proper voltage, which is then fed to the
engine ("BE_FAN"-line). Below 28°C ambient temp. the fan-
voltage is approx. 5V and is increased to 10V when the ambient
temperature goes up to approx. 38°C. The second part of the
Op-Amp. [7902-A] prevents damage of any temperature-
sensitive part in case the NTC or the wire in between is
damaged. It acts as a comparator and pulls the "BE_FAN"-
signal to 10V. As the fan has to be stopped in case the tray of
the drive is open this voltage is "killed" by the CC ("FAN_OFF"-
signal). The double-diode [6901] acts for both Op.-Amp.-
circuits. The circuit is also prepared for a set-fan (circuit around
the Op-Amp. [7902-C] ).
9.3 Analog board Europe
9.3.1 General
This PCB consists out of the following parts:
– Power-Supply-Unit
– Frontend (Audio & Video)
– Input-/Output-switching
– Audio ADC- & DAC-processing
– VPS/PDC- and Text-Data slicer
– Analog Follow-Me Circuit
All functional groups are either controlled via I2C-bus or via
separate signal lines by the Central-Controller on the µP-Sub-
Board. This sub board is directly soldered in onto the analog
PCB. During Stand-By mode of the set, several parts are not
supplied (Tuner, MSP, ...). The microprocessor is running and
maintains the clock of the set.
To avoid bus blockades the I2C-bus ("SCLSW" & "SDASW") to/
from these units is decoupled via transistors [7419], [7420]
from the general bus ("SCL" & "SDA").
Circuit-, IC descriptions and list of abbreviations
9.3.2 Power Supply Unit
Functional principle:
This power supply works in the way of a flyback converter. In
the mains input part [1931 to 2309], the mains voltage is
rectified and buffered in the capacitor [2309]. From this direct
voltage at [2309] energy is transferred into the transformer
[5300, pins 7-5] during the conductive phase of the switching
transistor [7307] and is stored there as magnetic energy. This
energy is passed to the secondary outputs of the power supply
in the blocking phase of the switching transistor [7307]. With
the switch-on time of the switching transistor [7307], the energy
transferred in every cycle is regulated in such a way that the
output voltages remain constant regardless of changes in the
load or mains voltage. The power transistor is driven by the
integrated circuit [7313].
Mains input part:
The mains input part extends from the mains socket [1931] to
the capacitor [2309]. The diodes [6301, 6302, 6305 and 6306]
rectify the AC supply voltage, which is then buffered by the
capacitor [2309]. The common mode coil [5302] and capacitor
[2302] work as a filter to block interference arising in the power
supply from the mains. Components [1302], [3306] and [3304]
protect the power supply against short-term over voltages in
the mains, e.g. caused by indirect lightning.
Start-up with Mains-on:
After connecting the power cord to the mains, the capacitor
[2325] is loaded via a current source between pin 8 and pin 1
in the IC [7313]. Once the voltage on [2325] and therefore the
supply voltage Vcc of the IC [7313] has reached approx. 11V,
the IC starts up and provides pulses at its output pin 5. These
pulses are used to drive the gate of the power transistor [7307].
The frequency of these pulses is depending on load and mains
voltage. The current consumption of the IC is approx. 5 mA at
Vcc in normal mode.
If Vcc drops to below approx. 9V (e.g. with power limitation) or
if Vac exceeds approximately 16V (e.g. interruption of the
control loop), the output of the IC [7313, pin 5] is blocked and a
new start-up cycle begins. (See also "Overload, Power
Limitation, Burst Mode" section)
Normal operation:
With the power supply in normal mode, the periodic sequences
in the circuit are divided primarily into the conductive and
blocking phase of the switching transistor [7307]. During the
conductive phase of the switching transistor [7307], current
flows from the rectified mains voltage at capacitor [2309]
through the primary coil of the transformer [5300, pins 7-5], the
transistor [7307] and resistors [3321, 3352] to ground. The
positive voltage on pin 7 of the transformer [5300] can be
assumed as constant for a switching cycle. The current in the
primary coil of the transformer [5300] increases linearly. A
magnetic field representing a certain value of the primary
current is formed inside the transformer. In this phase, the
voltages on the secondary coils are polarized such that the
diodes [6300, 6303, 6307, 6308, 6310, 6313, 6317 and 6319]
block. From the controller [7315] a current is supplied into the
CTRL input on the IC [pin 3, 7313] via optocoupler [7314]. Once
the switch on time of the switching transistor [7307] - that
corresponds to the current supplied into the CTRL input - has
been reached, the switching transistor [7307] is switched off.
When the switching transistor has been switched off, the
blocking phase begins. No more energy will be transferred into
the transformer. The inductivity of the transformer will still
attempt to keep the current flowing at a constant level (U=L*di/
dt). Switching off transistor [7307] interrupts the primary current
circuit. The polarity of the voltages on the transformer is
reversed, which means that the diodes [6300, 6303, 6307,
6308, 6310, 6313, 6317 and 6319] become conductive and
current flows into the capacitors [2305, 2312, 2319, 2322, 2326
and 2328] and the load. This current is also ramp-shaped (di/dt
negative, therefore decreasing).