Hitachi C28W440N, Service Manual

Page: 48 / 63

48
SMPS SWITCH OFF
When the mains is switched OFF, so long as the electrolytic bulk capacitor provides
energy to the SMPS the controller remains in the switching phase. Then the peak
current reaches its maximum peak value, the switching frequency decreases and all
the secondary voltages are reduced. The VCC voltage is also reduced. When VCC is
less than 6.5V, the SMPS stops working.
B) MICRO-CONTROLLER IC500
IC 500 controls all the functions of the receiver operated by the remote control and
the front panel customer controls. It produces the on screen graphics, operates tuning,
customers controls and engineering and servicing controls, and also incorporates all of
the Tele-text functions. It also controls the video processor, the audio processor, and
the tuner. The circuits just mentioned are controlled via the I²C bus. Also IC501
controls the video source switching, vertical position adjustment and the vertical
linearity adjustment via its ports.
An external 8K eeprom is used by the micro. The eeprom comes fully programmed.
The main clock oscillator is 6.0 MHz crystal X500 on pins 34 and 35. Reset is
provided on pin 33 via Q504. On switching on pin 34 becomes high and the controller
gets reset which stays valid till a low signal comes on that pin.
CONTROLS
Command information from the infra red remote controller is fed through the sensor
on the keyboard (front panel) in front of the cabinet to pin 24 of the micro-controller.
Operations on the customer front panel keys are detected by pin 4,5,6,7 those are
ADCs (analogue to digital converter). Pressing a switch will connect the 3.3V to the
ground through a particular resistor that makes a voltage on the dedicated pin for that
switch at that instant and, so micro-controller detects to operate which function and
the corresponding operation is performed.
IC500 automatically switches from TV mode to AV1, AV2 if a source is connected to
SCART-connectors, by detecting the signal coming from pin8 of the two SCART-
connectors, through pin16 and pin17 respectively.
TUNING
All the tuning functions are carried by the micro-controller IC500. Two tuning modes
are available for this chassis PLL tuning and frequency tuning. For both, both manual
and automatic modes are possible. If Auto Tuning Mode is selected the receiver tunes
Band 1, Band2, and UHF, putting into memory the channel, signal strength (video
indent for PLL), and tuning data of each TV station found. The memories are then
stored automatically to put the channels into frequency order from lowest frequency
to the highest one. In APS (Auto-Programming-System) TV sets the channels are
stored according to the standard tables provided for each country.
In both PLL and frequency tuning modes the tuning process is controlled by IC501
via the I²C BUS. In PLL mode a table for all the channels available is set according