Hitachi C28W40TN, Service Manual

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the base of Q2 via R1, the collector of Q2 holds the base of Q3 low via R3. This allows the collector of Q3 and pin 10 of IC2 to rise
through the pull up network consisting of R10, regulated to 5V6 by ZD2, R9 and R8, C7 is just to add a time constant.
A dual operational amplifier (IC6) is used with both comparator outputs (pins 1 & 7) joined together to mute the amplifiers by holding
the 5V rail low.
At power up pin 1 of IC6 holds the 5V rail low through D42 until the main supply rails settle. A reference voltage generated from the
38V rail by a potential divider consisting of R24 and R30 is applied to pin 2 of IC6, this is then compared to the voltage applied to pin
3 from the potential divider R104 and R100 with C107 adding the time constant.
The other half of the comparator is used at power down. Pin 6 has a reference level derived from the 38V supply from the potential
divider R24 and R102. Pin 5 is connected to the relay switching voltage supplied by the TV. When the TV is put into standby or
turned off the 16V drops to approximately 14V immediately causing pin 7 to be pulled low muting the amplifiers
At power down the 5V is pulled low turning off Q2, which allows the base of Q3 to rise via the pull up resistor R81 from the main
supply for IC2, this in turn pulls the collector low, discharging C7 and removing the voltage from pin 10 via R105 and D30A which
are used to speed up the turn off time of the amplifier. The pull up to the supply is used to make sure that the amplifier stays in mute
/ standby while the supply rail discharges.
DIGITAL SECTION
GENERAL DESCRIPTION
The main function of the digital section of the receiver is to accept COFDM ( C oded O rthogonal F requency D ivision M ultiplex) coded
signals modulated onto a RF carrier in the UHF band, and by demultiplexing, decoding and formatting the information carried, to
produce analogue outputs at audio and video suitable for presentation through a ‘TV’ display.
It may be integrated into a television receiver (IDTV) or a set-top box (STB) with a suitable analogue ‘motherboard’.
The graphics, menus and control functions for the digital receiver are also carried out within the digital module.
The digital section of the receiver is made up of two main PCBs and another associated PCB:
•
COFDM front-end PCB
•
MPEG Decoder and AV/Graphics PCB
•
Smartcard Interface PCB
All the interconnections between these PCBs are internal to the metal screening enclosure and using flat PCB ribbon connectors.
The connections from the digital PCBs to the analogue circuits also use the same type of flat PCB ribbon connectors, except for the
main power connector. The details are given in section 2.6.
The connections accessible to the user from the digital section are:
•
Tuner RF input connector
•
Tuner RF loop-through output connector
•
RS232 9 pin ‘D type’ connector
•
RJ11 modem connector
•
Common Interface sockets
A fan is attached to circulate air through the enclosure for the digital module on the integrated TV.