On-Screen Circuit; Video Input Circuit; Definition Converter Lsi Peripheral Circuit - NEC MultiSync LCD1550ME Service Manual

7.3 Regulation : If output voltage +5V goes up, the R terminal of I803 gets more bias, accordingly photo
transistor and photo diode flows more current. The voltage of pin 2 goes up too, making the pulse width
of pin 6 to become narrower. So the output voltage +5V will be pulled down to a stable value.
7.4 OVP : If +5V goes up too much, the induced voltage on pin 4 of T802 becomes large also. Suppose that
it is over 18 volts, ZD801 conducts, pin 3 of I801 is pulled up over 1 volt. The pulse train at pin 6 goes
down to zero, shutting Q803 off completely.
7.5 SCP : If output terminal is short to ground, photo transistor does not conduct, hence Q806 does not
conduct neither. The reference voltage +5V at pin 8 begins to charge on C829 and activates Q806 to
conduct either. Then oscillation of I801 is stop, shutting Q803 off completely.
Line Filter
3. On-screen circuit (Circuit diagrams Main PWB)
I306 (gmZAN1) Embeded function.
On-screen menu screen is established and the resultant data are output from I306 (Circuit diagram MAIN
PWB, gmZAN1)
4. Video input circuit (Circuit diagram MAIN PWB)
The AC-coupled video signal is used to clamp the black level at 0V.
5. Definition converter LSI peripheral circuit (Circuit diagram MAIN PWB)
I306 gmZAN1 is the definition converter LSI.
The analog R,G,B signal input entered from the video input circuit is converted into the digital data of video
signal through the incorporated A/D converter. Based on this conversion, this device performs interpolation
during pixel extension. The source voltage for this device is 3.3V and the system clock frequency is 20MHz.
The withstand voltage level for the input signal voltage is 3.3V and 5V.
LCD 1550ME Power Board Block Diagram
Rec.
&
Filter
PWM
Power
Transformer
Driver
FB
7-3
Output Rec. & Filter