Service manual

5、Work Principle
The product mainly handles with video signal, such as RF (cable TV),
CVBS(composite video), S-VIDEO(Y/C signal), YPbPr(progressive video), SCART, DTV
signal, VGA and HDMI, as well as audio signal amplifying output. AV includes S-VIDEO
and CVBS. In addition, it can input VGA and other signals to realize PIP function.
Because the product is comprised of three boards, the work principle will separate
into three parts as follows:
5.1、Work Principle of Power Board
1. When power is supplied, whether the TV is turned on or off is decided by the
voltage of STB pin in the mini MCU PIC16F72, the high voltage means standby, and
the low voltage means turning on.
In the standby state, the winding 5 and winding 7 of transformer T4 will supply
5VS voltage after regulated by D13, then the power is fed back to optical coupling,
After regulated by diode D14.the 5VS becomes steady. The circuit combined with
transformer T4 is a standby circuit, RE1 is off, then the power consumption could
be less then 1W. When STB pin in MCU sends out a low voltage order, the RE1 circuit
will work and the two main circuits output 24V、+/-12V，5V，3.3V.
2. When the RE1 is on, the power (90-264VAC) will be transformed into sine wave
by DB2. The C27 will be charged by D3 at first, not by T3, to avoid the C27 charge
saturation, which may cause hardware damages such as MOS JM1 failure.
At the same time, the pin 2、4 of U1 ML4800 will set up current and voltage
for the multiplication organ inside. Then C25 will be charged by R17 and R18,
supplying the power for the pin 13 (VCC) of U1. The PFC circuit begins to work,
PFC circuit includes pin 1, 2, 3, 4, 15, 16, 12 and correlative circuits, pin 13
of U1 (shares the power supply with PWM), pin 14(7.5V reference voltage), pin
7(brae pulse producing circuit, provides switch frequency), together with JM1,
T3 and D8.
When PFC begins to work, the pin 12 output switch waves, opposite to the half
wave sine waves, so when the voltage is highest, the duty cycle is lowest, when
the voltage is lowest, the duty cycle is highest. When JM1 is on, T3 charges, and
when JM1 is off, the stored energy of T3 is released to C27 by the loop circuit
consist of D8, C27, R81, and R83. In this way, the voltage of C27 is stabilized
to 380VDC. feed back by the 15 pin of U1. R26, R27 and R23 compose the voltage
dividing resistances. After dividing, the voltage of R23 reaches 2.5V.There is
a 2.5V exact voltage source inside the pin 15. Comparing with it, equation means
the voltage reaches 380V steadily. If the voltage of C27 is lower than 380V, then
the voltage of R23 will be lower than 2.5V. Compared with of 2.5V voltage, pin
16 will output high voltage, making the duty cycle of pin 12 higher, then the
energy storage of T3 increases, and voltage of C27 is higher. Contrarily, the
voltage of C27 is lower.
The pin 3 of U1 4800 is the detecting pin for PFC average voltage mode. The
minus voltage peak value on the pin 3 by R81 should not exceed 1V. Otherwise, the
12