(2) Working principle:
+5VEXT voltage stabilizing circuit: after the AC power inputted externally is being rectified/filtered
through power board, +12V power inputs to main video board. +12V power is divided to multi-group
power supply. One group inputs to the +5V voltage stabilizing circuit composed by U503 (Ap1506)
(Ap1506 is a voltage drop type voltage stabilizing IC), after the 12V power inputted by power board is
being voltage stabilized through Ap1506, +5V power outputs directly. Pin 4 of Ap1506 is feedback end
and the resistance of its externally-connected resistor may adjust the output voltage. To facilitate to
differe5VEXT mark ion circuit, this group of power is mainly used for MST718 and each control
circuit and supply working power for the player when in standby working state.
Display screen working power control circuit: display screen power has +5V/+12V. Display screen
of this player 12V power supply, so L219 is not used in circuit but L16 is used for power supply.
U502 in the picture is a MOS pipe used for switch function to control power supply of display screen.
This MOS pipe is controlled by PANELON signal. When machine is in standby, PANELON outputs high
level signal and U502 cuts off.
After machine power on, pin 74 of MST718 outputs low level signal to make Q505 cutoff, Q504/U502
is connected, +12V power outputs to display screen through U502 and display screen begins to work.
Figure 3.2.5.3 Voltage stabilizing circuit and display screen Power control circuit diagram
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