Mitsubishi DPLUS 74SB -BKA Service Manual page 105

1.7 Power saving
Suspend mode
OFF mode
1) When the power switch is turned ON, and there is no sync pulses to the video cable, the video shifts
from the free-run mode to the suspend mode. Transistor Q107 turns OFF and the operation returns to
the OFF mode within a few seconds.
2) As for the sequence of steps that turn the operation to the OFF mode, if the keyboard is not touched
for a given period of time that a preset in advance, the CPU outputs the LOW level signal to transistor
Q107, then Q105 turns OFF, transistor Q108 and Q104 turn OFF. As a result the power is shut out at
that state.
3) When the user touches the keyboard in the OFF mode, the operation resumes, the video signals V-
SYNC and H-SYNC turn ON the CPU via resistors R129 and R173, then the transistors Q105 and
Q107 turn ON, transistors Q104 and Q108 turn ON. As a result the operation returns to the ON state.
1.8 DC/DC
The DC/DC voltage is DC 49 volts, and since the set-up voltage is variable from 62 volts to 160 volts,
it is variable depending on the horizontal sync. The frequency band is variable from 31 kHz to 69 kHz.
The voltage is fed back from the fly-back transformer (FBT). The DC/DC output voltage is used as high-
voltage input of FBT T301.
1) DC/DC is a step-up circuit, and consists mainly of the choke L304, the transistor Q319, the diode
D335 and U302.
2) PWM controls U302 IC TDA4857, a driving pulse is generated at the gate of transistor Q319 turning it
ON. During the ON cycle, the energy is stored in the choke L304.
Transistor Q319 turns OFF when the driving pulse disappears from the gate.
As a result, the voltage at the dot terminal of the winding flows in the positive direction and goes to the
fly-back rectifier. The energy stored in the choke L304 enters the FBT, passing through choke L304,
diode D335 and capacitor C373.
3) The feedback is detected by the FBT via diode D310, capacitor C326, resistor R349, variable resistor
VR102 and fixed resistor R382, and is connected to U302 pin 5.
This is loop is the regular type one.
4) The frequency of the sync signal coming from video H-SYNC is variable from 31 kHz to 69 kHz. The
circuit consists of the R30X.
5) The soft start circuit consists of resistor R30P, capacitor C309, diode D303 and Q326.
: Every DC voltage operation of the CRT is turned OFF.
The color of the power LED101 switches from green to orange.
: This is the mode in which the CPU control unit turns OFF the power supply, but
the power turns ON when the user touches the keyboard.
The power LED switches to dark orange color.
7-4