NEC MultiSync 75F-3 Service Manual page 43

‡
Power saving
Suspend mode
OFF mode
1) When the power switch is turned ON when there is nothing being entered in the video cable. At that
time, if the video is shifted from the free-run mode to the suspend mode, the 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 the transistor Q107, then Q105 turns OFF, the transistor Q108 and Q104 turns 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 is resumed, the video signals
V-SYNC and H-SYNC turn ON the CPU via resistor R129 and R159, then the transistor Q105 and
Q107 turn ON, the transistors Q104 and Q108 turn ON. As a result the operation returns to the ON
state.
ˆ
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 synchronism. 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) When the PWM controls U302 IC TDA4857, a driving pulse is generated at the gate of the transis-
tor Q319, and the transistor Q319 turns ON. During the ON cycle, the energy is stored in the choke
L304. The transistor Q319 turns OFF when the driving pulse disappears from the gate of the tran-
sistor Q319. 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 is entered in the FBT,
passing through the choke L304, the diode D335 and the capacitor C307.
3) The feedback is detected by the FBT via diode D310, the capacitor C326, the resistor R349 the
variable resistor VR302 and the fixed resistor R382, and is connected to the U302 pin 5.
This is loop is the regular type one.
4) The frequency of the synchronization 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 the resistor R30P, the capacitor C309, the diode D303 and the
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.
40