NEC Diamond Scan 51, Service Manual

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DC/DC
The DC/DC voltage is DC 45 volts, and since the set-up voltage is variable from 60 volts to 160 volts,
it is variable depending on the horizontal synchronism. The frequency band is variable from 31 kHz to
65 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 L106, the transistor Q102, the diode
D114 and U102.
2) When the PWM controls U102 IC KA3843, a driving pulse is generated at the gate of the transistor
Q102, and the transistor Q102 turns ON. During the ON cycle, the energy is stored in the choke
L106. The transistor Q102 turns OFF when the driving pulse disappears from the gate of the tran-
sistor Q102. 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 L106 is entered in the FBT,
passing through the choke L106, the diode D114 and the capacitor C129.
3) The feedback is detected by the FBT via diode D130, the capacitor C145, the resistor R145 the
variable resistor VR102 and the fixed resistor R146, and is connected to the U102-2 pin.
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 capacitor C132, the resistor R139, the diode D115, the resistor
R138 and the resistor R140.
5) The soft start circuit consists of the resistor R178, the capacitor C164, the diode D133 and the
diode D117.
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