Hitachi CM771ET, Service Manual

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3.2 High Voltage Regulation Circuit
The output pulse from Q701 is also fed to the high voltage regulation circuit with the same design of consonant circuit as
the horizontal deflection circuit. High voltage of 27.0 kV is obtained by the step-up windings of the flyback transformer to
drive the CRT anode. The high voltage is monitored by the E
HV
error detection circuit. The error detection circuit
functions by stepping the high voltage down and comparing it with the reference voltage of inside I740 whose output
controls I740, the Integrated Circuit included with E
HV
switching transistor.
3.3 Dynamic focus drive circuit
This monitor’s CRT includes a dynamic focusing electron gun to achieve sharp and uniform focus throughout the display
area. The CRT’s Focus anode receives a DC component of approximately 27% of the CRT anode voltage, combined
with the AC voltage parabolic wave form of magnitude of 600 Vp-p horizontal, and 180 Vp-p vertical. DC focus voltage is
obtained from a tap of the flyback transformer’s bleeder resister, and fed to G3 focus electrode. Horizontal and vertical
parabolic output pulses are amplified at Q501
Q508, T560 and fed to the flyback transformer where they are combined
with the DC component (27% of anode voltage). The potentiometers (focus 1, focus 2) at the flyback adjust the DC
focus voltage. The focus 2 potentiometer mainly adjusts horizontal beam shape (vertical line width), and the focus 2
potentiometer mainly adjusts the vertical beam shape (horizontal line width) by optimizing the DC component of the
parabolic waveform.
3.4 High Voltage Hold-Down Circuit
DJ72- Chassis uses a system that stops H/V DRIVE SIGNAL output when abnormal high voltage is detected. So that
the high voltage output will be declined to zero. The circuit operation in detail is as follows.
When an input voltage to I740 (pin15, it is determined by R792 and R793) exceeds the specified level, the high voltage
is declined to zero.
FIG.10 HIGH VOLTAGE HOLD-DOWN CIRCUIT DIAGRAM
For the models CM771U-511, CM771ET-301
D790
+
-
C791
R793
R792 R791
L791
CRT
FBT
HIGH
VOLTAGE
8
E
HV
+B
I740
E
HV
module
R756
R747
R746
10