Mitsubishi Electric HITACHI VS-50111 Troubleshooting Manual page 43

The output of Q5A51 is the drive signal for the Flyback
transformer, T5A51. In the Flyback, the signals are
stepped up and rectified to generate the HV and Focus
voltages for the three CRTs.
The amount of HV generated depends of the con-
duction time of Q5A51, the longer the conduction
time the more energy supplied to the Flyback, and
HV increases. HV is regulated by controlling the
duty cycle of the drive signal to Q5A31.
A sample of the HV, HV-DC-FB, is derived from an
internal resistive divider in the Flyback and is output
at pin 13. The HV sample is applied to the non-
inverting input of an OP-Amp in IC5A01. The HV ADJ
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voltage in applied to the inverting input of a second OP-
Amp in IC5A01. The outputs of both OP-Amps are
combined and directed to pin 4 of IC5A00.
DO NOT measure the HV-DC-FB voltage at pin 13
of the T5A51. The meter may load down the internal
resistive divider, resulting in excessive HV.
If HV is high, the voltage at pin 4 of IC5A00 in-
creases. Changing the duty cycle of the drive to
Q5A51, decreasing its' conduction time and reduc-
ing HV. If HV decreases just the opposite occurs.
A HV-AC-FB sample is added to the voltage at pin
4 of IC5A00, to speed up the circuits reaction, reduc-
ing any raster breathing effect.
6-5
and Troubleshooting Guide