An active amplifier is responsive to a vertical parabola voltage developed across a trace or S-shaping capacitor for developing a DC voltage at an increased voltage level than can be obtained by rectifying the parabola voltage. The increased voltage level is developed in a capacitor, during part of the period of each scan. The capacitor is discharged through a parallel resistor. During normal scanning, the capacitor voltage, alternately, increases and decreases, but remains above a threshold level as detected by a comparator. Upon scan loss, the capacitor discharges below the threshold and the comparator triggers blanking of the kinescope driver. In this way, the phosphors of a cathode ray tube of a television, especially a projection television, are protected from burning due to a loss of electron beam deflection.
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