When controlling the working point of video output stages in monitors, data display cathode ray tubes (CRTs) (10) and the like, in which the scanning raster is applied to the screen of the CRT entirely on the visible portion of the screen, and to permit control of the working point of the video output stage by strobing the beam current at a high brightness level, the vertical deflection of the beam during not-needed lines is so controlled that the beam is deflected beneath a still remaining frame or beyond the phosphor layer of the screen. The deflection is obtained by an additional auxiliary deflection pulse derived, for example, from a monostable multivibrator (45) receiving a pulse close to the beginning of the selected line, and superimposing its pulse to the deflection of the deflection current for the vertical coil 42.
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