Philips 20PT643F Service Manual page 92

SCAVEM (if present)
The SCAn VElocity Modulation (SCAVEM) circuitry is implemented in the layout of the
picture tube panel. It is thus not an extra module. This circuit influences the horizontal
deflection as a function of the picture content. In an ideal square wave, the sides are
limited in slope due to a limited bandwidth (5 MHz). SCAVEM will improve the slope as
follows:
At a positive slope , a SCAVEM current is generated which supports the
deflection current. At the first half of the slope, the spot is accelerated and the
picture is darker. At the second half of the slope, the spot is delayed and the
slope becomes steeper.
At the end of the slope , the SCAVEM-current decays to zero and the spot is at
the original position. An overshoot occurs which improves the impression of
sharpness.
At the negative slope , the SCAVEM-current counteracts the deflection. During
the first half of the slope, the spot is delayed and the slope becomes steeper.
During the second half the spot accelerates, the SCAVEM-current is zero at the
end of the slope.
The R/G/B signals are fed into the SCAVEM circuit and differentiated by
C2364/2365/2366 and the input impedance of 7360 stage. Diode 6364 (Schottky diode)
is the coring component, which blocks all the signals below 0.3 V so that the noise is not
amplified and all the signals larger than 0.3V are differentiated and amplified.
After differentiation, the signal is amplified by Q7360 with 3369 as the collector resistor.
The biasing of the 7360 stage is done by 3369, 3361, 3360, 3362, and 3363.
Items 6367, 2367, 3367, 3361, and 2360 work as the clipping components that limit the
SCAVEM current at a certain level, to prevent SCAVEM over correction.