Philips 20PT643F Service Manual page 96

across measuring resistor R3471 that provides internal feedback information. The
voltage across this measuring resistor is proportional to the output current, which is
available at pins 4 and 7 where they drive the vertical deflection coil (connector 0222) in
phase opposition.
IC 7471 is supplied by +13 V. The vertical flyback voltage is determined by an external
supply voltage at pin 6 (VlotAux+50V). This voltage is almost totally available as flyback
voltage across the coil, this being possible due to the absence of a coupling capacitor
(which is not necessary, due to the 'bridge' configuration).
Deflection Corrections
The Linearity Correction
A constant voltage on the horizontal deflection coil should result in a sawtooth current.
This however is not the case as the resistance of the coil is not negligible. In order to
compensate for this resistance, a pre-magnetized coil L5457 is used. R3485 and C2459
ensure that L5457 does not excite, because of its own parasite capacitance. This L5457
is called the 'linearity coil'.
The Mannheim Effect
When clear white lines are displayed, the high-voltage circuit is heavily loaded. During
the first half of the flyback, the high voltage capacitors are considerable charged. At that
point in time, the deflection coil excites through C2465. This current peak, through the
high-voltage capacitor, distorts the flyback pulse. This causes synchronization errors,
causing an oscillation under the white line.
During t3 - t5, C2490//2458 is charged via R3459. At the moment of the flyback,
C2490//2458 is subjected to the negative voltage pulses of the parabola as a result of
which D6465 and D6466 are conducting and C2490//2458 is switched in parallel with
C2456//2457. The high-voltage diodes are conducting this moment. Now extra energy is