Philips 27PS60S321 Vertical drive, Deflection corrections, Linearity correction, Mannheim effect

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the CRT, while theline output circuit drives the horizontal deflection coil.

Vertical drive

A divider circuit performs the vertical synchronization.The vertical ramp generator needs an external resistor (R3245, pin 20)and capacitor (C2244, pin 21). A differential output is availableat pins 16 and 17, which are DC-coupled with the vertical outputstage.

During the insertion of RGB signals, the maximum vertical frequencyis increased to 72 Hz so that the circuit can also synchronize onsignals with a higher vertical frequency like VGA.

To avoid damage of the picture tube when the vertical deflectionfails, the guard output is fed to the beam current limiting input.When a failure is detected the RGB-outputs are blanked. When novertical deflection output stage is connected this guard circuitwill also blank the output signals.

These ‘V_DRIVE+’ and ‘V_DRIVE-‘ signalsare applied to the input pins 1 and 2 of IC 7471 (full bridge verticaldeflection amplifier). These are voltage driven differential inputs.As the driver device (IC 7200) delivers output currents, R3474 and R3475convert them to voltage. The differential input voltage is comparedwith the voltage across measuring resistor R3471 that provides internalfeedback information. The voltage across this measuring resistoris proportional to the output current, which is available at pins4 and 7 where they drive the vertical deflection coil (connector0222) in phase opposition.

IC 7471 is supplied by +13 V. The vertical flybackvoltage is determined by an external supply voltage at pin 6 (VlotAux+50V).This voltage is almost totally available as flyback voltage acrossthe 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 deflectioncoil should result in a sawtooth current. This however is not thecase as the resistance of the coil is not negligible. In order tocompensate for this resistance, a pre-magnetised coil L5457 is used.R3485 and C2459 ensure that L5457 does not excite, because of its ownparasite capacitance. This L5457 is called the "linearity coil".

The Mannheim effect

When clear white lines are displayed, the high-voltagecircuit is heavily loaded. During the first half of the flyback,the high voltage capacitors are considerable charged. At that pointin time, the deflection coil excites through C2465. This current peak,through the high-voltage capacitor, distorts the flyback pulse.This causes synchronisation errors, causing an oscillation underthe

7/8/2004