Daewoo DWP-28W2ZZF Service Manual download pdf (Page 51)

Appendix Service manual WP-795

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The SECAM decoder contains an auto-calibrating PLL demodulator which has two references, via the

divided 12 MHz reference frequency (obtained from the

-Controller) which is used to tune the PLL to

the desired free-running frequency and the bandgap reference to obtain the correct absolute value of the

output signal. The VCO of the PLL is calibrated during each vertical blanking period, when the IC is in

search or SECAM mode.

The base-band delay line (TDA 4665 function) is integrated. This delay line is also active during NTSC

to obtain a good suppression of cross colour effects. The demodulated colour difference signals are

internally supplied to the delay line.

RGB output circuit and black-current stabilisation

In the RGB control circuit the signal is controlled on contrast, brightness and saturation. The ICs have a

linear input for external RGB signals. The signals for OSD and text are internally supplied to the control

circuit. The output signal has an amplitude of about 2 Volts black-to-white at nominal input signals and

nominal settings of the various controls.

To obtain an accurate biasing of the picture tube the 'Continuous Cathode Calibration’ system has been

included in these ICs. A black level off set can be made with respect to the level which is generated by

the black current stabilisation system. In this way different colour temperatures can be obtained for the

bright and the dark part of the picture.

The black current stabilisation system checks the output level of the 3 channels and indicates whether the

black level of the highest output is in a certain window or below or above this window. This indication is

read from the status byte 01 and is used for automatic adjustment of the Vg2

voltage during the production

of the TV receiver.

During switch-off of the TV receiver a fixed beam current is generated by the black current control circuit.

This current ensures that the picture tube capacitance is discharged. During the switch-off period the

vertical deflection is placed in an overscan position so that the discharge is not visible on the screen.

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Daewoo DWP-28W2ZZF Service Manual Page 51