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Service Manual CP-785F
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.
Summary of Contents for DTE-29U1TH
Page 67: ...Service Manual CP 785F 66 7 Exploded view ...
Page 68: ...Service Manual CP 785F 67 8 PCB Layout 8 1 Main PCB ...
Page 69: ...Service Manual CP 785F 68 8 2 AV PCB ...
Page 70: ... 69 Service Manual CP 785F 9 Circuit Diagram ...
Page 71: ...NEC Corporation 7 1 SHIBA 5 CHOME MINATO KU TOKYO 108 8001 JAPAN ...