Daewoo DTL- 25G6F Service Manual download pdf (Page 41)

RGB output and black current stabilization

The colour difference signals (R-Y, B-Y) are matrixed with the luminance signal (Y) to obtain the RGBout output

signals (pins 19, 20, 21).

In the TDA8844 the matrix type automatically adapts to the decoded standard (NTSC,PAL) .

Linear amplifiers are used to interface external RGB in signals (pins 23,24,25) from the SCART connector.

These signals overrule the internal RGB signals when the data insertion pin 26 (FBI) is switched to a level between

1.0V and 3.0V.

The contrast and brightness control and the peak white limiter operate on both internal and external RGB signals

R,G and B each have their own, independent gain control to compensate for the difference in phosphor efficiencies

of the picture tube: so called “white point” adjustment.

The nominal amplitude is about 2V black to white, at nominal input signals and control settings.

TDA8844 has a black current stabilization loop, that automatically adjust the black level to the cut-off voltage of the

picture tubes three gun cathodes.

Since no current is flowing when the voltage the cathode is equal to the cut-off voltage of the tube, the loop stabilizes

at a very small gun current.

This “black current” of the three guns is measured internally and compared with a reference current, to adjust the

black level of RGBout.

The black level loop is active during 4 lines at the end of the vertical blanking.

In the first line the leakage current is measured (max. acceptable 100 ).

In the next three lines the black levels of the three guns are adjusted.

The nominal value of the ‘black current’ is 10

The ratio of the ‘black currents’ for the 3 guns tracks automatically with the white point adjustment, so the

back-ground colour is the same as the adjusted white point.

At switch-on of the TV receiver the black current stabilization circuit is not yet active and RGBout are blanked.

Before the first measurement pulses appear, 0.5 sec delay ensures that the vertical deflection is active, so the pulses

will not be visible on the screen.

During the measuring lines RGBout will supply 4V pulses to the video output stages.

The TDA8844 waits until the black current feedback input (pin 18) exceeds 200 , which indicates that the picture tube

is warm-up.

Then the black current stabilization circuit is active.

After a waiting time of about 1.0 sec, the blanking of RGBout is released.

TDA8351 vertical deflection.

The TDA8351 is a vertical deflection circuit.

It can be used in 90 deflection systems with frame frequencies from 50 up to 120 Hz

With its bridge configuration the deflection output can be DC coupled with few external components.

Only a supply voltage for the scan and a second supply for the flyback are needed.

The TDA8351 can drive max.2A.

The vertical drive currents of TDA8375A pins 47 and 46 are connected to input pins 1 and 2 of the TDA8351.

The currents are converted into a voltage by a resistor between pins 1 and 2.

Pin2 is on a fixed DC level (internal bias voltage) and on pin l the drive voltage can be measured (typical 1.8 Vpp).

The drive voltage is amplified by ‘A’ and fed to two amplifiers ‘B’ and ‘C’, one is inverting and the other is a non

inverting amplifier.

The outputs (pins 4 and 7) are connected to the series connection of the vertical deflection coil and feedback resistor .

The voltage across feed back resistor is fed via pin 9 to correction amplifier ‘D’, to obtain a deflection current which is

proportional to the drive voltage.

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Daewoo DTL- 25G6F Service Manual Page 41

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