Video Circuit - Nokia 447ZiPlus Service Manual

6-12. Video Circuit

There are three the ICs applied for the video circuit.
I901 is a three channel pre-amplifier IC; I903 is OSD display IC.
I904 is for video output amplifier IC.
The analog R. G. B. video input signal are supplied through the cable, which is terminated at P901, these
input signal are approximately 0.7Vp-p in amplitude.
R901/R931/R961 give a resistance of 75 ohm for impedance match. The R, G, B video signal are AC
coupled via C901/C931/C961 then fed into the video pre-amplifier I901 at pin11, pin6 and pin2
respectively.
After being pre-amplified, the R, G, B video signals are output from pin35, pin32, and pin29 of I901
respectively. The amplitude of the signal at theses output are about 3 ... 4 V p-p. Those video signals are
connected the I904 at pin 11, 8, 9 respectively.
The I904 is video output amplifier to amplify the mixed R, G, and B signal. They offer about 40 V p-p
signals output in amplitude to drive the cathodes of CRT.
L912, L942, L972, are the peaking coils, for the compensation of high frequency response.
The contrast adjustment is controlled by SCL and SDA which fed into the pin20, 21 of the I901 through
the internal D/A converter to control the subcontract and the main contrast, so that we can have different
output in amplitude on pin35 (R), pin32 (G), pin29 (B).
The beam currents limit circuit that is composed of D941 and its peripheral components automatically
responds to the dynamic or static variation of beam current of the CRT. The ABL voltage is fed into pin15
of the I901. If this voltage is lower enough, it will activate the limitation to the internal main contrast to
prevent the picture from being too bright.
The DC brightness bias voltage derived from R902, R903 is applied to pin30 of I901 to give a constant DC
bias for the video pre-amplifier outputs.
The cathode cut-off setting voltage required for white balance is obtained from pin23(R), pin25 (G), pin26
(B) respectively. These output magnitude control the degree of transistor conducting (Q903, Q933, Q963)
individually so that the DC bias of the cathode are then varied to optimize the color temperature of back
ground.
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