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 (
O
n
S
creen
D
isplay) feature IC and I904 is for video output amplifier IC.
The analog R. G. B. video input signals are supplied through the cable, which is terminated at
P901, these input signals 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 signals are AC coupled via C901/C931/C961 then fed into the video pre-amplifier
I901 at pin11, pin6 and pin2 respectively.
After being pre-amplifier, 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 7,6,9 respectively.
The I904 is video output amplifier to amplify the mixed R, G, and B signal. They
offer about
38 Vp-p
signal in amplitude to drive the cathodes of CRT.
L912, L942, L972, are the peaking coils, also for the compensation.
The pin 26; 25 and 23 of I 901 are for CRT cut-off control. These pins can vary the cathode
cut-off voltage settings to meet color temperature setting of white balance by Q963; Q933and
Q903 and its peripheral components. The beam currents limit circuit, which is composed of
D941 and its peripheral components; will respond to the dynamic or static variation of beam
current of the CRT.
The DC brightness bias voltage derived from R902, R903 is applied to pin30 of I901 holding
the constant DC bias of the video amplifier during the reference portion of the black level of the
video waveform.
When the OSD open instruction is received from micro-controller I501 through SDA /SCL pins,
OSD pattern is set from pin13(R); pin14 (G) and pin15 (B) of I903 to pin13(R); pin9 (G) and pin4
(B) of I901.
