Video Block; Picture Signal Amplifier Circuit; Cut-Off Circuit - NEC FP1355 Service Manual

1.9

Video Block

1.9.1

Picture signal amplifier circuit

As for picture signal (video) amplification circuit, R, G and B is respectively the same circuit in structure.
G (green) video circuit is explained in this section.
There are two systems, i.e. SIGNAL-A and SIGNAL-B, in the video input terminal, and both have a D-
SUB connector.
SIGNAL-A input means to input from pin 2 of D-SUB connector J215 to pin 12 of analog switch IC216.
SIGNAL-B input means to input from pin 2 of D-SUB connector J216 to pin 4 of analog switch IC216.
(Refer to A point.)
Analog switch IC216 selects the signal when SIGNAL-A and SIGNAL-B are simultaneously input.
As for the method of selecting the signal, according to SELECT signal of pin 53 of microcomputer IC101,
input signal SIGNAL-A is selected when pin 13 of analog switch IC216 (SELECT SW) is HIGH, and input
signal SIGNAL-B is selected (refer to B point) when it is LOW. Either signal is output from pin 28 of
analog switch IC216.
Video signal output from pin 28 of analog switch IC216 is input to pin 10 of Pre-AMP IC211. (Refer to C
point.)
For video signal, voltage amplification, composite and amplitude control [Explanation 1] is performed in
Pre-AMP IC211, and the signal is output from pin 27. (Refer to D point.)
Video signal output from Pre-AMP is input to pin 8 of MAIN-AMP IC210 and is output from pin 5 of
MAIN-AMP IC210 after final amplification. (Refer to E point.)
(MAIN-AMP IC210 is an amplifier to amplify the video signal with voltage (GAIN: 12 to 15 times).
The video signal is coupled by AC to cut-off circuit (Refer to the cut-off circuit mentioned below.), and
then it is input to CRT socket J200 via connector J202 on CRT substrate through connector J210, and
supplied to the cathode of CRT. (Refer to F point.)
[Explanation 1] Duty of Pre-AMP IC211
•Voltage amplification of video signal (GAIN: 0 to 5 times)
•Composite of the video signal for adjustment screen (OSD) output from IC212 and the
blanking signal output from IC601
•Amplitude control of output voltage (ABL control) <Note 1>
•D/A output for bias control
The above is completely controlled by I2C bus (IC211 #3: SDA_I2C, #4:SCL_I2C) comes from micro-
computer IC101.
<Note 1>
According to detection of current by the flyback transformer on MAIN substrate, the upper limit value of
brightness when the screen is totally white, by controlling CRT anode current.
1.9.2

Cut-off circuit

The video signal amplified with voltage in the picture signal amplification circuit is coupled by AC (super-
impose the pulse into DC voltage) to the cut-off (diode clamp) circuit (DC bias control circuit) consists of
D250G, D251D, Q250G and Q251G at C210G.
The cut-off (DC bias control) circuit changes back raster brightness and chromaticity (bias) by brightness
control signal and bias control signal.
The brightness control signal, which is superimposing SUB-BRT signal (for factory adjustment) output
from pins 14 and 13 of OSD-IC IC212 and BRT signal (for user adjustment) output from pin 32 of Pre-
AMP IC211 at OP-AMP IC213, is output from pin 1 of OP-AMP IC213. When the superimposed bright-
ness control signal is applied to the emitter of the base ground transistor Q250G, the back raster bright-
ness changes.
The bias control signal is output from pin 30 of Pre-AMP IC211. The bias control signal is, as well as the
brightness control signal, applied to the emitter of the base ground transistor Q250G, and changes the
back raster chromaticity (BIAS).
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