6- 5 Dynamic focus circuit
The vertical dynamic focus come from pin 32 of I301 and feed to base of Q105 via R105 and C103. The
horizontal dynamic focus is taken between L402 and Horizontal S correction capacitors. It is step
up(amplified) to the certain level voltage (650V approx. at 80kHz Timing Mode) and combine with Vertical
dynamic focus waveform through L403, and then feed to PIN 12 of FBT. The dual trace parabola
wave-form plus the DC focus voltage gives sharp and clear display for the whole screen.
The two variable resistors on top of FBT provide focus setting.
The bottom one set Grid 2 voltage of CDT.
.
6- 6-1 Video and CDT drive circuit
7-6-1 VIDEO PREAMPLIFIER (I901)
The signals come from PC through P901 to CDT drive board (PWB-0414). Pin7 (Red), pin5
(Green), pin3 (Blue) feed to pin11, 6, 2 of I901 (video pre-amplifier). Pin 4, 9, 13 of I901 are OSD R,
G, B video signals input that come from OSD IC I903. Pin 12 of I901 is the input for OSD blanking
signal which blank the normal video as OSD video signal is active. Since the video inputs are AC
coupled to I901, so I901 need a clamping signal (at pin 19) from H. sync (IA01 pin 34) to give DC
restoring at I901 outputs. The horizontal blanking signal is provided from PWB-0334. Via P903, it
connects to pin 27 of I901. The video outputs of I901 will have the level darker than the video black
level during the horizontal retrace time. Pin 20 and 21are I
2
C input for MCU (IA01) to control video.
By IIC bus MCU will initiate I901 and control the contrast, brightness, R/G/B gain, and R/G/B bias.
Pin15 is an ABL control input. By detecting the beam current, it will reduce the contrast when the
beam current exceeds the setting point.
6-6-2 VIDEO DRIVE AND OUTPUT CIRCUITS
After being pre-amplified, the R. G. B. video signals set output at pin35, pin32, and pin29 of I901
respectively. The amplitude of the signal at theses output are about 2.8Vp-p. Those video signals
are connected to I904 at pin 7, 6, 9 respectively.
The I904 is video output amplifier to amplify the R, G, and B signals. They offer about 50 Vp-p (H.B)
signals in amplitude to drive the cathodes of CRT.
L912, L942, L972, are the peaking coils, for the compensation of high frequency response.
The DC off-set bias voltage is set up by R902, R903 at pin30 of I901.
The cathode cut-off setting voltages required for white balance are obtained from pin23 (g), pin25
(r), pin26 (b) respectively, these outputs magnitude can control the conduction of transistors Q903,
Q933, Q963 individually. The DC bias of the cathode is then varied to optimize the color
temperature of background.
Brightness control: Pin 2 of IA01 provides Brightness control. It feed through P903 to base of Q910
to vary R, G, and B bias at cathodes simultaneously.
ViewSonic Corporation
18
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