ViewSonic A91f+-1, Service Manual, Page: 25 / 83

4
﹒
Video
﹒
4.1 Video amplifier
（
IC601
）﹒
The video amplifier module is composed of three amplifiers for Red
﹐
Green
﹐
Blue channel
﹒
The video input signal is fed to the video preamplifier IC601
（
KA2500
）
pin8 Red
﹐
pin5 Green
﹐
pin10 Blue
﹐
through AC coupling capacitor
﹐
C601
﹐
C601
﹐
C603
﹒
The clamping pulse comes from IC301 pin33
﹒
IC603
output amplifier for B
﹑
R
﹑
G channel respectively
﹒
4.2 On Screen Display
（
OSD
） （
IC602
）﹒
IC602
（
NT6827-0047
）
is a on screen display generator, pin5 for H-sync input
﹐
pin10 for V- sync input
﹒
The IC602 is controlled by IC301 via SCL
﹐
SDA bus IC602
（
pin7
﹐
pin8
） ﹒
The on screen display signal is output from pin15
（
r
）﹐
pin14
（
g
）﹐
pin13
（
b
）
and OSD BLK to IC601 pin2
﹐
pin1
﹐
pin3
﹐
pin4
﹒
4.3 Auto Beam Limit CKT
（
ABL CKT
） ﹒
When beam current pass through VR4402 over 500uA
﹐
the voltage build at base of Q4406 will be low enough
to turn on Q4406
﹐
then the voltage of pin12 of IC601 will be pulled down accordingly to reduce the video
preamplifier gain output
﹒
4.4
Brightness Control
﹒
Brightness is controlled by varying the DC voltage of G1 with the IC301 PIN1
（
PWM
）﹒
`
4.5
Blanking CKT
﹒
IC501
（
TDA8172
）
pin3 vertical blanking pulse are fed to the base of Q501
﹐
The blanking pulse O/P is coupled
to G1 by C507
﹒
Horizontal blanking pulse are fed to IC601 pin19 and let video O/P amp cut off during the period of horizontal
retrace
﹐
while mode change
﹐
IC301 pin19 will pull high to turn off
﹒
The G1 voltage will down to –180V then
CRT will cut off the video output
﹒
5
﹒
Tilt
﹒
IC301 pin2 PWM to tilt circuit
﹐
Q211 and Q213 control flow of current of Rotation coil
﹐
when the base of C215
increase from 0 to 12V
﹐
As Q211 will turn on gradually
﹐
the current starts from 14V to 6.3V via Rotate coil
﹒
When C215decrease from 12V to 0V
﹐
the current will change from 6.3V to GND
﹒
22
ViewSonic Corporation
Co
nfidential - Do Not Copy A91f+-1