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8-5
3) Video Bias Circuit
The video signal is amplified maximum 45Vpp in IC721 output, then the signal is done DC bias by video
bias circuit through C729G. This DC voltage varies when cut off adjustment and brightness adjustment.
The video bias circuit consists of fixed voltage circuit construction with op-amp. Q741G is used high-
voltage transistor due to get approx.80V to 170V at cathode black level voltage.
SUB BIAS, G BIAS and G BRT which are output from DAC IC751 are mixed at the op-amp IC742.
(Fig 1.3) Brightness and Cut-off Control
4) ABL Circuit
The ABL circuit consists of the part (Q771,Q772 and peripherals) which detects and amplifies the current
flowing to the CRT anode, and the other parts (IC711) which control the gain of the video signal (R,G and B).
The ABL circuit limits the amplitude of the video signal and serves to keep the maximum average current
flowing to the CRT up to 750
μ
A. When the anode current is less than 750
μ
A, Q772 does not operate. And
the anode current attempts to exceed 750
μ
A, Q772 operates and the voltage of IC711 pin 15 drops, the
gain of IC711 drops. After saturation of IC711 pin 15, D713 becomes ON, then IC711 pin 30 drops. This is
the voltage that decides the black level of IC711 video output. Therefore back raster becomes dark.
(Fig 1.4) ABL Circuit
Cathod
R74GG
IC801
CPU
C729G
R742G
D741G
I
2
C BUS
-
+
+12V
C744G
R747G
R748G
Q741G
R749G
Auto Adjust
Blanking Circuit
R74AG
R744G R743G
R746G
+12V
IC751
M62393FP
DAC
15
13
14
G_BRT
G_BIAS
SUB_BIAS
C743G
R745G
R741G
+220V
IC721
VPS16
14
R74FG
R74EG
C741G
TP-G
CRT
BLANKING
PULSE
C7E4
G1
R904
R903
C902
C901
L901G
R901G
R902G
SG901G
SG902
HIGH VOLTAGE
PROTECTOR
R5U3
R5U2
R5U1
+80V
T5R1
F.B.T
8
+15V
IC711
M52742ASP
15
30
23
ABL IN
GAIN
ADJUST
MAIN
BRIGHT
-NESS
+12V
+12V
C5F2
ZD5F1
Q5F2
R771
D771
C771
R772
R773
R774
Q772
Q771
R775
D713
R713
R714
D712
D711
SW/HV PWB
VIDEO PWB