Video Bias Circuit; Abl Circuit - NEC MultiSync FP1370 Service Manual

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.
The voltage from DAC IC751 input
IC801
CPU
2
I C BUS
G_BRT
15
IC751
G_BIAS
14
M62393FP
SUB_BIAS
5
+12V

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 750μA. When the anode current is less than 750μA, Q772 does not operate. And
the anode current attempts to exceed 750μA, 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.
SW/HV PWB
+80V
T5R1
8
F.B.T
HIGH VOLTAGE
+15V
C5F2
IC721
14
R745G C743G
R74AG R744G R743G
C744G
R749G
-
R74FG
+
R74EG
R746G
(Fig 1.3) Brightness and Cut-off Control
R5U1
R5U2
R5U3
Q5F2
ZD5F1
TP-G
C729G
R741G
D741G
+12V
Q741G
R747G
R748G
VIDEO PWB
C771 D771
+12V
R772
R771
Q771
R773
(Fig 1.4) ABL Circuit
8-5
Cathod
R901G
R902G
L901G
+220V
R742G
SG901G
R74GG
SG902
C741G
C7E4 R904 R903
BLANKING
C902
Auto Adjust
IC711
15
ABL IN M52742ASP
VIDEO
GAIN
PRE AMP
23
R775
D713
Q772
R774
CRT
G1
C901
+12V
R713
30
MAIN
BRIGHT
R714
-NESS
D711
D712