Mitsubishi Electric 1786FD2 Service Manual page 64

MEA 1786FD2 CRT Monitor Service Guide
30. Video CKT
VIDEO C.K.T. BLOCK DIAGRAM ¡ G
R OSD
G OSD
(FROM
IC 1901 )
B OSD
CUT
R
G
VIDEO
B
CONTRAST
(FROM IC801)
BEAN CURRENT
(FROM F.B.T.)
H-PULSE
(FROM F.B.T.)
31. OSD Preamp CKT:
(a) AS shown in the block diagram:
The R/G/B signals will generate an enough amplitude of Vpp to show up on the
CRT screen after the amplification of two amplifiers. The first one, preamp CKT,
process the signal and mix up the OSD, and the second one does the power
amplification.
(b) OSD preamp IC101, LM1279, will output the R.G.B signals seperately. The R.G.B
driver will control the gain of these three guns individually to approach the white
balance of CRT.
(c) The signal H-Blank is to let the output of LM1279 down to 0.2V while non-display
duration. Then the CRT driver CKT will generate a level higher than Black Level. (i.e.
SYNC TIP), therefore the video signal will be blanked in order to prevent the fold over
to occure while adjusting H-phase. Besides, the SYNC TIP is used for
the DC Restoration of cascode CKT.
(d) LM1279 is equipped with OSD mixer. when signal CUT is Low, the output of
LM1279 is video signal when signal CUT goes high, the output will be OSD signal.
32. CRT DRIVER CKT:
DRIVE
(FROM IC1901)
B
G
R
PREAMP
CKT
LM1279
IC101
ABL CKT
Q322,323
H-BLANK
CKT
Q320
Circuit Operation Theory
BIAS
(FROM IC 1901)
B
R G
DC RESTORE
Q121,122
Q141,142
Q161,162
CRT DRIVER
CKT
LM2439
IC102
BRIGHTNESS CKT: Q203
CHANGE MODE BLANK CKT:Q202
V BLANK CKT: Q202
SPOT KILLER CKT: Q202
R
G
B
G1
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