Mitsubishi Electric 1786FD2 Service Manual page 65

MEA 1786FD2 CRT Monitor Service Guide
Output stage adopts CRT driver LM2439 to amplify the signal which has been
recessed by LM1279 to a enough amplitude of Vpp, then display on the CRT. The IC
contains three high input impedance, wide band amplifiers which directly drive the
RGB cathodes of a CRT. The gain of each channel is internally set at -15 and can
drive CRT capacitive loads as well as resistive loads presented by other application
limited only by the package's power dissipation.
33. DC Restore CKT:
(a) The video signal amplified by the output stage is coupled to CRT by way of AC
coupling. SO DC restoration CKT is needed to do the white balance adjustment.
(b) This DC restoration circuit adopts SYNC TIP CLAMP, In the duration of
SYNC TIP the capacitor charges, and the capacitor discharge in the other time. The
Black Level is kept to the level of DC restoration setted by UC.
34. ABL CKT: (Auto Brightness Limit)
ABL is a protection circuit. When the anode current goes higher than the setting
value of ABL circuit . ABL will pull down the voltage of contrast to limit the anode
current. This is helpful to protect CRT.
35. H-BLANK CKT:
Affair the collect pulse comes from FBT being shaped and inverted, it will be sent
to preamp CKT and used as the H-Blank.
36. Brightness, V-blank, change mode blank, spotkiller CKT:
(a) About the cut off voltage , while the voltage, cathode to G1 , over the cut off ,
voltage, the picture will disappear, If cut off voltage off the CRT G Gru is
setted at 110V and the black level of cathode is 60v, the picture wont show,
the signals higher the black level once the G1 voltage is lower than-50v.
(b) As described above, we may using the voltage control G1 as the brightness
control. Generally the G1 control range is about 10~15V if the raster
brightness is form 0 to 1.5 ft-L.
(c) Similarly, we may overlap a negative pulse of vertical duration on the G1
voltage to prevent the vertical retrace line from showing on the picture , This
is to keep the voltage cathode to G1 over the cut off voltage during the
period of vertical retrace.
(d) In order to avoid the picture occur transiently while change mode, pull
down the G1 voltage and let the voltage cathode to G1 over CUT OFF voltage.
This will make the picture blanking.
(e) While monitor turned off , the discharge speed of high voltage circuit is slow
since there is no deflection scan act on the electronic beam, a spot which will
destroy the phosphor of CRT. So the SPOT KILLER circuit will generate a
Circuit Operation Theory
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