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7277e Monitor Service Guide
signal when signal CUT goes high, the output will be OSD signal.
2.2.4.2 CRT DRIVER CKT
Output stage adopts CRT driver LM2407 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.
2.2.4.3 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.
2.2.4.4 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.
2.2.4.5 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.
2.2.4.6 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 set 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, The 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 CUP 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 negative voltage higher than CUP OFF to the G1 to beam this
is to protect the CRT.
2.3 Microcontroller Circuit Operation Theory
2.3.1 Introduction
This model, 7277e, will support powerful OSD function to help end user fine adjustment. The
Microcontroller circuit of the 7277e can determine what mode it is by detecting the frequency of horizontal
and vertical synchronous and the polarity of horizontal synchronous, and provide DC voltages to control
the picture and save the adjusted value into the EEPROM by using the OSD, "On Screen Display control",
that means the user can get any information of the picture display or adjust it and save the status values
into the EEPROM by choosing and pressing the proper key according to the indication of the OSD. In
addition, user can press i-key to do auto-calibration.
