Technicolor - Thomson ICC19, Principles And Maintenance

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AUTOMATIC CUTOFF CONTROL
Automatic cutoff control is performed for Lines 21 through 24 and 334
through 337. To compensate for the tube leakage current, the RGB outputs
are erased during Lines 21 and 334. The leakage current measurement
determines a compensation current within IV001 that creates a voltage of
1.5 V at the terminals of RV092 (Pin 43 of IV001). This compensation
current is memorised for an entire frame, and is taken into account for the
automatic control of the tube cutoffs. This compensation current is added to
the RGB gun cutoff measurement currents.
During Lines 22 and 335, the blue channel is in operation. The blue gun
measurement current plus the compensation current produce a voltage at the
terminals of RV092. This voltage is compared to an internal 3-V reference,
and then determines the black level of the blue signal.
During Lines 23 and 336, the green channel is in operation. Its automatic
control is identical to that of the blue channel.
During Lines 24 and 337, the red channel is in operation. Its automatic control
is identical to that of the blue channel.
Transistor TB18 and its associated components limit the cutoff measurement
voltage to approximately 4.2 V.
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