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Chapter 2—Functional Descriptions
2-56 Model 330. 340SC, and 370SC Service Manual
Beam Current Sense
The cathode current is sensed at the output of the video amplifier, then filtered
and amplified so that the output is a 1mV/1uA signal averaged over several
horizontal lines (the number of lines depending on the horizontal frequency). This
current sense signal is then sent to the VPB as the signal BEAM.
DC Restore
The DC Restore function is accomplished on the Video Amplifier Board when
commanded by the CLAMP signal from the VPB. When the CLAMP signal
arrives, the video signal has the offset values (Brightness and Threshold) removed
so that they will not be changed by the DC Restore circuit. The CLAMP signal
causes the output voltage to be sampled and compared to a reference. The
difference is sent to the input of the video amplifier to set the new black level for
the duration of the next horizontal line.
Arc Protection
The Arc Protection Circuit functions to drain off any energy that arrives at the
Video Output Amplifier board due to arcs on the cathode or grids. The arc energy
is suppressed by sending it directly to the HVPS chassis via the Arc Ground
terminal on the
board.
Blanking
The Blanking circuit operates by using the BLANKING signal from the VPB. The
BLANKING signal causes an amplifier to drive an AC coupled pulldown of
GRID1. G1 voltage is normally -81V but will be pulled down to -111V during the
blanking interval. The cathode voltage is unaffected by blanking.
Enable Circuit
The Enable circuit is controlled by the /ENABLE signal from the VPB. The
/ENABLE signal, when pulled high, causes both GRID1 and GRID2 to be pulled
low: G1 is pulled from nominal -81V to the disabled value of -200V, G2 is pulled
from its usual value of between +800V to +1200V to its disabled value of +15V.
Focus
The FOCUS voltage is a pass through from the HVPS to the CRT. It is not in any
way modified by the VOB.
Filament Supply
For the CRT Filament supply, 6.3Vdc from the System PS is simply filtered and
sent to the CRT.