HP 9020 Service Manual page 66

2-40
Theory of Operation
Video Board
The video board receives red, green, and blue video streams from the graphics/digital video board.
It contains three identical video amps which amplify each video stream to the voltage level required
to drive a CRT color gun.
Adjustments on the video board include black level and white level for each of the video streams:
red, green, and blue.
Sweep Board
The sweep board receives horizontal and vertical sync signals and vertical blanking from the
graphics/digital video board, and produces the current required for horizontal and vertical deflec-
tion of the electron beam.
The sweep board generates the voltages required by the CRT control grids and for accelerating and
focusing the electron beam. A flyback transformer is utilized for this function.
Potentiometers on the sweep board control the focus, intensity, vertical centering and height, and
horizontal centering of the CRT display. A coil on the sweep board controls the horizontal width.
CRTNoke Assembly
The CRT/yoke assembly displays the video signals. The resulting display is 25 lines by 80 charac-
ters per line. The graphics is 512 dots horizontal by 390 dots vertical.
The CRT/yoke assembly has adjusting magnets for pincushioning. They are set at the factory and
normally do not need adjusting.
Display Power Supply
The function of the power supply is to change 100-240V ac into filtered, regulated dc levels of
+
5V,
+
12V,
+
75V, and - 12V, and to provide overcurrent and overvoltage protection.
The display has an ac fan which runs off the power transformer primary.
The display power supply remains turned off until it senses the
+
12V supply line in the base. It
turns on and remains on until the base
+
12V supply drops.