Repair manual

2-66
THEORY OF OPERATION
To maintain a stable CRT beam (image, brightness, and focus) with changing
Digital Palette loads, the Cathode Driver circuit keeps the applied CRT cathode
voltage constant. It does this by maintaining a constant current flow through its
network.
6. Horizontal Deflection
The Horizontal Deflection circuit generates an output signal that powers the CRT
and deflects the CRT beam horizontally across the display screen. At the end of
each horizontal scan, the Vertical Deflection circuit moves the CRT beam
vertically down one scan line. Refer to paragraph 7 in this Section of the Service
Manual for a functional description of the Vertical Deflection circuit.
When the Horizontal Sync Generation circuit on the Logic Controller P.C. Board
generates the horizontal sync pulse (MONHS), it is processed by the horizontal
processor. This signal is applied via connector J9 to the input of the horizontal
processor. The horizontal processor converts the MONHS signal into a flyback
pulse that is used to drive the horizontal output driver and the flyback
transformer. The horizontal output driver deflects the CRT beam horizontally
across the display screen. Refer to paragraph 2 in this Section of the Service
Manual for a functional description of the flyback transformer and how its output
voltages are used to drive the CRT.
a. Beam Deflection
The flyback pulse from the horizontal processor is applied to the input of
an inverter network. The output from each inverter in the network is
coupled to the horizontal output driver. The signal from the inverter
network drives the horizontal output driver between cut-off and saturation.
When the horizontal output driver is turned on, the current in the
HORIZONTAL YOKE increases linearly. This action moves the CRT
beam from the left to the right side of the CRT display screen. At this
time, the horizontal output driver turns off causing a large inductive pulse
to develop in the HORIZONTAL YOKE. As this inductive pulse
develops, it charges a capacitor across the horizontal output driver net
work and the horizontal yoke’s internal capacitance. The current in the
horizontal yoke changes direction, and the CRT beam is returned to the
left side of the CRT display screen.
As the horizontal yoke current decays to approximately zero, the beam
returns to the left side of the display screen. At this time, horizontal
output driver turns on again and the deflection cycle is repeated.