Repair manual

2-65
THEORY OF OPERATION
d. +850 VDC
This voltage drives accelerator grid 6 and focus grid 7 of the CRT.
When the Horizontal Deflection circuit applies a flyback pulse to the
primary winding of the flyback transformer, a voltage output pulse from
tap 8 of the flyback transformer secondary winding is applied to a voltage
rectifier. The voltage rectifier converts the voltage output pulse into a
positive dc bias voltage. This positive dc bias voltage is applied to:
o Accelerator circuit
o Focus circuit
Accelerator circuit provides a dc output voltage that powers accelerator
grid 6 of the CRT.
Focus circuit and focus adjustment potentiometer R36 provides a dc
output voltage that powers focus grid 3 of the CRT. Potentiometer R36
adjusts the CRT beam for its sharpest image.
5. Cathode Driver
The Cathode Driver circuit turns the CRT on and off. It also maintains a constant
drive voltage on the cathode of the CRT when it is turned on. Keeping the
cathode drive voltage constant maintains a stable CRT beam with changing
Digital Palette Loads.
When the Logic Controller P.C. Board applies a digital VIDEO signal to the input
of the cathode driver circuit, the CRT turns on. A high signal turns on the CRT
beam and a low signal turns off the CRT beam. The VIDEO signal comes from
the Logic Controller P.C. Board via connector J9. It is is a two-level digital
signal with no composite sync. Refer to Section D of this Service Manual for a
functional description on how the VIDEO signal is generated.
A positive dc bias voltage from tap 4 of the flyback transformer secondary
winding is applied to the Cathode Driver circuit. This positive dc bias voltage
drives the CRT cathode when the Cathode Driver circuit turns on. The voltage
from the secondary winding of the flyback transformer drives the CRT cathode
more negative, causing an increase in its electron flow. This electron flow is
converted into a stream of electrons (beam). The CRT gun and a high anode
voltage accelerates this beam until it strikes the phosphor of the CRT display
screen.