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-90 V, the adjustment range is adequate to change a 45 V Control Gate dark threshold signal
into the desired cutoff voltage at the CRT grid.
To turn the CRT on, the grid voltage must be raised more positive toward the cathode from
the cutoff voltage. The Control Gate signal, the analog of the desired CRT brightness,
comes from the AlA XYZ Driver Amplifier board. The Control Gate signal has the correct
relative-voltage amplitude to drive the grid but it has a 0 V to 110 V signal range and the
grid is at -2400 V co (V cutoff). The 45 V dark threshold voltage can be translated to the
CRT cutoff voltage by adjusting the INTENSITY LIMIT pot on the AlA board. In order for
the beam to be turned on to the desired intensity, the grid voltage must be more positive.
The CRT has a maximum drive voltage requirement of 55 V. Drive voltage is defined as the
voltage, more positive than cutoff, that is required to bring the CRT to maximum allowable
intensity. The AlA Z Axis Driver Amplifier gain is adjustable, giving approximately 60
V above the 45 V dark threshold (105 V total). This signal is coupled onto the grid signal
two ways. If the instantaneous level of the control gate signal is raised, the peak-to-peak ac
voltage at TP3 is reduced by an equal amount. This raises the grid voltage by the same
amount that the ac voltage is reduced. The result is an Offset dc coupling of the Control Gate
signal onto the grid. However, the response time is fairly slow, requiring several cycles of the
30 kHz ac signal to establish the dc level. This problem is solved by C6, which capacitively
couples the Control Gate signal directly onto the grid signal. The result of these two coupling
methods is that the fast edges are capacitively coupled and then the dc level is maintained
with the Offset dc coupling. This circuit accurately follows the 40 ns rise- and fall-times of the
Control Gate signal and also maintains dc level for the longest direct video vectors (10 ms).
Neon glow lamps, DSl and DS2, act to limit the grid-to-cathode voltage to approximately 150
V. This prevents arcing inside the CRT if a circuit failure should occur and on power-down
discharges of C5 and C6 as the cathode voltage drops. There are eleven spark gaps, El
through Ell, that are designed in as part the PC board traces. These spark gaps are used to
protect various low level signals against leakage or shorts in any of the capacitors that have
high voltage across them.
Focus Grid Level Shift @
The focus grid in the CRT needs to be approximately -1500 V dc to properly focus the beam.
It is necessary to modify this voltage to correct for beam intensity and beam position.
The basic operation of the Focus Grid Level Shift is the same as Control Gate Level Shift @
with a few exceptions. The Focus Gate signal works just like the Control Gate signal, it uses
the same ac and "Offset dc" coupling technique to couple the focus amplifier signal onto the
focus grid. The basic level of the focus grid id determined by a resistive divider formed by R2,
R14, and R15. R14 allows for the initial adjustment of the low brightness center screen focus
voltage. This is the voltage that all corrections are added to. Instead of the Intensity Limit
signal, the Focus Grid Level Shift circuit has a fixed dc level clamp voltage of 82 V set up by
VR2.
This function block also serves as an interconnect for the +18 V dc and the DRIVE signal for
the High Voltage Module from the AlA High Voltage board.
4 AlA
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