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(Serial Number Prefixes: 3004A and Above)
The High Voltage Regulator PC board couples the grid drive and focus control signals to their
dc bias voltages to drive the CRT. It also provides mounting and interconnect for the AlASAl
High Voltage Module to AlVl CRT and the AlA High Voltage Power Supply drive and
control circuitry.
Control Grid Level Shift @
The filament voltage from the AlASAl High Voltage Module is connected to the CRT
through this function block. The filament voltage is adjusted by adjusting the drive to the HV
module so as to get 6.0 V 20.05 V ac. TPlA and TPlB are provided to measure the filament
voltage.
Warning
High Voltage. Test points TPlA and TPlB are at -2400 V dc.
The cathode voltage is connected to one of the filament leads inside the AlA3Al HV Module.
Cl provides primary filtering of the cathode voltage while Rl and C3 provide a secondary
stage of filtering. The filtered voltage is fed to the CRT through current sensing resistor, R26.
The High Voltage Feedback is fed to the AlA High Voltage Power Supply regulator circuitry
through R3 and C2.
The control grid modulates the beam current and thus the brightness of the CRT. The grid
cutoff voltage of the CRT, the voltage that will turn the CRT off completely, is between -60
V and -90 V relative to the cathode. This dc offset on the grid is generated by capacitively
coupling an ac signal onto the dc bias voltage and rectifying it into dc signal. The amplitude
of the ac voltage is clamped to control the amount of dc signal that is added to the cathode
voltage. The source of the ac voltage is a winding in the High Voltage Module that generates
about 400 V p-p at 30 kHz. One side of this winding is grounded at J2-2 (COM). The other
side connects to J2-3, HOT. It is capacitively coupled through Cl3 and current limited by R9.
The amplitude of this signal is clamped between two dc levels, Intensity Limit and Control
Gate.
Intensity Limit is an adjustable dc voltage (56 to 120 V dc) from AlA XYZ Driver Amplifier.
Control Gate is the output of the Z Axis (brightness) amplifier on AlA2. The Control Gate
signal is set to a dc level that represents the dark threshold level, the cutoff voltage. The
currents are small enough that the voltage drops across R12 and R13 can be ignored. CR4
is biased so that if the voltage at TP3 tries to drop below 45 V, CR4 will conduct and hold
the voltage at 45 V (AlA XYZ Driver Amplifier is designed so that the dark threshold is
approximately 45 V). The anode of CR4 is at a voltage between 56 V and 120 V. VRl adds
34.8 volts to the anode voltage of CR4 so that CR3 will conduct at somewhere between 90 V
and 155 V. This determines the upper value of the ac voltage and sets the adjustable range of
the peak-to-peak voltage of TP3 at between 45 V to 110 V. This ac voltage is coupled through
C4 to the node between CR1 and CR2. As the ac voltage swings negative, current flows
through R8 and CR2 to charge C5 to a negative voltage; as the ac voltage swings positive,
CR1 is turned on and the node is held at the cathode voltage. This results in a process
that charges C5 to a dc level more negative than the cathode voltage by the peak-to-peak
amplitude of the adjustable ac voltage. Since the cutoff requirement is between -60 V and
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