Circuit Description—Type 422 AC-DC
Drive
signal
from
Power
Supply
Positive High-
Voltage
Multiplier
D810-D816
Negative High-
Voltage
Multiplier
D821-D823
High-Voltage
Regulator
V829
Unblanking
gate
from
Sweep
Generator
circuit
Chopped
blanking
from
Vertical
Switching
circuit
External
horizontal
unblanking
from
Sweep
Generator
circuit
Fig. 3-11. CRT Circuit detailed block diagram.
in parallel. Terminals 1 and 4 are connected together and
terminals 3 and 6 are connected together. The output across
the secondary of T801 at terminals 7 and 8 is about 700
volts peak. This output becomes the source voltage for two
voltage-multiplier circuits to produce the positive and
negative accelerating potential for the CRT.
Diodes D810-D811-D812-D813-D814-D815-D816 along
w ith capacitors C810-C811-C812-C813-C814-C815-C816
form a voltage pentupler multiplier8 network to provide a
positive accelerating potential of about +4900 volts at the
CRT anode. Ground return for this supply is through the
resistive helix inside the CRT to pin 5, and then to ground
through R844. Diodes D821-D822-D823 along with capaci
tors C821-C822-C823 form a voltage tripler multiplier net
work to provide the negative accelerating potential. The
output of the Negative High-Voltage Multiplier is regulated
by the High-Voltage Regulator V829 and connected to the
control grid of the CRT through D839-R839. The parallel
network of D839-R839 compensates for the CRT cathode
potential changes as the CRT cathode current is changed.
This network in series with the internal impedance of V829
has a voltage-current characteristic equal and opposite to
that of the CRT grid-cathode structure. Therefore, the over
all accelerating potential of the CRT remains constant.
8Tektronix Circuit Concepts booklet, “Power Supply Circuits”,
Tektronix Part No. 062-0888-00, pp. 14-16.
Secondary terminals 9 and 10 of T801 provide filament
voltage for the CRT. The filament voltage can be obtained
from the high-voltage transformer since the CRT has a very
low filament-current drain. Diodes D849 A-D provide
rectified filament voltage and C849 filters the filament
voltage to hold it constant at about 6.3 volts. The CRT
cathode and the filament are connected together through
R846. This raises the potential on the CRT filament to
about the same level as the cathode to prevent
cathode-to-filament breakdown. Neon bulb B846 provides
further protection, since it ignites if the voltage difference
between the cathode and filament exceeds about 65 volts.
CRT Control Circuits
The regulated negative accelerating potential is applied
across the divider R831-R832-R833-R834-R837-R838. The
INTENSITY control R837 determines the current through
this divider to set the leyel at the cathode of the CRT. The
cathode is more positive (less negative) than the control
grid because of the voltage drop across R838 and the
INTENSITY control R837. As the level at the CRT cathode
is changed with the INTENSITY control, the beam current
and therefore the display brightness is changed also. The
FOCUS control R833 is also part of this divider and it sets
the level on the focus grid of the CRT. The added voltage
drop across R884 and the FOCUS control R833 sets the
focus grid at a more positive level (less negative) than either
the control grid or the cathode. The ASTIGMATISM con-
3-19
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