Tektronix 422, Instruction Manual

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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 o f T801 at terminals 7 and 8 is about 700
volts peak. This output becomes the source voltage fo r tw o
voltage-multiplier circuits to produce the positive and
negative accelerating potential fo r 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 m ultiplier8 network to provide a
positive accelerating potential o f about +4900 volts at the
CRT anode. Ground return fo r this supply is through the
resistive helix inside the CRT to pin 5, and then to ground
through R844. Diodes D821-D822-D823 along w ith capaci­
tors C821-C822-C823 form a voltage trip le r m ultiplier net­
work to provide the negative accelerating potential. The
o utput o f the Negative High-Voltage M ultiplier is regulated
by the High-Voltage Regulator V829 and connected to the
control grid o f the CRT through D839-R839. The parallel
network o f D839-R839 compensates fo r the CRT cathode
potential changes as the CRT cathode current is changed.
This network in series w ith the internal impedance o f V829
has a voltage-current characteristic equal and opposite to
that o f the CRT grid-cathode structure. Therefore, the over­
all accelerating potential o f 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 filam ent
voltage fo r the CRT. The filam ent 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 filam ent voltage and C849 filters the filam ent
voltage to hold it constant at about 6.3 volts. The CRT
cathode and the filam ent are connected together through
R846. This raises the potential on the CRT filam ent 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 filam ent 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 o f the CRT. The
cathode is more positive (less negative) than the control
grid because o f 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 o f this divider and it sets
the level on the focus grid o f 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-
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