Tektronix 7623, Instruction Manual

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Circuit Description —7623/R7623 Service
a larger collector current. This results in a larger induced
voltage in the secondary of T1225 to produce a more
negative level at the —1475 V test point to correct the
original error. In a similar manner, the circuit compensates
for output changes in a negative direction. Since the
amplitude of the voltage induced into the secondary of
T1225 also determines the output level of the positive
High-Voltage Supply and the Control-Grid Supply, the total
high-voltage output is regulated by sampling the output of
the negative High-Voltage Supply.
High-Voltage Supplies
High-voltage transformer T1225 has two output
windings. One winding provides filament voltage for the
cathode-ray tube. The other winding provides the negative
and positive accelerating potential for the CRT and the bias
voltage for the control grid. All of these voltages are
regulated by the High-Voltage Regulator stage to maintain a
constant output voltage as previously described.
Positive accelerating potential for the CRT anode is
supplied by the voltage doubler. The applied voltage from
the secondary of T1225 is about 3.5 kilovolts peak-to-peak.
This results in an output voltage of about +7 kilovolts at
the CRT anode. The negative accelerating potential for the
CRT cathode is also obtained from this same secondary
winding. Half-wave rectifier CR1232 provides an output
voltage of about —1.475 kilovolts which is connected to the
CRT cathode through R1234. The cathode and filament are
connected together through R1275 to prevent cathode-to-
filament breakdown due to a large difference in potential
between these CRT elements. A sample of the negative
accelerating voltage is connected to the High-Voltage
Regulator to maintain a regulated high-voltage output.
The network consisting of diodes
CR1269-CR 1268-CR1270-CR1264-VR 1264 provides the
negative voltage for the control grid of the CRT. Output
level of this supply is set by CRT Grid Bias adjustment
R1261. Approximately 600 volts peak-to-peak from the
secondary of T1225 is connected to the Control-Grid
Supply through C1266 and R1266. Diodes CR1268 and
CR1264 clip this signal to determine the operating level at
the control grid. CR1268 limits the negative excursion of
the signal; quiescently when the CRT is blanked, the anode
of CR1268 is set at about +15 volts by the Z-Axis
Amplifier stage. The positive clipping level at the cathode
of CR1264 is set by CRT Grid Bias adjustment R1261.
R1261 is adjusted to bias the control grid of the CRT just
enough negative so the trace is blanked between sweeps.
Under normal conditions, this biases the control grid about
80 volts more negative than the cathode.
The negative level at the CRT cathode is connected to
the cathode of CR1270. This level is held constant by the
High-Voltage Regulator as described previously. The
clipped voltage developed by diodes CR1264 and CR1268
is peak to peak rectified by diodes CR1269 and CR1270
and super-imposed on this negative voltage to result in a
level at the grid of the CRT which is more negative than the
CRT cathode level. C1269 acts as a filter to provide a
constant voltage output level. The unblanking gate level
developed by the Z-Axis Amplifier stage is applied to the
anode of CR1268 through R1157. The fast rising and
falling portions of this signal are coupled directly to the
output through C1269. The overall effect of the unblanking
gate is to further clip the negative excursions thereby
reducing the voltage difference between grid and cathode of
the
CRT. This allows the cathode current of the CRT to
pass to the anode so the display can be viewed.
CRT Control Circuits
The focus of the display is determined by the FOCUS
control R1045. This control and the Auto Focus amplifier
maintains a well-defined display for fast changes in the
intensity of the display. The network consisting of
CR1255, CR1254, CR 1253, CR 1258, and VR1258 provide
the negative voltage for the focus grid of the CR F.
Approximately 600 volts peak to peak from the secondary
of F1225 is connected to the focus grid supply through
C1257 and R1257. The positive clipping level at the anode
of CR1258 is set by the FOCUS control setting. This
determines the operating level at the focus grid. Under
normal operating conditions the voltage applied to the
focus grid is more positive (less negative) than the control
grid or the cathode of the CRT. The signal developed by
the Auto Focus amplifier is coupled to the focus grid by
C1254. When there is a sudden change in intensity levels
the focus grid level will change to maintain a well-defined
display. Astigmatism adjustment R1193, which is used in
conjunction with the FOCUS control to obtain a well
defined display, varies the positive level on the astigmatism
grid. Geometry adjustment R1184 varies the positive level
on the horizontal deflection-plate shield to control the
overall geometry of the display.
Two adjustments control the trace alignment by varying
the magnetic field induced by coils around the CRT. Y-Axis
Alignment R1190 controls the current through L1098,
which affects the CRT beam after vertical deflection, but
before horizontal deflection. Therefore, it affects only the
vertical (Y) components of the display. Beam Rotation
adjustment R1181 controls the current through L1099and
affects both the vertical and horizontal rotation of the
display.
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