Tektronix 7834

Theory of
Operation-7834
The
CRT circuit provides the high voltage and control cir-
cuits necessary for operation of the crt (cathode-ray tube) .
This circuit also includes the Z-Axis Amplifier
and the Auto-
Focus Amplifier (diagram 17, at the rear of this
manual) .
Figure 3-45 shows a detailed block diagram of the CRT
circuit
. A schematic of this circuit is shown on diagram 16,
at the rear of
this manual .
Power Transformer
Power
Transformer T2010 provides pre-regulated voltages
for the
high-voltage supplies, and 6 .3 volts ac for the crt
heater
. The crt heater is elevated to the cathode potential
through 132074 . The high-voltage winding of T2010
provides
a two-kilovolt peak square-wave voltage to
the Anode
Voltage Multiplier and
CRT Cathode Supply . A 600 volt
winding supplies ac to
the Control-Grid DC Restorer stage
and is also rectified to
supply +600 volts dc . Taps on this
winding provide
the +130 volts, through CR2118 and
CR2119
in full-wave configuration, and a means of current
sensing through resistor 132121 .
Anode Voltage
Multiplier
Positive
accelerating potential for the crt anode is supplied
by the
four-times voltage multiplier contained within
U2012 . The applied
voltage to the input of U2012 from the
high-voltage secondary
of T2010 is about two kilovolts
peak . This results
in
an
output voltage of about +8 kilo-
volts at
the crt anode . The internal resistance of this supply
is about
15 megohms and subject to meter loading when
measured .
CRT Cathode Supply
in the full scan mode (front-panel
REDUCED SCAN push
button out) the crt cathode supply voltage
is about -2 kilo-
volts ; in the reduced scan mode, about --4
kilovolts . The
negative potential for the crt cathode
is generated by a half-
wave rectifier
(CR2012) for the full scan mode and modified
to a
doubler in the Reduced Scan mode by activating
K2014
which connects CR2014 in the circuit . Diode
CR2023 provides the return path for the rectified current
in the full scan mode ; the reduced scan mode, and
the X4
Multiplier require an ac return path
which is provided by
C2021, CR2021, CR2022 and VR2021 .
Cathode-Supply Regulator
The Cathode-Supply Regulator maintains the potential on
the crt cathode and actively reduces the ac ripple through
C2034 . A sample
of
the output of the CRT Cathode Supply
is connected to amplifier U2224 through the resistor
divider network 132218 . High-frequency changes from the
CRT Cathode Supply are coupled through C2282 and
132282 . A tetrode voltage, which is maintained at about
+2 kilovolts above
cathode potential in either full or re-
duced scan, is also obtained from the high-voltage secondry .
Filtering for this voltage is accomplished by 132015 and
C2014
. Resistor 132014 limits current under fault condi-
3- 62
Z AXIS & CRT
tions . Diode CR2016 clamps
the tetrode voltage a few volts
above ground in the
Full
Scan
mode . Components C2016
and 132017 are an isolation
filter .
The output of U2224 drives 02132 . The collector of 02132
is in
the
do
current return path of the Cathode Supply . Any
change in collector voltage directly changes the cathode
voltage . Diodes VR2133 and CR2133 prevent the voltage
on the collector of 02132 from exceeding safe levels .
The -1955 V (full scan cathode voltage) adjustment
132285
sets the crt cathode voltage (-1955 V in Full Scan
mode) .
The Reduced Scan Horiz Gain adjustment
132220 sets crt
horizontal sensitivity by
setting the crt cathode voltage
when in Reduced Scan
mode .
Z-Axis Amplifier
The Z-Axis Amplifier controls crt intensity
by varying grid
drive . High speed changes are coupled through
C2289 . Slow
changes and do levels are coupled through the Control
Grid
DC Restorer . The Logic circuit and
the Readout System
provide input signals to the Z-Axis
Amplifier .
Transistors 02206 and 02216 are
common-base amplifiers
to establish a low input impedance for the
Z-Axis Ampli-
fier . Transistors 02236, 02242, 02254, 02264 and 02
.274
form a current to voltage amplifier with feedback resistors
132233-132248 . The Z-Axis Amplifier is compensated by
132235 and C2235 for optimum square corner .
Control Grid DC Restorer
The Control Grid DC Restorer couples do and low
frequency
components of the Z-Axis Amplifier
signal
to
the crt
control grid, where difference in potential
prohibits direct
coupling . The do restorer is actually a
cathode-referenced
bias supply for the crt control grid .
Quiescently, its output
voltage
is more negative than the cathode by an amount
set by the Crt Bias (grid) adjustment 132135 .
The
Control
Grid DC Restorer is current driven from
the
+600 volt winding of T2010 through C2041-132041-112042
.
This approximately 25-kilohertz drive signal is connected to
the junction of CR2045-CR2055-C2052 . Diodes CR2045
and CR2055
limit the peak-to-peak amplitude of the drive
to
the difference between their forward-bias levels . The crt
bias adjustment 132135 and the output level of the Z-Axis
Amplifier set the forward-bias levels of CR2045 and
CR2055 respectively . Capacitor C2052 couples the limited-
amplitude drive
to
the junction of CR2052-CR2054 . During
positive half cycles of the drive, CR2054 charges the con-
trol grid side of C2055 to a level equal to the peak-to-peak
value of the drive signal . The resulting control-grid voltage
is more negative than the cathode by an amount equal to
the difference between the CRT Bias adjustment setting
and the Z-Axis Amplifier output level .