Tektronix 7834

Theory of Operation--7834
Focus Supply
The Focus Supply provides a regulated do voltage to the crt
focus and astigmatism elements . The supply voltage is pro-
duced by the half-wave rectifier CR2155 . In the Reduced
Scan mode the contacts of K2155 close to connect CR2152
in the circuit to form a half-wave voltage doubler .
Regulation of the Focus Supply is accomplished by 02156,
02160 and 02162 . Normally the voltage at the base of
02162 is +15 volts . Any error in this voltage changes the
voltage at the collector of 02156 to correct the Focus Sup-
ply voltage . The Focus adjustment 82165
determines the
quiescent operating voltage of the Focus Supply
for
a
well-
defined crt display . In the Full Scan mode, feedback
from
the focus grid electrode is provided by two parallel
resistor
networks 82166-82167 and 82141-82142
. In the Reduced
Scan mode 02140 turns on,
routing the current flowing
through 82141 and
82142 through 82140 to ground . This
action doubles the effective feedback
resistance, increasing
focus grid voltage . The
Reduced Scan Focus adjustment
82140 is operational
only in the Reduced Scan mode . All
of the load
current for the Focus Supply flows through
02156
and 82153 . If the load current exceeds about 8
milliamperes the voltage across 82153 causes 02152 to
conduct . The additional current in the I
Sense line will
cause the low-voltage supply to
turn off, preventing com-
ponent damage .
CRT Control Circuits
The ASTIG adjustment, 82195 (used in conjunction with
the FOCUS control to obtain a well-defined display) varies
the potential on the astigmatism element of the crt . The
range of adjustment of 82195 is determined by the current
path
through 02188 and 02182 . In the Reduced Scan
mode the current path
is through 02178 and 02172, acti-
vating the
Reduced Scan adjustment for optimizing the
display
.
The Stigmator adjustment, 82110, affects the focus of the
crt beam . The Vert Shield adjustment, 82105, sets the
voltage level on the vertical shield, which provides isolation
for the vertical deflection plates .
The Reduced Scan Vert Gain adjustment 82175 determines
the vertical sensitivity of the crt when the crt is operated
in
the Reduced Scan mode .
Two
coils control the trace alignment by varying the mag-
netic field around the crt . The Y-Axis alignment coil L2201,
affects the crt beam after vertical deflection but before
horizontal deflection . - Therefore, only the vertical
(Y)
com-
ponents of the display is affected . The
TRACE ROTATION
coil, L2200 affects both the vertical and horizontal
rotation
of the crt beam .
3- 64
The Auto Focus Amplifier provides control voltages
to
maintain optimum focus of the crt display with
various
settings of the front-panel A INTENSITY,
B INTENSITY
and READOUT - INTENSITY controls .
Figure 3-46 shows a
detailed
block diagram of the Auto Focus
Amplifier . A
schematic for the Auto Focus Amplifier is
given on diagram
17 at the rear of the manual .
Data
Switch
AUTO FOCUS AMPLIFIER

10
The output of the Data Switch is determined by
the Display
B and X-Y Inhibit input signals . The Logic Table
given in
Figure 3-46B shows the intensity controls that
determine
the output current of this stage for
different combinations
of Display B and X-Y Inhibit
input levels .
Current to Voltage
Amplifier
The Current to
Voltage Amplifier stage, consisting of
02322, 02328,
and 02332, converts the current output
from the Data Switch to a voltage in order to
drive the
Non-Linear Amplifier stage .
Non-Linear Amplifier
The
Non-Linear Amplifier stage, consisting of 02336
and
CR2335,
inverts the input signal in a nonlinear manner
.
When the input
signal is too low to forward bias the emitter
base junction of
02336, no current flows, and the collector
of
02336 is at its most positive level . As the signal
becomes
more positive 02336 will conduct current causing
the col-
lector voltage
to decrease . When the voltage at the base of
02336
becomes positive enough to cause CR2335 to con-
duct, the
emitter degeneration is reduced, which increases
the gain of
the stage . Variable resistors 82365 and
82366
determine the gain of the Non-Linear Amplifier stage
in the
Full Scan and Reduced Scan modes .
Output
Amplifier
The signal from the Non-Linear Amplifier stage
is applied
to the Output Amplifier for final amplification
and then
connected to pin 4 of the crt .
Emitter Follower
The signal applied to pin 4 of the crt is connected
through
a voltage divider to the Emitter Follower stage
and applied
to pin 10 of the crt . Variable resistors
82425 and 82435
determine the ratio of the divider in the
Reduced Scan and
Full Scan modes .