Auto Focus; Crt Circuit; Control Grid Dc Restorer - Tektronix 7140 Instruction Manual

Theory of Operation-7104
Readout and Logic inputs in addition to providing
termination of the input coax cables . The collector
current out of this stage is fed to the Auto Focus
amplifier through 81606, developing a voltage signal to
CRT CIRCUIT
drive the Z-Axis preamplifier . Transistor 01626 limits the
voltage drive to the preamplifier by clamping the output
of 01608 at a level determined by Clamp Level
A schematic diagram of the CRT Circuit is given on
adjustment 81226 .
diagram 13, in Section 8 of this manual (Diagrams and
Circuit Board Illustrations) . The schematic is divided by
gray shaded lines separating the circuitry into major
stages . These stages aid
in locating components
The Z-Axis preamplifier, a transconductance amplifier,
mentioned here . Sub-headings in the following
consists of 01632, 01648, and 0~ 652, which provides a
discussion use the stage names to further identify
current drive for the output stage.
Z-Axis Gain
Portions of the circuitry on diagram 13 .
adjustment, 81637, sets the gain of this stage and is
used to set the gain for the entire Z-Axis system .
Adjustments 81635, C1635, and 81651 provide current
peaking to the output stage for optimum transient
CONTROL GRID DC RESTORER
response . The Z-Axis Level adjustment 81645 is used to
adjust the Z-Axis baseline to the proper level. The
The purpose of the Control Grid DC Restorer stage is to
preamplifier is disabled by a shutdown signal from the
elevate the two low voltage grid control signals to a large
Intensity Limiter circuit, which saturates 01644.
negative potential. These inputs are the output signal
from the Z-Axis amplifier, coupled through 81680 to the
first input of the Control Grid DC Restorer stage and the
crt grid bias (with its associated crt grid cut off warmup
The output driver is a shunt feedback stage with gain set compensation circuit) coupled through 81748 to the
by 81660. 01658, 01668, 01666 and 01676 form a second input of the Control Grid DC Restorer stage.
direct coupled amplifier with a high open loop gain ; thus
the input at the base of 01658 is a virtual ground . These
The Control Grid DC Restorer stage is current driven from
transistors are capable of providing
high
speed
the square wave at the high voltage winding through
transitions in the negative going direction only . Fast
81788, 81789, 81812, and 81811 . When the voltage at
positive transistions are achieved by peaking the base of
terminal 9 of transformer T1770 goes positive diode
01676 via 01664 and T1664. Capacitor C1663 adjusts
CR1749 conducts at the voltage determined by the first
the drive to 01676 for optimum response . Transistor
input, the Z-Axis amplifier output level . This clamping
01672 establishes the collector voltage supply for
action establishes the positive swing of the do restorer
01676. VR1671, 81677, CR1675, and CR1678 provide
drive. On the negative swing of T1770 diode CR1747
protection from high transient voltages .
conducts at the voltage determined by the second input;
the crt grid bias voltage. This clamping action establishes
the negative swing of the do restorer drive. The ac swing
of the do restorer is coupled from the low voltage section
to the high voltage section by capacitor C1793. On the
negative swing of the do restorer drive, the high voltage
AUTO FOCUS
end of C1793 is clamped to the -2400 volt supply by
CR1794 . On the positive swing of the do restorer drive;
The Auto Focus stage maintains optimum focus of the crt CR1792 changes the high voltage end of C1792 to a
display over a range of sweep speeds . The crt needs voltage more positive than the -2400 volt supply by an
focus correction only at high Z-Axis drive conditions . amount equal to the sum of the absolute value of the
Consequently, the output of the Auto Focus amplifier is voltages of the two inputs ; the Z-Axis amplifier output
ac coupled to the focus grid . The collector current of Z- voltage level and the grid bias voltage level.
Axis
impedance matching stage, which consists of
01618 and 01608, is fed to the input of the Auto Focus
The crt cathode voltage is 135 volts more positive than
amplifier, 01603. The emitter voltage of emitter follower
the -2400 volt supply, as determined by the Grid Bias
01607 is held steady at -8 .6 volts. At zero volts, drive of
the Z-Axis the collector current of 01603 is maximum (8
SuPPIY stage. Therefore, the grid is negative with respect
milliamperes) which causes clamping diode CR1609 to
to the cathode by 135 volts (minus the sum of the
conduct. At midrange Z-Axis drive, the collector current
absolute values of the voltages of the Z-Axis amplifier
of 01603 drops to 6 milliamperes and the voltage at the
output voltage level and the crt grid bias voltage level) .
anode of CR1609 is -8 .0 volts which causes the clamping
The CRT Grid Bias adjustment 81746 is set with the Z-
diode CR1609 to barely conduct. For a Z-Axis drive over
Axis amplifier output at the low level for the proper crt
midrange, diode CR1609 becomes reverse biased and a
cutoff voltage. The CRT Grid Bias adjustment has a range
negative going signal appears at the base of 01617. The
of 50 volts.
amplifier, consisting
of 01617 and 01620, is
noninverting and has a voltage gain of approximately
The Grid Bias Supply stage not only helps to protect the
four . The negative going signal at the collector of 01620
crt during turn-on of the instrument (see description of
is connected to emitter follower 01629 and then ac
the Grid Crow Bar circuit) but also when a malfunction of
coupled through C1628 to the focus grid .
the Control Grid DC Restorer stage occurs . In the case of
3-60 ADD NOV 1985