Z-Axis Amplifier - Tektronix 7140 Instruction Manual

fresh start. The waveform at pin 8 of 01986 is differentiated
01956 will increase and the voltage level of the intensity
by C1993 and inverted by 01997 . Flip-flop 01992 is trig-
reference at TP1956 will increase from -10 volts to a more
gered on a positive going pulse, so, at the end of the 10
positive value. The intensity reference is connected to the A
second timing interval 01992 is triggered and the output at
and B INTENSITY control network (see Mode Switch and
pin 8 goes HI .
Calibrator, diagram 2) . If this reference goes more positive
the input drive to the Z-Axis logic is reduced . Therefore, the
This output goes high to cause the following to occur :
beam current of the crt is reduced which results in a lower
average screen current. If this feedback loop reaches equi-
(1) The Z-Axis turns off.
librium, then the voltage level of the intensity reference is
such
that the average
screen current equals
1 .8
microamperes . Singleshot screen currents are not limited to
(2) The red LED is turned on steady, to indicate the 1 .g microamperes since the feedback loop has a long time
shutdown of the Z-Axis system . Transistor 01994 turns
constant .
off when the Z-Axis Off signal goes HI and the red LED
DS1 .944 is turned on steady from the +15 volt supply
through R1994. Pin 2 of 01986 goes HI to inhibit the 10
When the intensity limiter is limiting the average screen cur-
second timer.
rent to 1 .8 microamperes the output of U1958A is high, with
the result that 01970 is driven into saturation . This causes
The Intensity Limiter stage can be reset several ways :
the yellow LED to flash (indicates that the intensity is
limited) .
(1) By either pressing the RESET button S1988 before
shutdown occurs, or by reducing the display intensity so
that the average screen current is less than 25 nA .
If the RESET button is pressed before shutdown occurs,
the base of 01980 is shorted to ground, 01982 turns off,
and 01978 conducts to discharge timing capacitor
C1971 .
If the screen current is below 25 nA, the output of the
operational amplifier U1958B goes LO and 01974 turns
off, thereby discharging timing capacitor C1971 through.
R1974 and R1971 .
(2) By either pressing the RESET button S1988 during
the 10 second delay before shutdown (when the red LED
is flashing), or by reducing the intensity so that the aver-
age screen current is below 25 nA .
If the RESET button is pressed during the 10 second
delay before shutdown occurs, the 10 second timer
01986 is reset and the reset pin of flip-flop 01992 is held
low preventing Z-Axis shutdown .
If the screen current goes below 25 nA during the 10
second delay before shutdown occurs, the output of
operational amplifier 019588 goes LO, the base of
01998 is pulled LO, 01998 conducts and 01999 is satu-
rated. The saturation of 01999 has the same effect as
pressing the RESET button .
(3) By pressing the RESET button S1988 after shutdown
occurs, the 10 second timer is reset . Also, flip-flop 01992
is reset and the Z-Axis Off signal goes LO turning the Z-
Axis back on .
If the average screen current exceeds 1 .8 microamperes the
output of operational amplifier 01952 exceeds 1 .8 volts.
Since the noninverting input of U1958A is biased at
1 .8 volts, the output of this operational amplifier goes posi-
tive and diode CR1963 conducts . The current through
ADD NOV 1985
Theory of Operation-7104
Figure 3-23 illustrates two operating conditions of the Inten-
sity Limiter. The maximum viewing time period (approxi-
mately 1 hour) is shown in Figure 3-23A; this condition
occurs with the minimum average screen current of 25 nA
required to actuate the limited viewing time circuitry. The
minimum viewing time (approximately 1 minute) is shown in
Figure 3-23B ; this condition occurs when the intensity is
limited to an average screen current of 1 .8 microamperes .
Diodes are connected to the A and B INTENSITY controls
to prevent interaction between the intensity controls when
the HORIZONTAL MODE switch is set for ALT or CHOP
operation (see Mode Switch and Calibrator, diagram 2) . This
interaction may occur when the display is alternating be-
tween time-base units. If either diode CR2019 or CR2009 is
shorted, the following will occur : If the A INTENSITY control
is advanced to the point where the Intensity Limiter stage
limits the average screen current to 1 .8 microamperes, the
voltage level of the intensity reference becomes more posi-
tive than -10 volts . Also, the B intensity level is reduced. In
the extreme case the reduction of the B intensity could be
such that the B trace is not visible. Present limiting of the B
intensity will only occur if diode CR2019 conducts . Diode
CR2019 will conduct if the B intensity level is set high
enough that the cathode of CR2019 is more negative than
the intensity reference voltage level at the anode of CR
2019 .
Z-Q~XIS AMPLIFIER
The Z=Axis Amplifier stage controls the crt display intensity
by varying the crt grid drive . The Logic circuit and the
Readout System provide input signals to the Z-Axis Amplifi-
er at J1606 and J 1632 respectively .
The Z-Axis Amplifier consists of three stages ; an impedance
matching stage, a preamplifier, and output driver . The im-
pedance matching stage consists of 01618 and 01608.
This stage provides isolation between the
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