Sawtooth Amplifier; Gate Amplifier; Graticule Illumination; Vertical Channel Switch - Tektronix 7140 Instruction Manual

stages . These stages aid in locating components
mentioned here .
Sub-headings in the following
discussion use the stage names to further identify
portions of the circuitry on diagram 7.
REV NOV 1985
CHANNEL SWITCH
Theory of Operation-7104
VERTICAL CHANNEL SWITCH
+ SAWTOOTH AMPLIFIER
The Vertical Channel Switch circuit selects the vertical
The sawtooth signals from the A and B time-base units
deflection signal from the output of the LEFT and/or
are connected to the Sawtooth Amplifier stage through
RIGHT VERT plug-in compartments) for display on the
series resistors R93 and R95 respectively (see diagram 3) .
crt .
The front-panel selector switch, S1930, determines
whether the A-sweep or the B-sweep sawtooth signal
A schematic diagram of the Vertical Channel Switch is
provides the + SAWTOOTH signal . The unused sawtooth
given on diagram 8, in Section 8 of this manual
signal is terminated by R1941 .
(Diagrams and Circuit Board Illustrations) . The schematic
is divided by gray shaded lines separating the circuitry
Transistors 01943-01942-01946 form an inverting
into
major stages . These stages aid in locating
feedback amplifier. Gain of the stage is about 2, as
components
mentioned here . Sub-headings in the
determined by the ratio of feedback resistor R1944 to the
following discussion use the stage names to further
input resistance (made up of R1940 and on diagram 3,
identify portions of the circuitry on diagram 8 .
either R93 or R95 depending on which sawtooth source
is selected) .
The vertical deflection signal from the left and right
+ GATE AMPLIFIER
vertical plug-in units is either terminated within the
stage or coupled through the stage as determined by the
The front-panel + GATE switch (S1940) selects the gate
Vertical Channel Selector stage. The Channel Switch
signal from either A or B time-base unit . The unused stage is made up primarily of integrated circuit U668 .
+ gate is terminated in R1931 . Before a gate occurs, Inputs 7 and 9 provide a differential input for the signal
01934 is biased off and 01938 is conducting ; its from the right vertical plug-in unit . Input pins 17 and 19
collector potential is low enough to cut off 01928. When provide a differential input for the signal from the left
a gate occurs, it is coupled to the base of 01934, causing vertical plug-in unit . The differential output signal at pins
it to conduct thereby cutting off 01938. The current 3 and 13 is connected to J694 and J592 respectively .
through R1911 now flows through 01928 to produce the
+ GATE signal . The signal at the collector of 01934 is
Components U682, 0682, 0676, and 0672 supply
picked off to drive the Graticule Illumination stage and/or
standing current to U668 and maintain the output
the readout system in PULSED mode .
common-mode do level at +8 .5 volts for all Channel
Switch modes . The common-mode level at pins 3 and 13
GRATICULE ILLUMINATION
of U668 is sensed by R559-R659, and compared with a
reference level determined by divider R680-R681 .
Variable resistor R1900 (GRAT ILLUM) determines the
Assume, for example, that pin 2 of U682 is lower than pin 3,
brightness of the graticule lights (except when in the indicating an output level below +8 .5 volts. The output
PULSED position) by controlling the output of the graticule
of U682 at pin 6 will be driven positive and current will
light supply (see LV Regulators, diagram 15). Variable
flow in R683 . This current must be supplied from the +15
resistor R1902 (GRAT ILLUM PRESET) determines the
V supply via R682, thereby lowering the base voltage of
brightness of the graticule lights when the GRAT ILLUM
0682 . This increases the collector current in 0682 .
Transistor 0676 operates as a common-base amplifier
control is set to PULSED . In the PULSED mode, the
and passes along the increased collector current to pin
graticule lights are gated on for approximately 0.5 seconds .
3a of 0668 . This increases the output common-mode
Programmable unijuction transistor Cl1908, in conjunction
level, thus bringing U682 into balance. The voltage at pin
with Q1910, generates the pulse to turn the graticule lights
3a of U668 depends on the Channel Switch mode : in
on . A negative signal (from the MAN pushbutton, the + LEFT, RIGHT, ALT, or CHOP pin 3a is at +10.5 V; in ADD it
Gate or from an external input) will cause Q1908 to conduct
is +12 .5 V; when X-Y Inhibit is HI, pin 3a is +8 .5 volts. In
and start discharging C1908. At this time, 611910 turns off,
all
modes,
the current supplied by 0676 is 160
which allows R1902 to control the output of the graticule
milliamperes plus or minus small variations required to
light supply . Capacitor C1908 discharges until Q1908
keep the output level at +8 .5 volts.
cannot maintain conduction . As Q1908 turns off C1908
begins to charge positive until the zener voltage of VR1910
VERTICAL CHANNEL SELECTOR
is reached which turns on Q1910; its collector then goes
The Vertical Channel Selector interfaces the Channel
negative to turn the graticule light supply off. When in the Switch, U668, to the logic signals arriving from the Main
PULSED mode and operating from the + GATE source, the
Interface . The Channel Switch stage requires two pairs of
graticule lights will turn on momentarily at the trailing edge
complementing control voltages ; one pair for each
of the + GATE (end of each sweep) .
channel. The HI control voltage is +4 .0 V,
the
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