Tektronix 7B51 Instruction Manual page 31

Circuit Description-Type 7B51
SLOPE
\
\
\
\
Slope Selector
and
Level Comparator
0224,0234,0244
0254 026
POP
Auto
Trigger
Level
0353,0356
0364,036
0373,037
0384,038
0333
+50
V
+
-15V
These voltage levels are present
only if the Type 7B51 is used as a
Delayed Sweep Time-Sase.
~~.5V
Delay Mode Control " 1":3 V
So~
Level
Center
Sweep Gate
~
Multivibrator
0305,0315
CR299 0293 0296 SWEEP START
, , GATE to Sweep
Generator
LOCKOUT
from Sweep
Reset Multivibrator
Bright Line
Auto
0273,0275,0285
0324,0326,0328
SWEEP TRIG'D
LAMP
+5V
Fig. 3-3. Trigger Generator Block Diagram.
R221 establishes the emitter current for 0224 and
0234. The transistor with the most positive base controls
conduction of the comparator. For example, assume that
the trigger signal from the input stage is positive-going and
0224 is forward-biased. The increased current flow through
R221 makes the emitter of 0234 more positive and since
the base is held constant by the level control voltage, the
current through 0234 decreases. The increase in current
through 0224 makes the base of 0244 less positive and the
decrease in current through 0234 makes the base of 0254
more positive. Refer to Fig. 3-4 for current paths for posi-
tive slope triggering. Notice that the signal currents at the
collectors of 0224 and 0234 are opposite in phase. The
sweep can be triggered from either the positive or negative
3-4
edge of the input signal. The selection is made by the
SLOPE switch, S260.
When the LEVEL control is set to midrange (MODE
switch to NORM) the base of 0234 is at about 0 volts. This
corresponds to the 0 volt level at the input of 0213A, and
the comparator switches at the zero volt level of the trigger
signal. As the LEVEL control is turned clockwise from 0/+,
the voltage on the base of 0234 becomes more positive.
Now the trigger signal must rise to a more positive level
before comparison takes place. The resultant CRT display
starts at a more positive point on the displayed signal. When
the LEVEL control is turned counterclockwise from 0/+
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