Tektronix 422, Instruction Manual

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Circuit Description—Type 422 AC-DC
Fig. 3-7. Current path for (A) positive-slope triggering, (B) negative-slope triggering.
control is turned clockwise from midrange to produce a
display which starts at a more positive level on the selected
slope. When the LEVEL control is turned clockwise toward
+, a more positive quiescent DC level is established at the
base o f Q364 which results in more output current from
the Trigger A m p lifie r stage. Since the Trigger A m p lifie r
stage is operating as a current shunt fo r positive-slope trig
gering, the signal at the base of Q364 must go farther nega­
tive before enough current returns to the Trigger TD and it
can switch to its high-voltage state. Therefore, the resultant
CRT display starts at a more positive point on the displayed
waveform. When the LEVEL control is turned toward —,
the DC level at the base o f Q364 goes negative to reduce
the quiescent current at the output o f the Trigger A m plifier
stage. Now, the trigger signal does not have to rise as far
positive before D375 switches to its high-voltage state and
the resultant CRT display starts at a more negative level on
the displayed waveform.
Auto Multivibrators
The basic configuration o f the A uto M ultivibrator stage
is a monostable m ultivibrator6 comprised o f transistors
Q345 and Q347. This stage produces the control gate to the
Sweep Generator circuit fo r auto-trigger operation. Under
quiescent conditions (no trigger signal) Q345 is biased on
b y th e p o s itiv e voltage applied to its base from
C342-R340-R342 through T377 and R341. The base o f
Q347 is referenced to ground through R344, and both
diode D344 and transistor Q347 are reverse biased. The
collector level o f Q347 depends upon the setting o f TR IG ­
GERING Mode switch SW355. When this switch is in the
AUTO position (pushed in), the level at the collector of
Q347, and therefore the output level to the Sweep
Generator circuit, is determined by the absence or presence
of trigger pulses at the input of this stage. For the quiescent
condition, Q347 is o ff and its collector rises positive to the
level determined by voltage divider R348-R402-R407. The
base level o f Q400 as determined by divider
R348-R402-R407 is about +1.5 volts and 0400 is forward
biased. The voltage drops across the base-emitter junction
of Q400 and across D408 balance out to provide an auto
gate output level to the Sweep Generator circuit o f about
+1.5 volts (this level corresponds to the free running
condition of the Sweep Generator; see the discussion on
Auto Trigger Mode Operation).
6Millman and Taub, pp. 405-438.
3-12