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 of Q364 which results in more output current from
the Trigger Amplifier stage. Since the Trigger Amplifier
stage is operating as a current shunt for 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 of Q364 goes negative to reduce
the quiescent current at the output of the Trigger Amplifier
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 of the Auto Multivibrator stage
is a monostable multivibrator6 comprised of transistors
Q345 and Q347. This stage produces the control gate to the
Sweep Generator circuit for auto-trigger operation. Under
quiescent conditions (no trigger signal) Q345 is biased on
by the positive voltage applied to its base from
C342-R340-R342 through T377 and R341. The base of
Q347 is referenced to ground through R344, and both
diode D344 and transistor Q347 are reverse biased. The
collector level of Q347 depends upon the setting of TRIG
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 off and its collector rises positive to the
level determined by voltage divider R348-R402-R407. The
base level of 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 of 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
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