Circuit Description—Type 422 AC-DC
understand the action, assume that the trigger signal at the
base of Q323 is positive-going. This produces a positive
going signal at the emitter of Q323 which forward-biases
Q324, and its collector goes negative. The signal at the
collector of Q324 is connected to the Trigger Amplifier
stage and the feedback networks through C325-D325.
Zener diode D325 reduces the DC level of the output signal
without appreciable attenuation of the trigger signal. If the
signal is less (lower in amplitude) than about —0.3 volt,
negative feedback is provided to the base of Q323 through
feedback network C353-R353 to establish a stage gain of
about 1.4. Variable capacitor C353 provides high-frequency
compensation for this stage.
As long as the output signal is less than about —0.3 volt,
the limiting network is effectively disconnected. Diode
D331 is reverse biased, since its anode is held at about +0.3
volt by the forward voltage drop of germanium diode
D332. Likewise, D334 is reverse biased since its cathode is
held at about —0.3 volt by the forward voltage drop of
germanium diode D333. When the output signal exceeds
about —0.3 volt, silicon diode D331 is forward biased since
its anode is held at about +0.3 volt. Negative feedback is
now provided to the base of Q323 through C332-R332.
Since R332 has a lower resistance than R353, more feed
back voltage reaches the base of Q323 to lim it the stage
gain to about 0.14. This limiting for higher amplitude sig
nals provides about the same output signal from this stage
for small input signals as for large input signals. Action is
similar for negative-going signals with D334 being forward
biased to provide limiting feedback through C333-R333 at
about +0.3 volt.
LEVEL control R355A determines the point on the trig
ger signal where triggering occurs. When the LEVEL control
is set near midrange, the output level from this stage is at
about zero volts. This results in a CRT display which starts
near the zero-volt level of the displayed waveform (near the
average DC level for AC trigger coupling). As the LEVEL
control is turned clockwise toward + (variable arm of
R355A moves toward —12 volts), a more positive voltage
level is produced at the output of this stage. This results in
a CRT display which starts at a more positive point on the
displayed waveform. The action is similar but opposite
when the LEVEL control is turned counterclockwise to
ward —. See the Trigger TD discussion for more detail on
how the output level from this stage changes the trigger
point on the displayed waveform.
Trigger Amplifier
The output signal from the Non-linear Feedback Ampli
fier stage is connected to the base of Trigger Amplifier
Q364. Diode D363 provides a reference at the emitter of
Q364 of about —0.6 volts. Therefore, as the signal at the
base of Q364 goes more negative than about zero volts, the
collector current of Q364 is reduced, and vice versa. This
output current at the collector of Q364 is in phase with the
trigger input signal current at the base of Q323. The output
signal from the Trigger Amplifier stage is connected to the
Trigger TD stage through the SLOPE switch.
Trigger TD
The Trigger TD stage shapes the output signal from the
Trigger Amplifier stage to provide a trigger pulse with a fast
leading edge. For positive-slope triggering, the Trigger Amp
lifier stage operates as a current shunt for tunnel diode5
D375 (see Fig. 3-7A). The cathode level of tunnel diode
D375 is set at about +6.2 volts by zener diode D364. When
a positive-going trigger signal is applied to the Sweep Trig
ger circuit, the output current of the Trigger Amplifier
stage decreases. This allows the current through tunnel
diode D375 to increase and it switches to its high-voltage
state. The reactance of L373 is high as D375 switches, and
therefore L373, R373 and T377 do not load D375. This
allows a majority of the D375 switching current to reach
the Sweep Gate circuit (through T40I). R373 establishes
the DC bias on tunnel diode D375 and C377 blocks this DC
current from flowing in the primary of T40I. The tunnel-
diode switching signal is connected to the Sweep Generator
circuit through transformer T401. Resistor R378 lowers the
"Q " of T401 to reduce ringing. Diode D401 in the second
ary of T401 allows only the positive going trigger pulses to
pass to the Sweep Generator circuit.
The circuit remains in this condition until the output
current of the Trigger Amplifier stage increases due to the
applied trigger signal. Then, the Trigger Amplifier stage
shunts the current from D375 so that it returns to its low-
voltage state. Notice that D375 switches to its high-voltage
state in phase with the applied trigger signal. This action
results in a CRT display which starts on the positive-going
slope of the displayed waveform.
When SLOPE switch SW365 is set to the negative-going
position, D375 is effectively inverted in the circuit (see Fig.
3-7B). Now the Trigger Amplifier stage acts as a series cur
rent source for tunnel diode D375. The anode of D375 is
held at about +6.2 volts by zener diode D364. As the trig
ger signal applied to the Sweep Trigger circuit goes positive,
the output current of the Trigger Amplifier stage decreases
and the current through D375 decreases also. Tunnel diode
D375 reverts to its low-voltage state, where it remains until
the applied trigger signal goes negative. When this occurs,
the Trigger Amplifier provides more current through D375
and it switches to its high-voltage state. Notice that for
negative-slope triggering, D375 switches to its high-voltage
state 180° out of phase with the applied trigger signal.
Therefore, the CRT display starts on the negative-going por
tion of the displayed waveform.
The LEVEL control determines the exact point on the
selected slope at which tunnel diode D375 switches to its
high-voltage state, and therefore determines the point on
the signal where the CRT display begins. The LEVEL con
trol varies the quiescent DC level at the base of Q364 (see
Nonlinear Feedback Amplifier discussion), and thus also
sets the quiescent current level at the output of the Trigger
Amplifier stage. This action occurs as follows (example
given for positive-slope triggering): Assume that the LEVEL
5Millman and Taub, pp. 452-455.
3-11
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