Circuit Description—Type 422 AC-DC
Time/Div Switch. The timing capacitor and resistor are
selected by the TIME/DIV switch to provide the various
sweep rates listed on the front panel. Diagram 7 shows a
complete diagram of the TIM E/DIV switch. VARIABLE
TIME/DIV control R440A (see Timing Switch diagram)
provides continuously variable, uncalibrated sweep rates
between the calibrated settings by varying the charge rate
of the timing capacitor.
Sweep Reset Amplifier. The positive-going sawtooth sig
nal at the collector of Q441 is connected to tunnel diode
D455 through voltage divider R451-R452. D455 is quies
cently biased in its low-voltage state. As the sawtooth out
put level reaches about +30 volts, the voltage level con
nected to the anode of D455 rises positive enough to switch
D455 to its high-voltage state. The positive voltage change
at the anode of D455 as it switches is connected to Q464
through R456 and Q464 goes into saturation (collector
drops to ground). This results in less current to tunnel
diode D405 and it reverts to its low-voltage state. The base
of Q414 also goes negative and it turns off to allow Q424
to return to conduction. When Q424 turns on, its collector
rises positive and Disconnect Diode D439 is forward biased
to quickly discharge timing capacitor C440 through R442.
This produces the retrace portion of the sawtooth signal.
Transistor Q464 turns off as tunnel diode D455 reverts to
its low-voltage state along with the negative-going retrace.
The Sawtooth Sweep Generator stage is now ready to pro
duce another sweep as soon as tunnel diode D405 is
enabled after the hold-off period and a trigger pulse is
received. The positive-going change at the collector of Q414
is also connected to the Unblanking Circuit so the CRT is
blanked during retrace time.
Sweep Lockout Amplifier. Quiescently before the
sweep is triggered, Q424 and Q434 are both conducting and
their emitter current is supplied through R415. The collec
tor of Q434 is held at about +0.3 volt as determined by the
auto gate level from the Sweep Trigger circuit (see Auto
Multivibrator discussion). The sweep gate from Q414
diverts the current from R415 at the start of the sweep and
both Q424 and Q434 turn off. In addition, the positive
going sawtooth output reverse biases D430 to interrupt the
base-current path for Q434. The collector of Q434 goes
negative toward —12 volts through R454 and this negative
level at the collector of Q434 holds D401 reverse biased
through R401 and T401 (see Sweep Trigger circuit) to pre
vent incoming trigger pulses from reaching tunnel diode
D405. The circuit remains in this condition until the sweep
gate from Q414 goes positive at the end of the sweep.
Then, the current from R415 is again available to Q424 and
Q434. Q424 is forward biased, but Q434 remains off since
D430 is held reverse biased by the positive sawtooth level at
the output of this circuit. Therefore, most of the current
available from R415 flows through Q424, producing the
retrace portion of the sawtooth. As the sawtooth at the
output of this circuit drops negative to its quiescent level,
D430 is forward biased as its cathode drops about 1.2 volt
more negative than the level at the emitter of Q434. Then,
Q434 is forward biased and its collector attempts to rise
positive. However, the RC networks C432-R432 and
C434-R434 along with capacitor C401 delay the voltage
change at the collector of Q434 to allow a hold off period
for all circuits to return to their original conditions before
the next sweep is produced. As these capacitors charge, the
anode level of D401 rises along with the collector of Q434
until it again can allow the positive-going trigger pulses to
pass.
Auto Trigger Mode Operation
Operation of the Sweep Generator circuit in the AUTO
position of the TRIGGERING Mode switch (LEVEL con
trol pushed in) is the same as for NORM when a trigger
pulse is present. However when a trigger pulse is not pre
sent, a free-running reference trace is produced in the
AUTO mode. This occurs as follows (refer to the Sweep
Trigger diagram for reference to the Auto Multivibrator,
C401, R401 and D402):
The auto gate level from the Auto Multivibrator stage is
connected to the collector of Q434. When the instrument is
triggered, the auto gate level from the Auto Multivibrator
stage is held at about zero volts and the sweep is produced
and reset as just described for NORM operation. However,
when a trigger pulse is not available, the auto gate level rises
positive to about +1.5 volts. Now, when the sweep is reset
and Q434 turns on, its collector attempts to rise positive
toward +1.5 volts as the holdoff network charges. Diode
D402 bypasses C401-D401-R401-T401 and connects the
collector level of Q434 directly to the anode of D405. The
more positive collector level for Q434 set by the auto gate
allows more current to flow through Sweep Gate tunnel
diode D405, so that it switches to its high-voltage state and
automatically retriggers the sweep. Therefore, the Sweep
Generator circuit is automatically retriggered at the end of
each holdoff period and a free-running sweep is produced.
Since the sweep free runs at the sweep rate of the Sweep
Generator as selected by the TIME/DIV switch, a bright
reference trace is produced even at fast sweep rates.
External Horizontal Operation
When the TIME/DIV switch is in the EXT HORIZ posi
tion, the Sweep Generator circuit is disabled so it cannot
produce a sweep. In the EXT HORIZ position, the TIME/
DIV switch connects —12 volts to tunnel diode D405
through D403 and R403. This holds D405 reverse biased so
it cannot accept the applied trigger pulses. A t the same
time, a negative unblanking level is connected to the CRT
circuit through diode D404 so the external horizontal dis
play can be presented. In all other positions of the TIME/
DIV switch, both D403 and D404 are held reverse biased
by the positive voltage applied to their cathodes through
R404.
HORIZONTAL AMPLIFIER
General
The Horizontal Amplifier circuit provides the output sig
nal to the horizontal deflection plates of the CRT. In all
positions of the TIME/DIV switch except EXT HORIZ, the
horizontal deflection signal is a sawtooth from the Sweep
Generator circuit. In the EXT HORIZ position, the hori-
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3-15
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