Circuit Description—Type 422 AC-DC
Trigger pulse
from Sweep
Trigger circuit
Auto gate
from Auto
Multivibrator
stage
C440
Sweep to
Horizontal
Amplifier
circuit
Fig. 3-8. Sweep Generator detailed block diagram.
The sweep gate signal from the collector of Q414 is also
connected to the Vertical Switching and Output Amplifier
circuit through R470. This signal provides an alternate
sweep pulse to switch the vertical channels at the end of
each sweep.
Sweep Start Amplifier. Before this circuit is triggered.
Sweep Start Amplifier Q424 is quiescently conducting
through R415, R423 and L423. The current through R424
raises the collector level of Q424 positive to forward bias
Disconnect Diode D439. Transistor Q429 sets a constant
voltage at the base of Q424 to establish the quiescent levels
in the Sweep Start Amplifier, Disconnect Diode, Sawtooth
Sweep Generator, Sweep Reset Amplifier and Sweep Lock
out Amplifier stages. This constant voltage insures that the
sweep starts at the same point each time (see Sweep Reset
Amplifier discussion for further details). When the sweep
gate signal from Q414 turns Q424 (and Q434) off upon
receipt of a trigger, the collector of Q424 goes negative
toward the -12-volt supply through R424. However, when
the collector level of Q424 reaches about —2.2 volts, D438
is forward biased to clamp the circuit at this level.
Disconnect Diode. The Disconnect Diode D439 is quies
cently conducting current through timing resistor R440 and
R442. This prevents timing current from timing resistor
R440 from charging timing capacitor C440. The sweep gate
signal from the Sweep Start Amplifier reverse biases D439
and interrupts the quiescent current flow. Now the timing
current through the timing resistor begins to charge timing
capacitor C440 so the Sawtooth Sweep Generator stage can
produce a sawtooth output signal. The fast rising portion of
the sweep gate signal produced by the action of C418
reduces the switching time of D439, which improves the
timing linearity at the start of the sweep.
Sawtooth Sweep Generator. The basic sweep generator
circuit is a Miller Integrator circuit7. When the current flow
through the Disconnect Diode is interrupted by the gate
signal, timing capacitor C440 begins to charge negatively on
the R440-R442 side through timing resistor R440. As the
timing capacitor charges toward the voltage applied to the
timing resistor, the gate of FET Q443 goes negative also.
This produces a negative-going output voltage at the source
of Q443 which is connected to the base of Q441. Q441
amplifies and inverts the voltage change at its base to pro
duce an amplified positive-going sawtooth output. To pro
vide a Jinear charging rate for the timing capacitor, the
sweep output signal is connected to the positive side of
C440. This feedback opposes any tendency for a voltage at
the gate of Q443 to change, so the resultant change at the
gate of Q443 is actually very small. Therefore, the charging
current through R440 to C440 remains constant to produce
a linear sawtooth output signal. The output voltage conti
nues to rise in a positive direction until the circuit is reset
through the Sweep Reset Amplifier stage. The output signal
from the collector of Q441 is connected to the Horizontal
Amplifier circuit.
7Millman and Taub, 540-548.
3-14
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