Circuit Description—455/A2/B2
When both pins 13 and 14 of U2790 are LO, the Sweep
Start Comparator (inside U2790) minus input is pulled LO.
This causes pin 4 of the 1C to become a high impedance,
allowing Ct to begin charging through R t. As Ct begins
to charge toward the voltage applied to Rt, the gate of
Q2784 (connected to the junction of Ct - Rt), goes posi
tive by the same amount. This increases the current
through Q2784 and decreases the current through Q2782.
The decrease in the Q2782 drain current through R2787
produces a positive-going voltage at the base of Q2786.
The voltage at the collector of Q2786 and the negative side
of Ct is pulled down. This results in a negative-going voltage
applied across Ct, maintaining a constant charging current
and giving a linear rate of fall to the sawtooth output
signal.
The sawtooth output continues to fall in this manner until
it reaches —2.4 volts and initiates a Holdoff Start pulse at
pin 10. The Holdoff Start pulse starts the holdoff ramp in
U2750 and resets the sweep by causing pin 13 of U2790
to go HI.
The B sweep is dependent on the signal at the delayed gate
input (pin 14) and resets only when the A sweep ends.
The Delay Start and B Start controls, R2782, and R2682,
adjust the quiescent level of the A and B sweeps, thus de
termining the correct starting points for the sweep outputs.
Sweep Control 1C
The Sweep Control circuit is contained within a single 1C
U2750. Fig. 4-7 shows a block diagram of the circuitry
in this 1C.
The following is a brief description of the function asso
ciated with each pin of U2750.
Pin 1. Single sweep mode. When this pin is LO (ground)
and pin 4 is HI, the sweep is in the single sweep triggering
mode.
Pin 2. Single sweep reset. Pushing the SGL SWP push
button resets the sweep triggering circuit, making it
ready for another single sweep to be triggered. The
READY indicator lights when the single sweep circuit
is reset and any sweep in progress is terminated.
Pin 3. Automatic timing. In the automatic triggering
mode, when no trigger pulse occurs within about 100 ms
following holdoff, C2757 charges sufficiently to allow
pin 3 to be HI. This causes pin 6 to produce a sweep
gating pulse to trigger the sweep and produce a baseline
trace even when no trigger is available.
Pin 4. Automatic mode. Grounding the pin enables auto
matic sweep operation when pin 1 is HI.
Pin 5. Logic Gate. This is the gate input from the A
Trigger Generator that is used to initiate several sweep
control functions.
Pin 6. Automatic Gate. Provides a gate output, in the
automatic mode, to trigger the sweep when no trigger
signal occurs within about 100 ms following the end of
the holdoff.
Pin 7. A Gate. Provides a gate pulse to trigger the alter
nate trace circuitry in the Vertical Module.
Pin 8. Ground.
Pin 9. Holdoff output. This pin switches from LO to
HI to reset the sweep and trigger circuits.
Pin 10. Holdoff Timing. RC networks are connected
between this pin and pin 11 to vary the holdoff time,
depending on the setting of the A TRIGGER HOLDOFF
control and the A TIM E/DIV switch.
Pin 11. Holdoff Ramp. A negative-going ramp is pre
sent here that determines the holdoff time.
Pin 12. Hold Off Start. A positive-going pulse, which
occurs at the end of the sweep, is applied to this pin to
terminate any automatic gate present, start the holdoff
ramp, and initiate the reset pulse to the A trigger circuit.
4-16
REV. A, O CT 1975
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