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Theory of Operation
1780R-Series Service Manual
3–29
When the sweep ramp has reached approximately 3/4 of its final amplitude the
End of Sweep Detector outputs a high on the pin 1 input of U775A (OR gate) to
provide a high on one input of U780A (an AND gate). The next +Ref H sync
will AND U780A causing Q589 to turn on and force the Line Ramp Generator to
retrace and hold low for the remainder of the +Ref H sync pulse. In 2- and 3-line
sweeps the End of Sweep Detector output will be low during the first 3/4 of the
sweep and +Ref H sync will not AND U780A to turn on Q589 until the final 1/4
of sweep.
Line Ramp Generator. The Ramp Generator consists of an integrator-connected
operational amplifier, U385C. Its output is coupled back to the negative input
through the integrating capacitor, C587. The current source for the integrator is
through R489 from the voltage output of the DAC. Current always flows through
R489. When the ramp is to be terminated (for retrace), Q589 is turned on and
C587 is discharged. When Q589 is on, the output of the integrator is held near
ground by shorting the output to the input.
DAC. The DAC controls the slope of both the line and field ramps by controlling
the amount of charging current to the ramp timing capacitors. Pin 4 of U490 is a
current source for R487, R488, and R489, which are the timing resistors for the
ramp timing capacitors. The amount of current available to the resistors depends
on the 1L/F and 2L/F control line status.
In the 1-line or field sweep mode both the 1L/F and 2L/F control lines are high
and the maximum current is present at the output of U490, which produces about
–3 volts. In the 2-line or field sweep mode only the 2L/F line is high and the
output voltage drops to approximately –1.5 volts. In 3-line or field sweep mode
both 1L/F and 2L/F control lines are low and the voltage output at pin 4 of U490
is about –1 volt.
Field Sweep Gate. When the 1780R-Series Waveform Monitor is in a sweep mode
other than 1-, 2-, or 3-field, the FIELD control line is low. When FIELD is low
U795B, a D-type flip-flop, is preset with a high on the Q output. The high output
is ORed through U790D to turn on Q386 and hold the Field Ramp Generator
integrator near ground (no sweep).
In the field rate sweep modes the A input of U789B has to go low to enable the
retriggerable one-shot. In the 1-field mode, U790C ORs through a high from the
1L/F control line. When the End of Sweep Detector (U690A) output is high
(signifying the ramp is near maximum amplitude) U760B ANDs and goes high.
U880D inverts the output of U760B to provide the low on the A input of U789B.
Once the A input is low, the first rising edge of V sync, on the B input, triggers
the one-shot. The Q output goes high for approximately 148 ms after the one-shot
is triggered by V sync. The high from the Q output sets the D input of U795A
high so that the next two +Ref H sync pulses clock through a 2-line high to
U790D, which ORs to turn on Q386 for retrace.