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Theory of Operation
3–30
1780R-Series Service Manual
In 3-field sweep modes the FLD Retrace Gate is always high. The Field Ramp
Generator End of Sweep Detector outputs a high in the last third of the ramp.
ANDing through U760B does not occur until significantly after the second V
sync to ensure that the next V sync will again retrace and start the Field Ramp
Generator.
In 2-field sweep mode it is possible to trigger the field sweep on a selected
monochrome field. The FLD Retrace Gate line is high for only half a line and V
sync goes high during the last half of the line for field 1 and during the first half
for field 2. By changing which half of the line the FLD Retrace Gate is high, it is
possible to start the one-shot only for the selected field.
Because the line count scheme is different for PAL instruments the one-shot is
triggered on the trailing edge of V sync. J694 is moved to select /V Sync in the
PAL instruments to accommodate the difference.
When the instrument is in the Remote Sync mode and there is no horizontal sync
information in the remote sync signal, the /DIR REMSYNC EN line will be low.
This disables U789B (one-shot) and the sweep is controlled by U795B, a D-type
flip-flop. The DIR REMSYNC clocks a low to begin a sweep and then goes high
to begin retrace.
Field Ramp Generator. The Ramp Generator consists of an integrator-connected
operational amplifier, U386D. Its output is coupled back to the negative input
through the integrating capacitor, C485. The current source for the integrator is
through R487 from the voltage output of the DAC. Current always flows through
R487. When the ramp is to be terminated (for retrace) or is undesirable, Q386 is
turned on and C485 is discharged. When Q386 is on, the output of the integrator
is held near ground by shorting the output to the input.
Ramp Attenuator for External Staircase. When the /RGB EN control line is pulled
low the output of U880F goes high to turn on Q686 and ground one end of
R587. When Q686 is on, the output of the Ramp Generator is attenuated to either
20% for the 4-step staircase (YRGB) or 33% for the 3-step staircase (RGB) as
dictated by the position of the plug jumper on J585.
End of Sweep Detector. U690A and U690B are connected as voltage comparators.
The negative inputs are set to approximately 5 volts by a voltage divider made
up of R591 and R592. Until the Ramp Generator output (applied to the positive
input) reaches 5 volts, the End of Sweep Detector output is low. The 5 volt level
corresponds to approximately 3/4 of the sweep length, which ensures that a 2-,
3-line, field sweep will be past the middle one or two syncs before the sweep
gate is enabled again.