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Theory of Operation
1780R-Series Service Manual
3–25
U185D or U185A being gated through. The output of U185C is Reference Sync
and Direct Remote Sync.
V Sync Detector. Q385 and Q391 form an integrating Sync Detector that has an
output when the vertical broad pulse occurs. Q385 is saturated for all of the field,
except during the vertical sync pulse when it is turned off. When Q385 turns off,
C285 charges, causing the collector to ramp up in a nearly linear waveform.
When Q385 again saturates, a peak detector, CR385, catches the peak and holds
it. When Q385 is on, during the active field, the charge on C390 slowly decays.
When Q385 again turns off C285 ramps back up just enough to replace the
amount of charge that has decayed off. Q391 then turns on during the time that
C390 is charging (while the decayed charge is being replaced) to output a
vertical sync pulse.
V Sync Free Run Generator. U291 is a timer circuit that follows the output of the
V Sync Detector, when there is vertical sync. If sync is missing, U291 free runs
at about a 30 Hz rate (V/2) to keep the instrument operating until there is
incoming video or remote sync. U190C provides a complement vertical sync.
Loss of Ref Sync Detector. U297B is a non re-triggerable one-shot that outputs a
36 ms pulse. It is triggered by the negative-going pulses output from the Ref or
Remote Sync switch. The timing of this one-shot is long enough to keep it from
triggering on vertical equalizing pulses, but well short of the horizontal sync
pulse rate.
Q282, which is driven by the Q output of U297B, has a long time constant
circuit in its base circuit; when Remote Sync is enabled it provides one input to
U185B, which generates the /Dir Sync Enable. Having a long charging time
prevents noise or the field or field/2 square wave repetition rate from disabling
the /Dir Sync Enable.
U297A is a re-triggerable one-shot with a period of 320 ms. As long as there is a
direct sync output from U297B it remains triggered and Q will be high and /Q
low. If the direct sync output from U297B is not present, U297A reverses
outputs (Q low and /Q high) causing the Direct or AFC switch to switch to AFC.
When AFC Sync is selected on the Waveform CRT menu the DIR/AFC line goes
low, which clears the one-shot and forces Q low and /Q high to force the switch
to the AFC mode.
Direct or AFC Switch. U395 is an inverting data switch that is controlled by the
outputs from U297A. When Q is high and /Q low the switch connects the direct
sync from U297B Q to the Ref H Sync Generator. When Q goes low and /Q high
the switch inverts and connects the AFC Sync, from Diagram 37, to the Ref H
Sync Generator.