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Theory of Operation
1780R-Series Service Manual
3–27
too long for PAL signals, a jumper (on Diagram 10) is provided to realign the
Fld Retrace Gate with V sync. See Field Sweep Gate description, on Diagram
10, for details.
Clamp Pulse Generator for Waveform Display. U858B is a retriggerable one-shot
multivibrator that generates a 2 s clamp pulse at either sync tip or backporch as
dictated by U958. Both + and – clamp pulses are generated when the B input is
high, which occurs when the DC Restorer is enabled, Clear is high, and the 1Y
input of U958 goes low.
U958 is a dual AND-OR-INVERT gate performing the timing logic for the
output one-shot (U858B). It makes the timing determination on a combination of
sync, backporch or sync tip selection and status of vertical lockout. Vertical
Lockout goes low during the vertical sync pulse to prevent the sync tip clamp
pulses from moving to backporch timing when the narrow serrations in the
vertical sync pulse occur.
U858A generates a pulse 5 ms after the leading edge of H sync. This delayed
pulse is used to time the start of the clamp pulse when backporch clamping is
selected. J866 can be set for clamp pulse timing reference from either the AFC
Sync or the Internal Sync Separator output.