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Theory of Operation
3–104
1780R-Series Service Manual
Diagrams 40 & 45 Scale Illum & Trace Rotation – Graticule Lights
A Triangle Generator is employed to provide a triangular waveform to drive one
input of the Pulse Width Modulators. Pulse Width Modulators are voltage
comparators that compare the triangle-shaped waveform to DC voltages and
produce a pulse width modulated square wave. The pulse width is changed by
varying the DC voltage. The brightness of the graticule illumination light bulbs
are varied by driving them with a variable duty cycle AC-coupled square wave to
provide longer bulb life. 50% duty cycle is maximum brightness. Anything
above or below 50% duty cycle reduces the value of illumination. There are four
sets of graticule lights, internal and external for both tubes. Each set of lights has
its own voltage comparator to produce a duty cycle modulated square wave.
Independent comparators make it possible for each set of graticule lights to have
a different intensity. The graticule lights are independently selected by enabling
or disabling their comparators.
The duty cycled square waves are AC-coupled to the Scale Drivers, which are in
turn AC-coupled to the graticule lights.
The Rotation Drivers buffer the trace rotation signals from the DAC and drive
the Trace Rotation coils around the CRTs.
Overview