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Theory of Operation
1780R-Series Service Manual
3–105
Rotation Drivers. The Rotation Drivers are simple push-pull, discrete transistor,
emitter followers. Each driver consists of a pnp and npn transistor pair (Q377 &
Q378, Q374 & Q375) between –15 volts and +15 volts. Their outputs drive the
Trace Rotation coils around the Vectorscope and Waveform Monitor CRTs.
Data Latch. 226 latches data from the Microprocessor Data Bus. Data values are
transferred to the Q outputs on the rising edge of the Grat Clock signal (pin 11).
The Q outputs are static lines that only occasionally change.
Triangle Generator. U478 is a voltage comparator whose output is a square wave.
The gain from the output (pin 5) to the negative input (pin 2) is time delayed and
greater than the gain to the positive input (pin 3), which causes it to oscillate.
C476 and R479 provide the time delay to the negative input and set the
frequency of oscillation. The negative input has a symmetrical exponential
waveform resembling a triangle waveform, which drives the Pulse Width
Modulators.
Pulse Width Modulators. U466C and U471C along with the diodes in their
outputs are the enabling/disabling circuits for the Pulse Width Modulators. They
select either internal or external graticule lights. U466A and D along with
U471A and D, when enabled, are the duty cycle comparators. They compare the
DC level output by Control Buffer Amplifiers, U466B and U471B, to the
triangle waveform. Comparator square-wave output is AC-coupled to the Scale
Drivers.
Scale Drivers. The Scale Drivers are simple push-pull, discrete transistor, emitter
followers, similar to those in the Rotation Drivers. Each driver consists of a pnp
and npn transistor pair between ground and +15 volts. Their output is AC-
coupled directly to the graticule lights.
Vector Grat Light. (Diagram <45>) Internal graticule is illuminated with three
bulbs, in parallel, between the output of the Vectorscope Internal Graticule Scale
Driver and ground. External graticule is illuminated with two bulbs, in parallel,
between the output of the Vectorscope External Scale Driver and ground.
S131 is a push-button switch with an LED that is directly beneath the Vector-
scope CRT. It assigns Focus, Intensity, Scale Illum, and Position controls to the
Vectorscope.
Waveform Grat Light. (Diagram <45>) Internal graticule is illuminated with three
bulbs, in parallel, between the output of the Waveform Internal Graticule Scale
Driver and ground. External graticule is illuminated with two bulbs, in parallel,
between the output of the Waveform External Scale Driver and ground.
Circuit Theory