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Theory of Operation
3–76
1780R-Series Service Manual
XY Input Amplifiers. U211 is a quad, J-FET input operational amplifier. U211A
and C are balanced Differential Input Amplifiers intended for audio use. In a
600 W system, R113 and R308 can be adjusted to normalize signals from 0 dBm
to +12 dBm (2 -9 volts p-to-p). The input impedance is greater than 20 kW to
ground.
P108 and P205 (on J108 & J205) can be installed to connect the inverting inputs
of U211A and U211C for high gain (X10) inputs. These inputs are provided for
special non-audio applications where a higher gain may be needed.
U211B and D drive the X and Y Deflection Amplifiers through a Master
MPU-controlled input switch. Position offset voltage for both the X and Y
Amplifiers is supplied from the Z-Axis Digital-to-Analog Converter (DAC) on
Diagram 39. R312 feeds a small amount of Y signal into the X Amplifier for
orthogonality adjustment.
Post Regulators. U764 and U864 are ground referenced post regulators. They
convert the 1780R-Series + and – 15 volts to the + and – 12 volts required to
power the Vectorscope circuitry.