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Theory of Operation
3–34
1780R-Series Service Manual
Readout Switching. When /WFM READOUT EN is high, the H RO DATA WFM
signal is disconnected from the Output Amplifier by turning on Q267 to short
the readout signal to the –2.2 volt supply. When Q267 is on, CR251 is reverse
biased and the horizontal readout data is disconnected from the Output Amplifi-
er. When Q267 is on, Q354 is open and allows the output signal from the
Magnifier Amplifier to flow through R354 and CR252.
When /WFM READOUT EN goes low, Q267 is open and the H RO DATA
WFM signal is coupled to the Output Amplifier through CR251. Q354 is turned
on by the output of U260F going high, which shorts the Magnifier Amplifier
output signal to the –2.2 volt supply and reverse biases CR252. R167 (WFM
Readout H Gain) determines the width of the readout display, while R170 (WFM
Readout H Center) controls the DC level to horizontally position the readout
display.
Output Amplifier. The Horizontal Output Amplifier has a paraphase output to
differentially drive the CRT deflection plates. The input is the base of Q547,
which is a summing junction near ground. R451 is a compensated (C352)
feedback resistor returning a differential current from the collector of Q555.
Common mode feedback from both outputs is returned to the emitters of Q547
and Q550. Bias current for Q544 and Q555 flows through Q643 and Q655.
A sample of the left deflection plate driving signal is taken from a voltage
divider in the base of Q550. It is coupled back to the Vertical Output Amplifier,
Diagram 6, to provide an orthogonality adjustment for the CRT.