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Theory of Operation
3–110
1780R-Series Service Manual
Diagram 42 Waveform High Volts
The Waveform Monitor high voltage supply generates all of the potentials
required to power the CRT, except for the deflection voltages (however,
+160 volts for the Horizontal Deflection Amplifier is generated by these
circuits). The high voltage is derived from an Armstrong Oscillator-generated
25-kHz signal. The Error Amplifier, which controls oscillator base current,
samples the cathode supply DC voltage. The voltage current limit circuit is a fail
safe, designed to prevent X-rays if the Error Amplifier should fail.
The PP Clamp provides the proper amplitude RF signal to the Grid Drive
Rectifier, which outputs a DC voltage proportional to the driving Z-axis signal.
The Focus Amplifier sets the voltage on the bottom of the focus divider. By
varying the Focus Amplifier output, the focus voltage higher up the divider
string can be varied a small amount. In line select the CRT is driven very hard.
The Line Select Focus circuit causes the focus voltage to change during the
selected line, and then return to normal the rest of the time so that the readout
signal, which does not drive the CRT as hard, will also be in focus.
The Cursor Shaper takes a positive-going TTL edge and turns it into a very
narrow negative pulse. This pulse is applied to the cathode of the CRT to provide
a short duration bright-up of the trace. The Cursor Shaper is used for timing
cursors and subcarrier dots mode.
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