Signal-Frame Sweep-Raster Scan Description

Signal Amplifier

This circuit receives video information at the Video I and Video 2 inputs, controlling the voltage to the CRT cathode at the video output (J902 pin 2). The input to Video 1 and Video 2 is supplied from the Display Gate Array on the Main PCB at TTL voltage levels. The outputs of U401 A, U401 B, and U4O1C provide biasing for Q401, causing Q401 to conduct and supply a nominal +40V to the video output. Four levels of video intensity are available by switching the outputs of U401A, U401B, and U401C. Brightness is adjustable through Video Gain, R402.

Frame Sweep Generator

A deflection hybrid IC, U101, drives the frame yoke to sweep the CRT during horizontal retrace. The Display Gate Array on the Main PCB supplies a nominal 60Hz synchronizing signal, Frame Sync, for this circuit.

To sweep the CRT accurately, U101 requires biasing by a number of external components. The RC combination, R105, R106, and C 102, sets the frequency of the hybrids internal oscillator. Horizontal Hold, R106, controls horizontal rolling. Horizontal Width, R111, controls the width adjustment and Horizontal Linearity, R113, calibrates the CRT beam linearity. R128 senses deflection current and provides feedback to U101 pin 10 to help control both width and linearity. R117 and C112 form a damping circuit to eliminate yoke ringing (oscillation).

Raster Scan Oscillator

The Raster Scan Oscillator sets the sweep frequency for the Raster Generator and the switching frequency for the Raster Voltage Generator power supply. The circuit is designed around voltage regulator U201.

The voltage regulator oscillator frequency is set to about 52kHz by the Raster signal. This signal is sent from the Display Gate Array on the Main PCB to Q201, causing Q201 to conduct and pass the signal to U201 pin 5. Resistor R217 and capacitor C203 function as an RC network, slowing the current at power-on. Resistor R209 decouples the +15V supply voltage so that display size will be minimally affected by changes in the +15V supply.

Input overvoltage protection is provided by Q201, Q202, and D201. If the input voltage rises to 18V or greater, D201 starts to conduct. This signal forward-biases Q203 which routes it to the gate of Q202. When Q202 conducts, it inhibits the internal oscillator in U201 by applying +5V to the input at pin 4.

Vertical Size, R224, controls the width of the pulse from the output stage of U201 which determines the amount of energy delivered to transformer T301.

Raster Voltage Generator

The Raster Voltage Generator supplies the input voltage for the primary of T301 as well as overall CRT bias voltages. The input signal for the circuit comes from the output of voltage regulator U201 pin 10 in the Raster ScanOscillator circuit.

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