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Vertical oscillator must convert the vertical sync pulse into a 50% duty cycle square wave for the horizontal output oscillator. Most chassis are based off of a heartly style voltage controlled oscillator with the flyback pulses and sync pulses being used to adjust the voltage going into the oscillator. if sync pulse is slow or fast the voltage raises or lowers changing the frequency.

The flyback circuit consists of a LC circuit and horizontal output transistor (HOT). When the transistor turns on current begins to build in the circuit relatively uniformly until the HOT turns off. When the HOT turns off the magnetic field collapses and sends all of its energy into the fly back capacitor. this then causes a spike in voltage causing the coil to begin conducting in the opposite direction. This is then repeated as the HOT turns back on.

Waveform of flyback circuit. Red = 15khz square wave driving HOT. Green = current going through coil. Blue = voltage at HOT Drain.

Example schematic of flyback circuit.

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