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computer monitor analyzing Understanding Horizontal Stages Of Multi-Frequency CRT Video Displays Fifth In A Series Of Articles Covering Multi-Frequency Horizontal Stage Analyzing omputer monitors and other multifrequency displays change operating I4 modes for higher display resolution. Changing modes increases or decreases the operating frequency of the horizontal output stage(s). Keeping the CRT high voltage and yoke current constant with horizontal frequency changes requires altering the operation of the horizontal output stage. This article looks at how the horizontal frequency impacts the high voltage and deflection produced by horizontal output stages. It further examines how multi-frequency monitors alter the operation of the horizontal output stage to regulate the high voltage and/or deflection current. frequency, consider the conduction time of the H.O.T. during the horizontal cycle. Recall that this is the time during the horizontal cycle that all the energy to produce high voltage and/or deflection is input to the output stage (see Fig. 1). When the H.O.T. is switched on, the B+ supply produces a rising current in the inductive transformer or coil winding. The rising current builds a magnetic field in the coil. The intensity of the magnetic field depends on the current buildup. The input energy (magnetic field intensity) produces induced voltage and alternating currents in the horizontal output stage during the remainder of the cycle. The level of induced voltage and alternating currents determine the high voltage and yoke deflection. For ease in comparing input energy at different operating frequencies, consider the rise in current in the flyback or coil to be linear. In normal operations, the current rise is not perfectly linear due to the resistance and inherent capacity of the flyback or coil. For example, with a frequency of 15 kHz, a B+ supply voltage of 100 volts may produce a flyback primary current reaching a 2 amp peak (see Fig. 1). The H.O.T. normally conducts approximately l/2 of the horizontal cycle. With a linear current increase and 50% conduction time, the average current in the coil calculates to 0.5 amps (5OW). A portion of this energy is transferred to the flyback secondaries and retrace capacitor Ct to produce high voltage and deflection current. 19 How Frequency Affects High Voltage/Deflection In The Horizontal Output Stage To understand how energy to the flyback transformer or coil changes with Fig. 1: Increasing the horizontal frequency reduces the average flyback or coil current and input energy to the horizontal output stage. Sencore N e w s 182 computer monitor analvzina With a drive frequency of 30 kHz, the conduction time of the H.O.T. each cycle is cut in half. The reduced conduction time reduces the current buildup to a 1 amp peak with the same applied B+ voltage and inductor. In comparison to 15 kHz, the H.O.T. completes two conduction cycles, but the average current calculates to approximately 0.25 am