Genius ISO XFR-100W, ISO XFR-75W Vertical Oscillator, Yolk, Voltage Feedback, Current Feedback

VERTICAL DEFLECTION CIRCUIT, FUNCTION, DESCRIPTION.

The LA7851 IC is used for the vertical oscillator. The LA7838 is a vertical deflection control and high efficiency vertical yoke driver IC. Together they form a compact and efficient vertical deflection system.

Vertical Sync		SIMPLIFIED VERTICAL DEFLECTION CIRCUIT								+23V to +27V
Vertical Oscillator	2	One		Current			Retrace	9
							Booster
LA7851		Shot	*	Source				13
								12
		5V Clamp				Drive	Output
Vertical
											Vertical
Linearity				+6V			LA7838
Fine Tuning											Yolk
				4	6	7
6V	Ramp Control Circuit								Voltage Feedback
Vertical					1uF		393	390		+
											449
Size							392
		375			401
				403		388
							391	Current Feedback			385

Clamp Enable Pulse *

Voltage on 401

Yoke Current

Yoke Drive

The vertical oscillator in the LA7851 supplies timing to the vertical deflection IC to maintain a raster with no sync present. Vertical sync supplies the timing when sync is present.

The one shot in the LA7838 clamps the ramp forming capacitor 401 to 5V during the first half of vertical retrace.

The ramp forming capacitor is supplied with current by a current source at pin 6. The current source has a fixed 6 volt input voltage at pin 4. A linear ramp is generated if a fixed resistor is connected from pin 4 to GND. Feedback from the yoke current, via resistor

403, is used to modify the linear ramp which helps correct for nonlinearity introduced by the voltage feedback circuit connected to pin 7. The vertical size control is connected to the current source input since adjusting the slope of the ramp adjusts the vertical size. This ramp with the clamp, as the discharge, produces a sawtooth waveform which is connected to the + input of the vertical control differential amplifier at pin 6.

The combination, voltage and current, feedback circuit senses the parabolic waveform on the yoke coupling capacitor

449and is connected to the yoke current sense resistor 385 . This circuit is then connected to the other input of the differential amplifier at pin 7. A capacitor

391smoothes the parabolic waveform and a voltage divider is used to set the output bias voltage. The time constant, of the capacitor 391 and resistor 392 , is set to produce good vertical linearity. An additional linearity correction circuit is added to fine tune vertical linearity. This circuit can be set to add or subtract deflection from the upper and lower portions of the raster.

The differential amplifier controls the power output stage which drives the vertical deflection yoke. The retrace booster is turned on when the ramp voltage is set to the clamp voltage and is reset when the yoke feedback voltage balances the ramp voltage.

79