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LA7851
Vertical Oscillator
Vertical Sync
LA7838
One
Shot
Clamp
5V
Current
Source
401
1uF
Retrace
Booster
385
Vertical
Yolk
+
390393
392
391
388
Linearity
Fine Tuning
Vertical
403
375
6V
+23V to +27V
Vertical
Size
Ramp Control Circuit
46 7
12
13
9
2
+6V
449
SIMPLIFIED VERTICAL DEFLECTION CIRCUIT
Current Feedback
Voltage Feedback
Drive
Output
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.
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
449 and is connected to the yoke current
sense resistor 193 . This circuit is then
connected to the other input of the differential
amplifier at pin 7. A capacitor
391 smoothes 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.
Clamp Enable Pulse
Yoke Current
Yoke Drive
Voltage on 401
*
*
401
449
385
392391
391
79
403