Vertical Deflection Circuit Description And Schematic - Ceronix 1493 Service Manual

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
2
Vertical Oscillator
LA7851
Vertical
Linearity
Fine Tuning
Ramp Control Circuit
6V
Vertical
Size
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
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
, is used to modify the linear ramp which
403 403
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.
SIMPLIFIED VERTICAL DEFLECTION CIRCUIT
Current
One
Source
Shot
*
5V
Clamp
+6V
4 6
375
403
*
to 5V during the
401 401
Retrace
Booster
Drive Output
LA7838
7
393
1uF
392
401
388
391
The combination, voltage and current,
feedback circuit senses the parabolic
waveform on the yoke coupling capacitor
and is connected to the yoke current
449 449
sense resistor
connected to the other input of the differential
amplifier at pin 7. A capacitor
smoothes the parabolic waveform and a
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voltage divider is used to set the output bias
voltage. The time constant, of the capacitor
and resistor
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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.
+23V to +27V
9
13
12
Voltage Feedback
+
390
Current Feedback
. This circuit is then
385 193
, is set to produce good
392 392
Vertical
Yolk
449
385
79