Vertical Deflection

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Vertical Deflection

• Review question How many horizontal lines are
  traced on the CRT image display for each VERTICAL
  scanning period? (top to bottom of display)
• 2 1 interlace for NTSC, PAL and SECAM
• Two types of vertical sweep circuits 1.)
  Oscillator or sweep generator (older units) and
  2.) Digital countdown circuits.

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Vertical Deflection contnd.

• OSCILLATOR TYPE
• Free running oscillator develops a 60 Hz sawtooth
  voltage waveform.
• Requires an in phase feedback path to maintain
  oscillations.
• Oscillator frequency can be adjusted usually by a
  variable component. (vertical hold circuit)
• A sync pulse is required to lock the vertical
  deflection oscillator. What is the origin of the
  sync pulse?

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Vertical Deflection

• OSCILLATOR TYPE contnd.
• Also requires a linearity control and height
  control
• Power output transistor drives the yoke.
• PINCUSHION distortion. Remedy a.) magnets for
  monochrome and b.) small amounts of vertical and
  horizontal feedback voltage for chroma systems.
• CONVERGENCE

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Vertical deflection contd.

• COUNTDOWN SWEEP GENERATOR TYPE
• A single crystal oscillator frequency is reduced
  to required H and V by frequency dividers.
  (inherent stability)
• Ex. 503.5 KHz to 31,469 Hz by divide by 16 and an
  additional divide by 2 brings it to 15,734 Hz.
  The output of the 16 divider is also reduced to
  59.94 Hz.
• Phase detection

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Vertical Deflection contnd.

• Vertical power output stage
• - requires a driver stage
  (usually)
• - power for a complete
  raster
• - class A amplifier or
  Class B push pull.
• - impedance matching with
  the yoke (therefore
• a low Z yoke)
• - yoke requires a
  trapezoid voltage waveform
• to produce a linear
  sawtooth current due to
• XL and R of yoke
  inductance.(linearity)
• - damping resistor
  across yoke

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Vertical Deflection contnd.

• YOKE COILS Horizontal (top and bottom)
• Vertical (left and
  right)

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Horizontal Deflection

• Tasks of the Horizontal Section
• 1. Develop deflection
  voltage for the yoke.
• 2. HV and focus voltage
  for picture tube.
• 3. Scan derived D.C.
  voltages for other circuits.
• 4. Sample voltages for
  horizontal blanking,
• pincushion correction
  and convergence.

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Horizontal Deflection contnd.

• Horizontal sweep generator (oscillator or
  countdown) operates at 15,750 Hz (15,734 Hz)
  locked to source.
• Requirements for a frequency and phase lock
• - horizontal sync pulses from a
  source
• - horizontal AFC circuit (HAFC)

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Horizontal Deflection contnd.

• Components of the Horizontal section
• 1. Horizontal oscillator (or crystal for
  countdown)
• 2. Horizontal Driver
• 3. Horizontal Output (Class C)
• 4. Feedback path (to implement sync lock
  mechanism)
• 5. AFC circuit (HAFC)
• 6. Flyback transformer
• 7. Tripler
• 6 and 7 part of IHVT

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Horizontal Deflection contnd.

• AFC SYSTEM controls frequency and phase of
  horizontal oscillator or sweep generator.
• What are we comparing? Sync pulse from a
  broadcast station and a reference signal
  generated locally.
• PHASE DETECTOR compares the reference signal with
  the incoming sync pulses for frequency and phase
  error.
• If they do not agree a d.c. error voltage is
  developed.
• The d.c. voltage is applied to the oscillator
  causing it to speed up or slow down.