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The top pulse mfg. A block compares the input ramp to a DC voltage to
produce a pulse. By changing this DC voltage, the pulse width can be
controlled for more or less dynamic focus correction. More correction is
required during a wide picture so a low input voltage produces a wider low
going pulse.
The second block is a current buffer (Q1501 and Q1510) that feeds the
pulse to driver Q7001 in the third block. The driver amplifies and inverts
the pulse so that a wider low going pulse results in longer conduction time
for Q7001 and consequently a higher supply voltage.
The fourth LPF block consists of a coil and capacitor that filters the pulse
into a DC voltage. This supply voltage is applied through DFT T7001's
primary to driver Q7012.
Bottom Circuit Blocks
The pulse mfg. B, inverter/buffer and Q7012 driver circuit blocks along the
bottom of the dynamic focus block diagram is discussed next. The pur-
pose of these blocks is to make a single pulse that identifies the left and
right sides of the screen for dynamic focus correction.
This location pulse is manufactured from the pulse manufacture B block.
A ramp and reference voltage are input to the first block. A pulse is cre-
ated that begins when the electron beam is at the right side of the screen.
After the beam has retraced and ventured into the left side of the screen,
the pulse ends. This pulse marks the area that dynamic focusing takes
place (at the left and right sides of the screen).
The second block inverts this location pulse and buffers it for voltage/cur-
rent gain. The third driver block amplifies this pulse and applies it to the
Dynamic Focus Transformer T7001. In this output stage, the location
pulse is shaped into a sine wave by L/C components. This sine wave is
coupled by T7001 to the DC focus voltage from the FBT, modulating it.
The modulation increases the focus voltage when the electron beams are
at the left and right sides of the screen.
115
Output Waveform
A lower DC voltage of 71Vdc in normal picture width causes just the peaks
of the bottom waveform to be added to the focus voltage for minimal focus
correction. In a wide width picture the focus point must be moved further
so a larger 117-supply voltage is produced. The higher voltage increases
the amplitude of the sine waveform for greater focus correction.
The resultant FV focus voltage is now a dynamic voltage that adjusts the
focus point to match the center, left and right sides of the flat TV screen.
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