Barco GRAPHICS 1209S, Service Manual

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Date : 10/02/98 R762271
Electromagnetic Focus+Hor Shift R762271
15
Sub module R762271S
Technical description
TECHNICAL DESCRIPTION “FOCUS - SHIFT”
Introduction. Two completely independent functions are performed on this board and its subunit, the
electromagnetic focus and the horizontal shift.
On the main board we find the Horizontal shift circuits and the power stages to drive
the focus coils.
The preparation and adjustment of the waveforms necessary for the focus coils are
accomplished on the sub-unit.
To obtain an independent left/right and top/bottom adjustment of the electromagnetic
focus, the waveforms are clamped during one half of a scan period.
principle.
In this form of focusing, a strong magnetic field produced by an electromagnet or a
permanent magnet mounted behind the yoke on the neck of the CRT forces diver-
gent electrons to take a helical (coil-like) path to reach the CRT faceplate. The
helical path is usually only one turn long. With the proper magnetic field strength
oppositely divergent electrons will move forward along coil-like paths that rotate in
opposite directions and meet at the phosphor screen, thereby bringing the elec-
trons together again to form a small spot.
a) At line frequency :
A sawtooth generator is built around Q300 / 301. C302 charges up via the variable (line
tracked) current generator Q300, and discharges via Q301 when a horizontal pulse
is sent to its base. This pulse starts slightly before the end of the horizontal scan in order
to obtain sufficient energy at the start of the horizontal scanning. The trigger pulse is
generated in two steps by the two monoflops of IC401. By an integration ( = averaging)
of the pulse train at the output pin 13, an automatic tracking of the trigger moment with
the line frequency is got.
C302 then charges, and its rate of charge is determined by the condition of the current
generator Q300. Q300 is supplied with the + FOC us voltage from the SMPS, which
is in some way proportional to the line frequency. A vertical parabola is added to this
charging current via C422.
Because of this, a tracking of the amplitude sawtooth signal with the line and vertical
frequencies is developed.
The sawtooth signal is now applied to :
- a multiplier IC301 in order to generate a symmetrical parabolic waveform,
regardless of the line frequency.
- a level detector in IC307 pins 3, 2 and 1 to produce a left-right squarewave which
is buffered by Q307 and feeds the switchers-clampers Q309 and Q310 with a
correct DC level (note that Q307 is supplied with + / - 12 volts).
- the inverting input of another level detector of IC307, pin 6, to produce a left-right
squarewave (DRIVER) opposite in phase to the one on pin 1 of IC307. This
squarewave is used on the main board to boost up the supply voltage of the line
power amplifiers during the second half of the horizontal scan (see further).
The parabolic signal from the multiplier, IC301 is buffered with Q302, clamped and
buffered. The signal then goes to the non-inverting inputs of two voltage-followers in
IC309 (MC34084).
A. Electromagnetic Focus