Barco GRAPHICS 1209S, Руководство по обслуживанию

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Horizontal Deflection
13
Date : 10/02/98 R7621085
R7621085
Technical description
HORIZONTAL DEFLECTION AND SCAN SWITCHING
MODULES BG1209S (R7621085 AND R762457)
Introduction. On this board we find the MOSFET switching transistors, acting as switches to start
and stop the currents through the deflection coils. As we need a very short retrace time,
the amplitude of the flyback pulse is bigger than the maximum of one Mosfet. Therefore,
two Mosfet switches are used in series. On the other hand, the amplitude of the current,
especially in the mode where the two half windings are in parallel, is so high that the
scan current must be divided over two Mosfets in parallel.
The very high scanning computer sources require a very short retrace time and thus
blanking time of the projector. The lower scanning frequencies do not require such a
short blanking time.
This is obtained by switching the two horizontal deflection windings of the deflection
yoke unit in parallel (we further call this the “Higher Freq Range” ) for the high
scanning frequency.
For the lower scanning sources however these same two windings on the yoke are
connected in series in order to double the retrace time compared with the parallel
connection (further called the “Lower Freq Range " range). This switching of the yoke
configuration is accomplished with three relays on the “Deflection Switching Module”
mounted on the CRT unit cover.
When switching from the low range to the high range, or vice versa, the deflection
vHTHD on the SMPS MUST be switched off (or at least reduced) the moment
the relays are active.
The circuits which inhibit the deflection voltage and the drive circuit which control these
relays are located on the Subunit of the Hor Defl. board.
Furthermore, on this board , we find the required protection circuits such as “scan hold
down” and “ scan failure”.
The horizontal deflection circuit uses two MOSFETS in series in order to be capable
of handling more than 1000 volt pulses. Therefore, two drive pulses on different ground
reference levels are required.
The “bottom” MOSFETS are driven by a pulse train referenced to ground or chassis
ground, the “top” MOSFETS by drive pulses referenced to the mid-point of the two
series connected MOSFETs, the HDM point. The Vcc of each opto-coupler must
therefore have a different ground reference.
The power supply for generating the “top drive pulses” is taken from the +17 volt via
diode D33 to block the pulses, as HDM, the reference ground for the top drive pulses,
carries line pulses.
The hor. drive pulses, prepared on the “UN SYNC+VERT DEFL ” board, are sent to the
amplifier Q13. By using a transformer T2, a "floating" drive pulse referred to HDM for
the top Mosfets can easily be obtained.
When the flyback pulse is present during retrace, D33 becomes reversed biased and
act like an open circuit to the 17VDC line. At that time, the drive circuit receives its
voltage supply from the charge stored in C48.
The "high" drive pulses reach the gate-source of the top Mosfets, and the "low" drive
pulses drive the bottom Mosfet switches.
Preparation of the drive
pulses.