Barco GRAPHICS 1209S, Service Manual

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Date: 10/02/98 R762518
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R762518
GREEN Convergence module
Convergence DRIVER module
R7625128
Preparation of the waveforms .
The voltage comparator IC16 (5-6-7) transforms the sawtooth into a squared Horizontal
'Switch Pulse' of 24 V
pp
(+ / - 12V). This switch signal is used to clamp either the
sawtooth or the parabola during the first or second half of the horizontal scanning. Note
that the clipping level is variable with the voltage that drives the current source of the
sawtooth generator. The multiplier IC20 (AD633) generates a horizontal parabola HP+
.
The next OPAMP shifts the DC level by clamping the middle of the parabola to a voltage
adjusted by P5 (approx. 0 volts) in order to compensate the tolerances of the multiplier.
The convergence at horizontal frequency with the ramp and parabola waveforms in the
zones 11 - 14 are adjusted in IC1-IC2-IC601-IC602.
The waveforms are each time clamped with a clamping transistor served by the
horizontal switch pulse. Two opposite phased waveform are each time applied to the
potentiometers in the Bella's (digital potentiometer or D/A convertors).
Vertical axis convergence The vertical sawtooth generator is generated in a similar way as the horizontal sawtooth
generator, discussed above. The much lower frequencies here allow the use of the
MUXDMUX IC46 (4053) in stead of the clampers.
The "Vertical Switch Pulse" is passed through the RS flip-flop IC39 which is clocked
with the pulse 1 in order to make the transition coincide with the trigger or the start of
the horizontal sawtooth. This avoids a jitter for interlaced signals.
North-South / East-West
and Midline corrections .
For these corrections we need horizontal waveforms with an amplitude depending on
the vertical position or vertical scanning. The horizontal waveforms must be modulated
on a vertical sawtooth or parabola. This modulation is performed by IC29 (MLT04). The
four different waveforms are all applied to the "X" and "Y" inputs and the modulated
waveforms are the "W1 - W4" pins.
The corrections , called GEOMETRY corrections, are added to the CORNER and AXIS
corrections in the " Summing Amplifiers ".
East - West corrections :
Two kind of corrections (referred to as "skew" and "bow") are required to correct the
projection angle and aberrations of the optical system.
The HSVS ( H orizontal S awtooth modulated on a V ertical S awtooth) and HSVP are
used for this purpose.
The HSVS is first amplified with an OPAMP in IC30 and then clamped with Q11 and
Q10 which are 'served' with the "R" and "S" switching signals. The LS+ and LS- (Left
Skew) and RS+ / RS- (Right Skew) are fed to two digital potentiometers in IC13 and
the adjusted outputs are sent simultaneously to the three convergence coils.
HSVP is first inverted and amplified and then clamped during the first or second half
of the scan. The RB+/- and LB+/- are applied to the digital potentiometers in the same
IC13 and the outputs also feed the three convergence coils.
North - South Corrections .
Two type of waveforms are modulated on a vertical sawtooth VS, Horizontal Sawtooth
(HS) and Horizontal Parabola (HP).The HSVS from IC29 (sheet 3) is capacitively
coupled to the MUX DMUX IC46 (sheet 2) and in stead of clamping, the signal it is
Technical description