Barco GRAPHICS 1209S, Service Manual

Page: 110 / 378

Date: 10/02/98 R762720
R-G-B Driver R762720
15
Technical description
one. This is got by swapping the Y1 and Y2 inputs of the multiplier IC403.
If we assume again a linear ramp input, the parabola of the output W of IC403 is
negative. The final result at output pin 6 of IC404 is a non-linear ramp with an increased
amplitude in the grey zones.
b) Blue correction circuit.
The blue phosphor saturates from some drive voltage onwards. This saturation point
depends on the CRT phosphor, the spot size, frequency of scanning, temperature,
etc...
To resolve this problem, the blue signal undergoes an increased gain from some level
onwards (the breakpoint level) .
This level can be adjusted by software and is the BLUE_BRKPNT voltage. As soon the
emitter voltage of Q402 exceeds the BLUE_BRKPNT level with 0.3V diode voltage
drop), the diode D412 gets forward biased and R456 is added to the emitter load. The
slope of the correction is further adjusted by means of the BLUE_CORR 1,2,3 voltages.
This will add to the emitter load one of the feedback resistor(s) R454, R455, R456 or a
combination of these.
c) DC stabilisation, additional clamping.
Without further measures, the DC level of the emitter voltage would not be very stable.
It is very much temperature dependent and it varies with the bias current through the
stage. An additional clamping is required.
This clamper compares the output voltage with the emitter voltage during the CP timing
window. The output voltage drives Q407 which is part of the biasing resistor (drain
connected to -12V).
Q408 provides a lower impedance for the source of Q407, as the current flow through
R437 and Q407 depends also on the video signal.
Blanking. The composite blanking pulses are the result of the sum of different individual blanking
informations added together in the NAND IC104 after modelling to the correct TTL
amplitude.
If there is no blanking necessary, the output pin 8 of the NAND is low level and this
forward biases Q107 and Q109. The current flowing through the latter installs around
+11V at the BL output.
In case of blanking, the TTL high level output of IC104 forward biases Q108 now and
through Q110 the BL output is at around -16V.
D114 - D117 avoid saturation of the transistors and hence improve the switching.
Following blanking informations are an input to the NAND gate IC104 :
- LP (line pulses) :
are dropped in amplitude with R116/R117/R140 limited with D102/D103, inverted
by IC103 (5,6) and input to pin 1.
(note : the same negative line pulses are inverted again with IC103(1,2) and then
referred to as HFB to be used in the optional Contrast modulation board.
- V BL (Vertical Blanking) :
these pulses are also dropped to TTL level, inverted and applied to pin 2.
- SPOT_KILL ( pin 5) :
The line SPOT_KILL is +12V during operation and drops to zero level at switch-
ing off the projector to blank the picture.