Barco GRAPHICS 1209S, Руководство по обслуживанию

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Date : 10/02/98 R7621127
Sync+Vertical Deflection R7621127
16
Technical description
TECHNICAL DESCRIPTION “UN SYNC + VERT DEFL “ (7621127)
Introduction. On this board and its sub-unit we find the sync separator, the horizontal autolock
circuit, the vertical oscillator and the vertical output stages.
The generation of the waveforms for the east-west correction signals, the left / right and
top / bottom blanking and the composite blanking.
The horizontal drive pulses, with its phase adjustment, for the MOSFET switchers on
the HOR DEFL module are also prepared on this board.
A Barco designed IC, called the “BELLA” , comprising four digitally controlled
potentiometers, is utilised for the adjustment of the amplitude of the waveforms, or for
the adjustment of some DC control voltages. This IC, custom made for Barco, is driven
by an I²C (serial data) bus from the microprocessor on the controller board.
a) Vertical Sawtooth Generator
The vertical sawtooth relaxation oscillator is built around the darlington Q403 / Q404
, C464 and the current generator Q408 (all on the subunit of the Vert Sync module).
The current that charges up C464 is simulated by another vertical relaxation oscillator
built around Q413 / C467 / Q409. The amplitude of the sawtooth of this oscillator is
maintained stable, irrelevant the vertical frequency. In order to stabilize the amplitude
regardless of the vertical frequency, the charging current must increase with
increasing vertical frequencies. The obtained current is then 'mirrored' to the real
oscillator.
The initial (minimum) charging current of C467 is taken from the +12V via R525.
Additional current is provided by the OPAMP output through the diode D411. This
output voltage is determined by the voltage difference between the pins 5 and 6.
Pin 5 voltage is adjusted with P401 and determines the "gain" of the loop system.
This is the current versus frequency behaviour. P401 is adjusted to obtain locking
for the full range of vertical frequencies (see Service manual).
The other input , pin 6, is a reference for the 'average' amplitude of the simulated
sawtooth. Therefore, the sawtooth across C467 is buffered and the average amplitude
is measured by the Miller integrator of IC407.
The output voltage changes as long both inputs have different potential, and settles
down as soon this condition is fulfilled.
The voltage adjusted by P401 determines consequently the average current of the
generators for one typical vertical frequency.
In free run ( no sync applied), the ramp voltage is limited by Z400. The OPAMP would
then go into saturation (in the free running mode) and the output drops to approx. -
10V. As soon "V drive" is applied the voltage now must increase from - 10V to around
+6 volts before D411 gets forward biased, as from that moment onwards the current
'follows' the frequency. That would take too long and therefore the output voltage of
the OPAMP is limited because Q407 gets in conduction as soon the OPAMP output
is 6 volts. Q406 is turned on then and the input current is deviated via this
transistor to avoid saturation of the output.
b) Synchronisation of the vertical oscillator
The TDA2595 performs well as sync stripper for videosignals because of its noise
integrating input. Such a (delaying) input circuit however has a bad performance for
computer signals. The LM1881 is better suited for computer sync signals and
1. Vertical oscillator -
Vertical pulses
(on the Subunit)