Barco GRAPHICS 1209S, Service Manual

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Horizontal Deflection
14
Date : 10/02/98 R7621085
R7621085
Technical description
The +HTHD voltage from the SMPS board is modulated in Q3 by the East-West
correction signal prepared on the “Sync + Vert Defl” board. Z2 protects the transistor
and SR2 limits the charging current of this coupling capacitor through the zener. Q2
transistor is used to discharge the boosting capacitor C31 at the start of a vertical scan.
A vertical flyback pulse, derived from VDH B (Vertical Deflection High Blue) is sent to
the gate. This minimizes keystone problems at the top due to a remaining charge on
C31 after the vertical retrace. It causes the charge on C31 to always start from the same
amplitude after each vertical retrace, regardless of the voltage that was built up at the
end of the vertical scan.
Modulation of the scan
voltage /
East-West correction
The problem we meet with such a big frequency range, is the frequency dependent
characteristic of the linearity coil. At a higher scanning frequency, the impedance of
the linearity coil would increase.
To overcome this, a second coil T1 is magnetically coupled to the standard linearity
coil. This current in this modulating coil is delivered by a Mosfet Q1.
The needed current for tracking is got via the biasing circuit of the gate of Q1 (LIN HIGH)
as follows.
The drive pulses trigger a one-shot in IC1 at the positive going transient input. The output
pulses are then applied to the gate of a Mosfet Q8 and at the drain split to two circuits
:
- the simple integrator R20 / C3, the obtained voltage across the capacitor is
consequently a voltage proportional with the line frequency labelled "LIN
REF".
- the push-pull Q5 / Q6 and the top/top detector just to obtain a negative voltage
to supply amongst others the OPAMP IC2.
The DC level of this LIN REF voltage is not correct to drive the Mosfet Q1 and a level
shift is realised with the OPAMP 1-2-3 of IC2.
This OPAMP receives at the inverting input a voltage that is proportional with the line
frequency, the amplitude adjustment does not affect this LIN REF voltage.
The other non-inverting input receives a voltage that is proportional with the scan
voltage. This voltage is proportional with the line frequency and with the amplitude
adjustment. The influence of the amplitude adjustment must be minimized and this
done as follows.
For one typical frequency, we obtain one typical LIN REF voltage. The HTHD voltage
however depends also on the horizontal amplitude. Any change in the emitter voltage
of Q4 is compensated via the feedback Q7 - base Q4.
Horizontal linearity track-
ing control.
Corrections in the "Higher
Freq. Range".
(on the subunit)
In the "High Freq. Range" mode, due to the change of the flyback time and the
appropriated scan voltage, adaptations on different line frequency dependent circuits
are required.
The HFM (Hor Freq. Mode) voltage, coming from the " UN SYNC + VERT DEFL" is
compared to the level set by R31/R32 in the level detector 5-6-7 of IC2.
The output changes from high to low or vice versa are detected by IC1 to turn off the
scan voltage (see later).
The output level is further inverted by a similar detector and the output pin 1 is used for
different functions :
a) To switch on and off Q45. When this transistor is "on", the relays on the "Deflection
Switching" module are "activated". These relays realize the commutation from
series to parallel. The same transistor also "serves" the relay "REL1" on the HOR