East/West Correction - Loewe Q2500B Serviceanleitung

Technology of Q 2500 colour TV set
A free DAC in I 2271 is used for horizontal
offset correction. The d.c. voltage felt on pin
55 can be set in the servicing mode. This is
fed via connector W 511 /W 1511, pin 8, to the
base of transistor Q 533 on the basic board.
Q 533 together with R 555 represents a cur-
rent source for the push-pull stage Q 531 and
Q 532. The operating point for the push-pull
stage is set with R 537 and the parallel circuit
of R 535/538/539. At rest the operating volt-
age of the line output stage is applied to the
base connections of Q 531/532 and both tran-
sistors are blocked.
If I 2271 increases the base voltage of transis-
tor Q 533 and this then conducts, the base
voltage of the push-pull stage drops and Q
531 conducts. This means the d.c. current is
made more negative by resistors R 516,
R 517 and R 518 and coil L 530, whereby the
deflection shifts to the left.
If, on the other hand, the voltage on pin 27 of
I 2271 drops, transistor Q 533 becomes high
resistance and the base voltage on the push-
pull stage increases. This means Q 532 con-
ducts and L 530 increases the voltage on pin
2 of the deflection coil. This leads to a shifting
of the d.c. current component in a positive
direction and deflection to the right. Coil L 533
serves as d.c. current coupling for the deflec-
tion.

2.5 East/west correction

In order to compensate for the pincushion
(distortion) in 110° units in an east/west direc-
tion, the horizontal deflection current in the
vertical centre must be increased with respect
to the vertical start and vertical end. The right
degree of correction is achieved by influencing
the horizontal deflection current with a vertical
frequency parabola in the east/west diode
modulator.
Document Q 2500
Two generators connected to each other by a
bridging circuit provide the correction signal
for the E/W modulator for the horizontal de-
flection current.
G1
C1
L1
C2
L2
G2
UB2
In the above drawing the two generators, G1 =
horizontal generator, and G2 = east/west gen-
erator, are represented only by their resonant
circuits. The deflection current in the deflec-
tion coil L1 is supplied by both generators. So
that the high voltage is not influenced by the
E/W modulator, the voltage on transformer TH
is determined only by the horizontal generator
G1. Therefore, for correct bridge compensa-
tion during the line trace, only E/W modulation
of the deflection current takes place in an al-
most non-reactive fashion on the horizontal
output level. For improved clarity the concep-
tual flow diagram above is shown in the fol-
lowing drawing with the actual component
locations.
26
D1
S
TH
D2
UB1
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