Flyback Generator - Loewe Q2500B Serviceanleitung

Technology of Q 2500 colour TV set
Vertical output stage
V-Flyback
W 511
R563
C564
Pin 4
6
D561
2
UV+
13V
4
UV-
-13V
These two control signals contain all vertical
correction information. This means that output
side S correction, which is necessary for a.c.
coupled output stages, can be dispensed with
completely. The component insert at the out-
put of the V output stage is also considerably
reduced. Symmetrical control almost fully
suppresses interference that can affect the
two V control signals due to the very high
common mode rejection ratio of the V-power
operating amplifier. The two V control signals
V+/V- are applied to the pin connector W 511,
pin 2/3 and reach the series resistors R 566
/R 578 via the inputs on pin 1/7 of the V output
stage I 561. The sawtooth, which is amplified
about 10 times, is output by pin 5 of I 561 and
forces the current through the vertical deflec-
tion coils.
The R/C combination 568, from the output of
pin 5 to earth, prevents the tendency of the
output stage to oscillate and protects it from
switching peaks that can be caused by the
deflection coins. To stabilise the V output
stage, part of the sawtooth from the base of
the deflection coils is fed back via resistor
R 579 to the inverted input pin 1 .
Dokument Q 2500
U-SVM
R559
C561
3
I 561
Flyback
Generator
STV9379FA
Picture formation is normally established by
the d.c. current component of the deflection
current. This means that for a d.c. coupled V
output stage, by simply changing the d.c.
components of the control sawtooth, the d.c.
component of the deflection current can be
changed, and with it the picture formation.
2.6.1

Flyback generator

The job of the flyback generator is to provide,
the switching voltage for the voltage increase
during the vertical flyback. The problem with
this is as follows:
The energy requirement of the vertical output
stage is at its highest during the flyback, as
the electron stream has to be directed rapidly
from the lower right corner of the screen to the
top left corner.
This transitory additional energy requirement
is achieved by doubling the operating voltage,
which is only available for the vertical output
stage. During the vertical trace, the bootstrap
capacitor C 563 is charged to approx. 26 V via
D 561. The output of the flyback generator at
pin 3 of I 561 at this point is UV-/-13 V. At the
time of flyback start, the flyback generator is
switched by the output stage output at pin 5
and applied to the output at pin 3 UV+/ +13 V.
Due to the d.c. shift on the minus pin of ca-
pacitor C 563, the operating voltage for the
output stage on pin 6 increases by the voltage
in C 563. At the same time, D 561 is blocked,
which prevents the charge in the power supply
leaking away. This means that for rapid fly-
back there is a transitory +40 V (approx.) op-
erating voltage available.
For many c.r.t. types the flyback pulse is not
sufficient to fully return the deflection beam,
so that flyback lines are visible.
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