Overbeam Current Fuse - Loewe Q2500B Serviceanleitung

Technology of Q 2500 colour TV set
The switch to limit the contrast and brightness
is controlled with a d.c value via pin 43.
The switch is designed in such a way that for
a voltage of >3.3 V there is no limitation on
contrast and brightness.
For a voltage between 3.3 and 2.2V the con-
trast is reduced in proportion to the voltage. If
the voltage drops below 1.8 V the brightness
as well as the contrast is reduced. At approx.
1 V on pin 15 the brightness and the contrast
are reduced by 100%, whereupon the screen
is black.
To achieve beam current limitation the base of
the diode split transformer influences the d.c.
value on pin 43 of TDA 9332. Inversely pro-
portional voltage information on the beam cur-
rent can be obtained at the base.
In order for the TDA pin to be at >3.3 V with a
beam current of zero, the base of the trans-
former is connected to U 200 via R 551/552.
The positive voltage in this operational state is
U3.3
D2596
LL103C
D2618
LL4148
R2586
22k
2.7.1

Overbeam current fuse

In correct operation the beam current meas-
urement input on pin 43 of TDA is at a d.c.
level of 2 to 4 V. According to the level, the
RGB output amplifier in this IC is controlled to
Document Q 2500
Overbeam current protective circuit
C2618
100n
R2618
R2617
10k
220k
Q2616
BC847BW
C2616
1u
charged via resistors R 2608, R 2609 and
R 2611 of Elko C 2603 and on pin 43 of the
TDA there is a voltage of >3.3 V; there is no
beam current limitation.
As the high voltage winding in the diode split
transformer operates with a virtual earth, the
base becomes negative with increasing beam
current, whereby C 2603 via R 2608, R 2609
and R 2611 can be correspondingly dis-
charged to a greater or lesser degree and the
beam current controlled.
Diodes D 2607 and D 2611 are provided so
that the circuit can also react to jumps in
beam current. They ensure the rapid dis-
charge of C 2603.
In addition, for a 4:3 display in a 16:9 TV set,
from pin 113 of I 2311 via Q 2612 and R 2601,
the voltage on C 2603 drops. This corrects the
beam current limitation for the smaller screen
area. (see also signal board deflection).
C2608
100n
R2616 R2608
220k 22k
a greater or lesser extent, which ensures that
the beam current limitation is implemented.
If the c.r.t. is controlled upwards to the full
extent, even though the gain of the amplifier
has been fully reduced , which can occur if
there is a fault in the RGB output stage or its
33
100R
R1043
100R
C1043
1n
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