Vertical Protective Circuits - Loewe Q2500B Serviceanleitung

Technology of Q 2500 colour TV set
In these devices the V output stage of
STV9379FA is used. For the flyback generator
this has an independent voltage supply. A
USVM/60V is used that is fed to the I 561 via
the fused resistor R 559 on pin 3.
If the V output stage is changed during service
then ensure it is replaced with the same type.
The two IC variants are not compatible.
2.6.2 Vertical protection circuits
Protection circuits are used to protect the c.r.t.
against burning if the vertical deflection fails.
Vertical protective circuit via CCU
U
40V
13V
t
6
D561
2
UV+
4
UV-
This VPROT signal is fed to the protective
circuit that consists of the dual operational
amplifier I 2651. Operational amplifier A works
on its non-inverting input pin 3, with reference
voltage of 3 V and operational amplifier B on
its inverting input pin 6 with 5 V. If the VPROT
signal - Diagram A – does not fall below or
exceed the reference values of 3V and 5 V,
then the V deflection and the vertical d.c.
component through the deflection coils is cor-
rect. If the d.c. component is too positive or
Document Q 2500
Basic board
C564
R563
C563
3
Flyback
Generator
I 561
TDA 8177
For a d.c. coupled V output stage a fault situa-
tion can theoretically occur in which the de-
flection saw tooth appears to be available, but
a faulty d.c. component directs the electron
beam to the upper or lower end of the c.r.t.
neck. This could cause the c.r.t. neck to melt
and lead to destruction of the c.r.t. neck.
To prevent this a V saw tooth is taken from
the base of the V deflection coils with R 573
and a d.c. voltage is applied to the signal
board with R 2651.
Signal board
W1731
Pin 4
R1036
D1769
R2593
C
D2591
2592/93
Bild 2 - 15
too negative - Diagrams B + C – this is inter-
preted as a fault. If, for example, the d.c.
component of the VPROT signal is too small
(<3V Diagram C), OP A controls its output on
pin 1 to H level. This is fed, via the two D
2667/2661, to CCU pin 94. H level on pin 94
represents a fault and the CCU switches the
device within about 2 secs to standby opera-
tion. If the d.c.level is too positive (>5V Dia-
gram B ) pin 7 goes from OP B to H level and
the CCU switches the device off.
30
U5
I 2801
CCU/Pin94
R2594
D2594
Q2594
R2586
R2594
© Loewe ProCollege
D2661
D2586