Horizontal Output Stage - Loewe Q2500B Serviceanleitung

Technology of Q 2500 colour TV set
It can be clearly determined from the line
driver circuit, as from the control of the line
output stage, that this is a case of low-
resistance current control. The driver stage is
equipped with a transistor, which supplies for
the drive transformer T 528 (conversion ratio
7:1) output stage the required base control
current of up to 0.9 Ass. To limit inductive
switching peaks an R/C combination is con-
nected, a.c. to earth, to the collector of Q 526
after diode D 527. The driver stage operates
with respect to the output stage in alternating
operation, i.e. if Q 526 is conducting, then
Q 534 is blocked and vice versa.

2.4 Horizontal output stage

As mentioned in the previous section control
of the horizontal output stage, or more pre-
cisely the horizontal switching transistor, al-
ternates according to the switching regulator
principle. Essentially the base resistance con-
sists of R 532 and the series secondary wind-
ing of the driver transformer T 528. The paral-
Document Q 2500
lel arranged resistor R 533 dampens switching
peaks arising in the inductivity.
An additional anti-parallel diode from collector
Q 534 to earth, which was used as a booster
diode in earlier forms of thyristor deflection
switching, is not required, as in this switching
concept the collector-base route of the switch-
ing transistor Q 534 fulfils the function of the
booster diode by a process of inverse opera-
tion. In practice the parallel O/W-modulation
diodes cause a perceptible load reduction of
the switching transistor. The drawing below
shows that this inverse operation of the tran-
sistor occurs during the first half of the trace,
until about the middle of the line. In the sec-
ond half of the run the transistor operates as
normal with conducting base-emitter diode.
Only during the relatively short flyback time is
the switching transistor blocked. The low re-
sistance base switching already described
enables the base peak current between +0.9
A and –0.9 A in both directions to be dissi-
pated quickly.
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