Loewe Q2500B Serviceanleitung Seite 162

Technology of Q 2500 colour TV set
This means that the traditional adjustment
controller and the associated adjustment work
is no longer required.
Directly after the vertical picture return, meas-
urements are taken of the leakage current of
the c.r.t. systems at ultra blank and then, one
after the other, the cathode currents of the
three systems after input of a specific blank
value.
These pulses are fed with the respective RGB
signals via the output stages of the c.r.t. sys-
tems. From the output stage IC's the respec-
tive blank currents of the control circuit in
video IC I 2521 on the signal board are sup-
plied via its pin 44.
From a comparison between the established
blank current and a reference value, which
represents the specified current, a difference
is obtained, which controls the cathode cur-
rent via the video output stages as a controller
output. It is dynamically stabilised just above
c.r.t. blocking current, up to control tolerances.
As small leakage currents in the output stages
of the c.r.t.s could lead to distortion of the cut
Document Q 2500
Cut - off measurement
LM
MR
LM
MG
LM
MB
LM = Leakage current measurement
MR
MG
Cut - off measurement pulse
MB
off control, measurement of the leakage cur-
rent is carried out during the vertical flyback
before measurement of the three blanking
currents.
Cut - off circuit principle
35 36 37 38
RGB
Insertion
TDA 9332
A consequence of a change to the UG2-
voltage is that the cut off control opposes this
change. Only a transient effect can be de-
tected on the screen, as long as the control
39
R - Channel
G - Channel
B - Channel
C2534
34 29
White Point Output
and Amplifier
Brightness and
Control Buffer
Beam
Continuous
Current
Cathode
Limter
Calibration
© Loewe ProCollege
40
R
41
G
42
B
W1051
Cut off
44
From
cathode
ray tube
PCB
43
BCL