Hflb Protective Circuit; Speed Modulator; General - Loewe Q2500B Serviceanleitung

Technology of Q 2500 colour TV set
power supply, then the c.r.t. could be dam-
aged if no protective measures are taken.
To prevent this, transistor Q 2616 is con-
nected to the beam current data. In normal
operation the transistor conducts and L level
is applied to its collector thus making the cir-
cuit inoperative.
If the described fault occurs, the base of the
diode split transformer becomes negative due
to the high beam current, Q 2616 blocks and
due to the 5 V on its collector diode D 2618
conducts.
Pin 94 of the CCU is set to H level and
switches the device to standby.

2.7.2 HFLB protective circuit

If the line output stage is too strongly over-
loaded by a fault, the protective circuits dis-
cussed cannot respond in any way. Excessive
overloading of the H output stage causes the
line flyback pulse to reduce in amplitude. The
HFLB pulse on pin 10 of W 1511 is therefore
monitored by a protective circuit. In normal
operation this pulse has an amplitude of
approx. 35 V. Via the 20 V zener diode
D 2581 this pulse makes Q 2581 conductive
in rhythm with the line frequency. Capacitor
C 2584 is switched to earth cyclically with the
line frequency via Q 581 and cannot therefore
charge up. This means that L level is felt on
C 2584l. If a fault in the deflection circuit
causes HFLB to fall below 20 V, zener diode
D 2581 and Q 2581 block. C 2584 charges to
H level. This H level is fed via D 2586 to pin
94 of the CCU, which switches the device
back to standby operation.
Document Q 2500

2.8 Speed modulator

The device is also equipped with a speed
modulator. It is located between the RGB out-
puts of the digital unit and the RGB output
stages on the c.r.t. board. It controls the
speed of the electron beam in the horizontal
direction, which increases picture sharpness
and prevents a defocusing effect at high con-
trast on vertical, very bright areas of the pic-
ture.
2.8.1

General

The speed modulator evaluates high fre-
quency changes in the picture signal and
sends them to an additional winding in the
deflector. The resulting magnetic field over-
lays the normal field produced by the deflec-
tion winding. By this method, the deflection
speed is adapted to the picture content, which
leads to a definite improvement in picture
sharpness.
A picture change in a positive or negative di-
rection produces a signal, as shown in the
following drawing and thereby supplies the
additional winding in the deflector.
The amplitude of the signal and the modula-
tion level of the contrast are therefore de-
pendent on the setting and the slope of the
signal change.
One could also modulate the deflection cur-
rent directly, which would however, be more
expensive than the second winding.
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