Mute Circuit - Loewe Q2500B Serviceanleitung

Technology of Q 2500 colour TV set
2
Via the I C bus it is also possible for MSP
3411 to switch off the sound in the loud-
speaker and headphone branch. At the same
time an internally generated 1 kHz rectangular
signal is applied to these channels. In the
headphone branch the level is dependent on
the headphone volume. In the loudspeaker
branch the level can be set, independently of
other volume, in 39 stages in the "Time ser-
vices" menu .

5.2.15 Mute circuit

As the only additional switching measure
there is an active mute circuit parallel to the
output amplifier inputs with transistors Q 1581
and Q 1586. It is controlled by transistors Q
2966 and Q 2961 from the ON/OFF command
of the CCU, or via transistor Q 2027 from pin
77 of MSP 3410 .
Noise associated with switching on and off, as
well as any crackling when switching over is
suppressed.
Mute stages are also incorporated for the
headphone branch and the Cinch sockets. If
an AC3 module is incorporated it also gener-
ates a mute if necessary e.g. if the sound de-
coder is switched.
On start up the L level of the ON information
of the CCU of transistor Q 2961 is blocked.
The voltage taken from the conversion trans-
former and rectified by D 491 is able to charge
Elko´s C 2967/68 very quickly via diode D
381. The charge process from C 2963 flows
via R 2963 extremely slowly, as D 2964 is
now blocked. Not until Elko C 2963 is charged
Document Q 2500
to almost the same potential as C 2967/68
does transistor Q 2966 conduct. A positive
voltage is felt on the base of transistors Q
1581/Q 1586 whereby this switches and the
VF lines are at earth potential.
Not until C 2963 is charged and Q 2966, Q
1581 and Q 1586 are blocked, can the VF
reach the output stage without hindrance.
At the point of switching off the voltage from
the power supply falls off relatively quickly.
Transistor Q 2961 is switched by the H level
of the OFF information of the CCU . This
means the charge from C 2963 can flow to
earth. Q 2966 becomes conductive, whereby
the charge from C 2967/68 controls transistors
Q 1581 and Q 1586 and the VF lines are at
earth potential.
The mute information that is active on start up
and shut down activates all the mute levels.
In the initialisation phase pin 77 is set to L
level by MSP 3411. With L level transistor Q
2027 switches. Via its emitter-collector path
U5 also reaches the mute line and all mute
stages are active.
To prevent the VF mute stages themselves
producing interference during the switching
phase, they are d.c. decoupled and fitted with
discharge resistors.
If a Dolby Digital Module is incorporated the
LFE channel from the mute function is con-
trolled silently via transistors Q 1551 and Q
1552/56.
The Dolby Digital Module receives the mute
function from the signal board on contact con-
nector W 2091 /pin 13.
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