Panasonic EURO 4 Chassis, Technical Manual

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10. AF. OUTPUT STAGE
Both amplitude controlled AF signals are output from
pins 24 and 25 of IC2101 of the MSP.
The signals are fed to the base of transistors Q2102
and Q2103 . The transistors are used for impedance
matching in order that the interference on the audio
lines between the MSP IC2101 and the audio output
I.C. IC251 is kept at a minimum.
The AF signals are fed to the audio output stage via
C266, R261 and C257 for the right hand signal and
C263, R256, C253 for the left. These capacitors may
be charged up very quickly as all control processes
(volume, balance) together with the base band
switch over are processed in the MSP IC2101 .
Both AF signals are fed to pins 2 and 5 of the output
I.C. IC251 . From here they are amplified and output
via pins 7 and 11. From here they are fed via the
headphone terminal to connectors E6 and E7 to the
internal speakers.
The usual negative feedback occurs from pin 11 to
pin 1 and from pin 7 to pin 6 of the I.C. via the R/C
network R252, C254 and R258 and C259. The
diodes at pins 2 and 5 D254 and D253 provide
protection for the output I.C. against any voltage
spikes by clamping the input pins. The output I.C. is
fed with a voltage of +29V to pin 10 of IC251 .
10.1. Active Mute
The active mute circuit is used in parallel to the
inputs of the audio output IC
IC251 . This active mute
circuit consists of two transistors Q251 and Q252
which is controlled by transistor Q2101 . Transistor
Q2101 is used to prevent POP during switch ON
and OFF times.
The POP mute circuit is fed a 5V supply and a12V
supply. The 5V supply which is fed via diode D2102
charges up capacitor C2129, while the 12V supply is
fed via R2113, charging up C2130. This 12V supply
is also fed to the emitter of transistor
Q2101 via
D2103 this positive supply to the emitter causes
Q2101 to conduct, as its emitter is more positive
with respect to its base, as a results Q2101 feeds a
HIGH level to the base of transistors Q251 / Q252
forcing them into conduction and grounding the
audio lines preventing POP.
This mute line is held HIGH until C2130 has charged
up to a level where the base of
Q2101 is no longer
negative with respect to its emitter and so Q2101
switches OFF, enabling the audio lines.
At switch OFF the 5V and 12V supply lines Fall
rapidly, resulting in C2130 discharging via D2104
into the falling 12V supply line. This causes
Q2101
to conduct allowing the charge of C2129 to
discharge via the Emitter /
collector junction of
Q2101 forcing the muting transistors Q251 , Q252 to
conduct grounding the audio lines.
This POP mute control is also fed to transistors
Q2301 , Q2303 which are used to mute the audio fed
via the RCA monitor out terminal.