Panasonic EURO 4 Chassis, Technical Manual

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9
Chapter 1 Common Circuits
4. AF Audio Output
Both amplitude controlled audio signals output from
pins 24 and 25 of IC2101 of the MSP are fed to the
base of transistors Q2102 and Q2103. These
transistors being used for impedance matching in
order that the interference on the audio lines between
the MSP IC2101 and the subsequent audio stages is
kept at a minimum.
The audio signals fed via transistors Q2102, Q2103
are then fed to connector E10 pins 1 and 2. Here the
audio signals are then fed to the Z-Board via
connector Z4 pins 1 and 2 where Acoustic Feedback
(AFB) processing takes place, as described in
chapter 1 section 3. After AFB processing the audio
signals are then fed via connectors Z4 / E10 pins 5 and
6.
The audio signals once on the E-Board are then 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 audio 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.
4.1. AF Audio Mute
The AF audio mute circuit located at the inputs of the
audio output IC IC251 consists of two transistors
Q251 and Q252 which is controlled by transistor
Q2101. Transistor Q2101 being used to prevent POP
during switch ON and OFF times.
At switch ON and OFF times the audio signals are
muted to prevent POP. The mute control is generated
by transistor Q2101 (located on the E-Board), its
operation being discussed in
section 10.1 of the
EURO 4 Technical Guide.
During the ON and OFF periods the mute control
signal is fed to the base of both transistors Q251 and
Q252.
The mute control line during the ON / OFF periods is
of a HIGH level which causes both transistors Q251
/ Q252 into conduction, muting the left and right audio
signals by pulling them to ground.
Audio Left Out
Audio Right Out
Audio Right In
Audio Left In
Switch ON Mute