Tait Orca 5000 Service Manual page 115

Note that the recommended method for
changing the gain of the audio power amplifier
is to adjust the ratio of the voltage divider
formed by R69 and R68.
The second path for the single ended audio
signal present at pin 7 of IC2:B is via R78 and
C52, to become the AUDIO-D25 signal. This
signal has its output at pin 6 on the D25
connector on the charger PCB. The minimum
input impedance of a circuit that connects to
AUDIO-D25 is 6k Ω. The recommended input
impedance would be 47k Ω.
The audio PA has three modes of operation
(standby, mute and on) which are set by the
voltage at pin 8. A pull-up for the on mode is
provided by R75, while Q3 controls the mute
mode, by switching in R73 to form a voltage
divider with R75. Q4 pulls IC4 pin 8 low to
control the standby mode. The following table
summarises the PA operation.
Table F-12: Vehicle kit option PCB - audio PA operation
Mode Voltage Level
On > 8.5 V
Mute 3.3 V - 6.4 V
Standby < 2 V
When the audio PA is in either the mute or on
mode, its outputs (pins 4 & 6) are biased with
a DC level of approximately 6.5 V.
A BUSY signal is created by looking at the DC
bias on the EXT+SPKR signal. IC2:A is config-
ured as a Schmitt trigger and is used to
produce the BUSY signal (pin 1). R67 and C27
provide filtering of the audio signal so that
IC2:A is not falsely triggered by large audio
peaks. The reference signal is produced from
+7V5-ACC via the voltage divider of R65 and
R66.
The 7V5-ACC signal is accessory power from
the radio and indicates if the radio is switched
June 2003 IPN: M5000-00-105
Controlled By
Q3: off
Q4: off
R75: Pull up
Q3: on
Q4: off
R75/R73: divider
Q4: on
on. If there is no 7V5-ACC signal then the
audio PA is held in its standby mode via Q7
and Q4. When the DC bias is absent from
EXT+SPKR, BUSY is high and the audio PA is
held in its mute mode via Q3. The audio PA
can also be put into standby mode via the SPK-
CUT control signal being high. Q4 will always
override Q3.
Putting the audio PA into standby is part of the
power save feature of the vehicle kit. LVSD is a
control line from the charger PCB which goes
high when the vehicle battery is too low
(<11V). If LVSD is high then the audio PA is
put into standby mode via Q6, Q7 and Q4.
A 3.5mm stereo phono socket is used to
connect the external speaker. When the mono
plug of an external speaker is attached, the
middle connection of the stereo socket
(SPKSENSE) is shorted to one of the audio PA
output signals. When the audio PA is operat-
ing there is a DC bias of approximately 6.5 V
(half rail) on both of its outputs. This bias is
used to turn on Q5, which pulls SENSE-0-ACC
low, disabling the radio's internal speaker.
The D25 connector has a control line called
SPKR-OFF on pin 2. If the accessory connected
to the D25 connector has a speaker, then by
pulling SPKR-OFF low all other speakers can
be disabled. The SPKR-OFF signal is inverted
by Q9 to produce SPK-CUT. If SPK-CUT is
high, then the audio PA will be placed in stand-
by mode via Q4 (which turns off an external
speaker if it is connected). SENSE-0-ACC is
pulled low to disable the radio's internal
speaker.
Microphone pre-amp
A capacitor multiplier formed by Q2, R2 and
C19 is used to filter the +5 V supply producing
+5V-FIL which is used to provide DC bias for
the microphones via R3 and R22.
The internal microphone in the radio is
disabled by an impedance to ground, which is
typically the electret microphone of a speaker
microphone. In the vehicle kit this is accom-
plished by R28, which is connected to the EXT-
Tait Orca vehicle kit F - 23