4-6 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
4.2.6 Audio Mute
The audio from the output of IC4a is looped back through the Motherboard
to the Audio and IF 1650kHz PCB and applied to the audio mute input
(R63/C37) and the mute gate (V12).
IC4b and IC3a are arranged as a squaring amplifier. The squared signal
charges C38 via D8 during negative excursions and the charge is transferred
to C39 by V13 during positive excursions. The resultant DC voltage on C39
is proportional to the frequency of the audio. IC3b and its associated
components form a low-pass filter with a cut-off frequency of approximately
10Hz. The output from IC3b is a DC voltage varying at the syllabic rate of
the received speech.
IC5a and IC5b form a window comparator whose window width is adjusted
by R73, setting the mute sensitivity. The divider formed by R75 and R76,
together with C42, averages the output of IC3b and provides the reference
voltage for the window comparator. If the output from IC3b rises above or
below this reference by the amount set by R73, then the output of either
IC5a or IC5b will go high.
The comparator outputs are diode ORed into C43 to provide a fast attack,
slow release (3 seconds) mute control signal.
The microcontroller on the Microprocessor Controller PCB can override the
mute control circuit in two ways:
• It can force the unmuted condition by grounding the inverting input of
IC6b (via J3 pin 9). This applies a high to the gate of V12 to switch it
on.
• It can force the muted condition by grounding the gate of V12 via J3
pin 8.
The control circuit is also over-ridden at D7 by the C rail being taken to 0V
in transmit. This forces the mute condition, thus preventing acoustic
feedback during transmission.
4.2.7 Volume Control and Audio Amplifier
The post-mute audio from V12 is passed to the volume control section. The
switches in IC9 control a resistive ladder network used as an attenuator.
Sixteen steps are available with each step, giving approximately 4dB
attenuation for a total of 64dB. The switches in IC9 are controlled by the
microprocessor on the Microprocessor Controller PCB.
IC6a buffers the attenuated audio and passes it to the audio amplifier, IC10.
The audio amplifier is capable of 8 Watts output into a 2 ohm load and gives
approximately 2 Watts into the internal 8 ohm speaker.
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