Technical Description 4-8 X-2 Technical Service Manual
Publication No: 15-02047
4.2.9 Volume Control and Audio Amplifier
The audio from the output of the mute gate passes through a ribbon cable to
the volume control on the front panel.
The output of the volume control is passed to the loudspeaker amplifier IC2.
The audio amplifier can supply 8 W into a 2 ohm load and supplies
approximately 2 W to the internally fitted 8 Ω speaker.
4.3 Transmitter Exciter
4.3.1 Microphone Compressor
The microphone is connected via connector P100 on the front panel to an RF
filter network R6 and C9 and then by ribbon cable to the main PCB (P401
pins 26 and 19). From here, the microphone speech passes to the input
network of the microphone compressor amplifier.
104-02907 Sheet 2
The input network includes an analogue switch IC305/D
[H2]
, to disable the
microphone when other transmit functions are selected.
The microphone compressor amplifier, consisting of IC303/B
[F3]
, IC306/A
[H3]
and IC306B
[G3]
, V303
[G3]
and associated components, provides a constant
output level for a large variation in speech levels applied to the input.
With no speech present, amplifier IC303/B is set to maximum gain
determined by feedback resistor R337 and shunt FET V303, functioning as a
variable resistor and set to minimum resistance (≈150 Ω).
The output of the amplifier IC303/B is connected to the inputs of IC306/A
and IC306/B which form a window comparator. The window is set by the
divider chain R330 to R342 to ± 0.25 V centred at 5 V.
When the level of speech applied to the microphone amplifier results in the
output exceeding 0.25 V peak, the window comparators produce negative
going output pulses lowering the DC charge on capacitor C238.
This effect reduces the voltage on the FET gate IC303, increases the
effective resistance of the FET (IC303) and lowers the gain of the
microphone amplifier.
The microphone amplifier is now in compression and the output level
remains constant for any further increase in speech level.
The microphone amplifier has a compression range of approximately 30 dB.
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