8525B/8528 Technical Service Manual Technical Description 4-7
Publication No: 15-02036 Issue 6
4.3 Transmitter Exciter
4.3.1 Microphone Amplifier (Refer Dwg 04-02093)
The input to the microphone amplifier is J1, pin 6. Transistor V7 is an audio
clamp which is released only when the microphone PTT button is pressed.
This prevents the microphone picking up background noise and sidetone
from the loudspeaker when transmitting emergency calls etc.
IC2a amplifies the microphone input to approximately 1V pp (gain=100).
The signal is then connected to the AGC detector (IC2b, V10) and the
modulator (IC8).
The output of IC2a to the AGC detector is fed directly to V10, and via
inverter IC2b, to V11. Transistors V10 and V11 form a full wave rectifier to
drive V9, which in turn drives V8 to complete the AGC loop by shunting the
input signal to ground.
The release time-constant of the amplifier is set by R37 and C25. Diode D4
allows the gain to rise exponentially to maximum. The three-terminal
regulator IC7 prevents power supply variations affecting the AGC circuit.
4.3.2 Modulator
The microphone amplifier output is capacitively coupled via C28 to the
temperature compensating resistor network R55 and R56. R55 is an NTC
resistor; as the temperature increases its resistance decreases and so the
audio signal level passed to IC8 increases. This is to compensate for reduced
gain at higher temperatures mainly in the 45MHz IF amplifier.
In the 8528S, the selection of the AM (H3E) mode forward biases V3 on the
Motherboard (04-02453). This switches R17, R27 and C23 (also on the
Motherboard) into circuit between the microphone amplifier output J1 pin 3
and ground. This attenuates the audio signal to the lower level required by
the modulator in this mode. Link Y is inserted in H versions which reduces
the audio drive by approximately 3dB to improve the exciter IMD products.
IC8 is the modulator during transmit and mixes the audio signal with the
1650kHz (1647kHz, LSB) local oscillator to produce a DSB signal output,
centred on the LO frequency, from pin 12.
This signal is then passed via the transmit/receive switch V1 and D1 to the
1650kHz crystal filter.
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