Technical description
HF SSB Transceiver 9323/9360/9390/9780 Technical Service Manual 4-11
Transmitter exciter
Microphone amplifier/compressor
1 04-02974 and 04-02976
The microphone insert is a moving coil device and is active when the PTT button
is pressed. This removes the short circuit across the microphone, and at the same
time, applying a ground to the PTT line on J1 pin 5.
The microphone is connected to the control panel via J3 pins 2 and 3 [1
04-02974] and to the PCB via P3 pins 1 (ground) and 5. From there, the
microphone audio is passed through an RF filter network R24 and C21 to the
input of the preamplifier IC10/B.
When the microphone is not in use, the input switch (consisting of transistor V10
and associated components) disables the audio by applying a short across the
input.
The audio output of amplifier IC10/B is fed through an RF decoupling network
(R40, C41 and R44) to connector P4 pin 2. In the front panel control version, the
audio is connected to P201 pin 2 on the Microprocessor and Audio PCB [1
04-02976 Sheet 2). From there it is fed via capacitor C219 and analogue switch
IC206/A to the combining amplifier IC204/A.
For the control head version, the Tx audio at connector P4 pin 2 is fed via a
control cable to P204 pin 11 on the transceiver. From there the output is connected
via a ribbon cable to P203 pin 6 on the Microprocessor and Audio PCB [1
04-02976 Sheet 2]. The audio path continues from pin 6 via a second RF
suppression network (R229/B and C242), through C218 and analogue switch
IC206/C to the combining amplifier IC204/A.
Additional audio inputs are applied to the combining amplifier IC204/A from:
• the tone generator IC106 via R213
• the optional PCB (P204 pin 3) via R214
• the option PS input (P303 pin 5) via C217, analogue switch IC206/B and R216
The microphone audio at the output of IC204/A is applied to the input of a
microphone compressor amplifier consisting of IC204/B, IC205/A and B, V202
and associated components. This provides a constant output for a large variation
in speech levels applied to the input (≈30 dB range).
When there is no speech present, the amplifier IC204/B is set to the maximum
gain. This is determined by the feedback resistor R208 and the shunt FET V202,
which functions as a variable resistor (minimum resistance is ≈150 Ω for
maximum gain).
The output of amplifier IC204/B is connected to the inputs of IC205/A and
IC205/B forming a window comparator. The window is set by the divider chain
R204–R207 to be centred at 5 ±0.25 V.
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