Technical description
4-10 HF SSB Transceiver 9323/9360/9390/9780 Technical Service Manual
Volume control and audio output amplifier
1 04-02974, 04-02976 Sheets 1 and 3
The volume control circuit is a 16-step attenuator and consists of R334–R340.
Each step has ≈4 dB of attenuation, giving a total of 64 dB.
The rotary volume control is a digital contacting encoder. When this control is
rotated in either direction, the microprocessor IC2 on the Display Panel PCB [1
04-02974] reads each position change. It sends commands via the external I
2
C bus
to the microprocessor IC101 on the Microprocessor and Audio PCB (see Sheet 1)
to select the appropriate audio attenuators. The microprocessor IC101 then sends
data via the local I
2
C bus to change the output ports of IC114 pins 3–6. This action
controls the analogue switches that switch the step attenuator in and out.
The audio is passed from the buffer amplifier IC304/A, through a resistor
attenuator R342 and R343, to the input of the loudspeaker amplifier IC306.
The maximum output of the audio amplifier is either:
• 2.5 W into an 8 Ω speaker
• 8 W into a 2 Ω speaker
Warning beeps are heard if the maximum or minimum limits of the volume
control are reached. The warning beeps are generated from the tone generator
IC106 pin 10 [1 04-02976 Sheet 1]. The tone is fed via a resistor attenuator
network to the input of the loudspeaker amplifier IC306 (BEEPS I/P line). The
beep audio level is varied by the volume control. The level is controlled by the
microprocessor, which selects the appropriate attenuator shunt resistors (R138,
R140 and R142) to ground (IC109 pins 8, 9 and 10).
Low voltage Tx inhibit
1 04-02972 and 04-03135 Sheet 2
The comparator IC211A on the Rx/Exciter PCB monitors the "A" rail. When the
supply voltage drops to 9 V, the output of IC211A is pulled low thus disabling the
PTT line connected from IC209 pin 6 via R275 to transistor switch V212. This
function prevents the transceiver latching in Tune mode when the supply voltage
drops below 9 V.
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