Audio Processing Section (Sheet 3) - RF Technology Eclipse Series Operation And Maintenance Manual

5.3

Audio Processing Section (Sheet 3)

Sheet 3 is a schematic, which itself refers to two other sheets.
Sheet 3 shows how the two Line inputs go to audio transformers T300 and T301, are
then optionally terminated by analogue switches U301B, and U301C, before being
passed to the audio input stages described by Sheet 4.
It also shows how the Direct Audio (TONE) signal is passed to the Tone circuitry (sheet
5).
It also shows how dc current in Line1 will cause the opto-isolator (U300) to generate
the CPU input LOOP_DET.
Relay RL300 is used to drive current back through an externally generated dc loop,
when the CPU output LOOP/VOLTS_SEL is high.
The output of the Tone circuitry and the Audio circuitry is mixed (summed) and
amplified by U302. It is then passed through a high order low pass filter (3.1kHz),
before being attenuated by digital POT U303.
The Digital POT (U303) sets the Maximum deviation.
U302C then adds 6dB of gain before sending the audio to the modulator.
R317, D307, and C304, act as an average peak detector. This enables the CPU to
determine the size of signals being handled by the audio section.
Note that the Line inputs, and the TONE input, are protected by transils and fuses
against accidental connection to damaging voltages. The fuses (F300, F301, and F302)
are not user replaceable.
authorised service personnel.
5.4 Line Input Processing Section (Sheet 4)
The two audio inputs are passed, after transformer coupling, to sheet 4.
In Sheet 4, the two Line Inputs are input to a transconductance amplifier (U402A, and
U402B). A transconductance amplifier is a current controlled, current amplifier, i.e. it
amplifies input current, but its level of amplification is controlled by the level of current
that is injected into pin1 or pin16. By converting a DAC output into a current, and
converting the input voltage into an input current, U402A and U402B are converted into
Voltage Controlled voltage Amplifiers(VCAs).
Two of the DAC outputs are converted to currents by U400B, U400C, Q401, and Q400,
and these currents are used to control the gain of the transconductance amplifiers.
The input voltages are converted to current by the input load resistors R402, and R403.
The output currents are converted to voltages by resistors R420 and R424.
Page 18
They are surface mount devices and must be replaced by
5 CIRCUIT DESCRIPTION
RF Technology T50