Broadcast Electronics Marti Electronics ME-40, Manual

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©2011 Broadcast Electronics
4 OVERALL THEORY OF OPERATION
This section presents overall theory of operation for the ME-40 FM Exciter.
For the purpose of definition, the ME-40 Exciter is divided into functional subassemblies in the following text.
A detailed description of each subassembly is presented later in this manual.
4.1 POWER SUPPLY/CONTROL CIRCUITS.
The power supply/control circuit board contains the exciter power supply and control circuitry. The
proceeding text will describe the power supply circuitry followed by the control circuitry.
4.1.1 POWER SUPPLY CIRCUIT
Primary AC power to the exciter is applied through a voltage selector and line filter module. This device
provides overload protection for the entire exciter and allows selection of a wide range of AC input potentials.
All the circuitry in the exciter operates from an unregulated potential of +30V DC and three pre ­ regulated
potentials of +20 volts, -20 volts and +5 volts. All supplies are full-wave rectified, filtered, and electronically
regulated to assure stable equipment operation.
The +20 volt, -20 volt, and +5 volt supplies are low-current circuits which are protected from over-voltage,
over-current, reverse-voltage, and short-circuit conditions. These potentials are distributed throughout the
exciter to various subassemblies and re-regulated to lower voltages on each circuit board. The +
13
volt supply
can be checked on the TEST METER.
The fi20 volt supply associated with the RF amplifier is regulated by the control circuitry in response to
preset level controls and feedback loops. This supply contains over ­ voltage, over-current, reverse-voltage,
short-circuit, and over-temperature circuitry to protect the exciter sub-assemblies.
4.1.2 CONTROL CIRCUIT
The control circuitry regulates operation of the RF amplifier within preset limits dependent upon several
parameters such as forward RF power output, reflected power, RF amplifier heat sink temperature, DC
current, DC supply voltage, an external mute control potential, and an external RF power adjust potential.
The control circuit assembly also contains amplifiers for the forward and reflected power directional
couplers, over temperature circuitry, and the VSWR circuitry.
The control circuit compares the sum of the forward and reflected powers to a reference for automatic
control of power output. If the reflected power becomes excessive, the power output will be reduced by
the amount required to maintain safe operation of the RF output transistor. If excessive VSWR exists, a
front-panel VSWR indicator will illuminate.
In addition, the control circuit monitors the total RF amplifier assembly heat sink temperature and limits
RF output accordingly. This assures operation at safe transistor temperatures under the worst case
conditions of high VSWR, high ambient temperatures, or failure of the cooling fan. If an over-temperature
condition exists, a front-panel TEMP indicator will illuminate.
Automatic protection of the RF devices from excessive voltage is provided by an MOV and crowbar circuit,
and short circuit protection is provided by foldback current limiting and a fuse. If an over-current
condition exists, a front-panel RF indicator will illuminate.
4.2 RFI FILTER NETWORK.
The RFI filter prevents interference from signals of 500 kHz and above by filtering and bypassing the audio,
control, and status input and output circuits.