Power Amplifier; Temperature Protection; Direct Coupled Audio Input - RF Technology Eclipse Series Operation And Maintenance Manual

5.3 Power Amplifier

The phase detector output signals of U6 are used to control two switched current sources. The
output of the positive and negative sources Q3 and Q6, produce the tuning voltage which is
smoothed by the loop filter components to bias the V.C.O. varactor diode D3.
5.3 Power Amplifier
The 10 mW output from the main board connects to the power amplifier board through a short
miniature 50Ω coaxial cable.
Q2 on the power amplifier board increases the signal to approximately200mW. The bias of
Q2 is controlled by Q1 and the power leveling circuitry to adjust the drive to the output
module U2.
U2 increases the power from the driver to 30 watts before it is fed to the directional coupler,
low pass filter and output connector. The directional coupler detects the forward and reverse
power components and provides proportional dc voltages which are amplified by U1a and
U1b.
The forward power voltage from U1a and U1b are compared to the present DC reference
voltage from RV1. The difference is amplified by U1c, Q3 and Q4. The resulting control
voltage supplies Q2 through R10, R12 and completes the power levelling control loop.
5.4 Temper atur e Pr otection
Thermistor RT1 on the power amplifier board is used to sense the case temperature of the
output module U2. If the case temperature rises above 90 degrees C, the voltage across RT1
will increase and transistor Q5 will be turned on. This reduces the dc reference voltage to the
power regulator which inturn reduces the outpower by 6-10dB.
5.5 600Ω Line Input
The 600Ω balanced line input connects to line isolation transformer T1. T1 has two 150Ω
primary windings which are normally connected in series for 600Ω lines. The dual primary
windings can be used to provide DC loop PTT signaling or a 2/4 wire hybrid connection. All
four leads are available at the rear panel system connector.
The secondary of T1 can be terminated with an internal 600Ω load through JP5 or left un-
terminated in high impedance applications.
5.6 Dir ect Coupled Audio Input
A high impedance (10kΩ) direct AC coupled input is available at the system connector. The
direct coupled input connects to U9a which is configured as a unity gain bridge amplifier.
The bridge configuration allows audio signal inversion by interchanging the positive and
negative inputs and minimizes ground loop problems. Both inputs should be connected, with
one lead going to the source output pin and the other connected to the source audio ground.
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RF Technology T800
5 CIRCUIT DESCRIPTION
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