User Manual

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VHF Amplifier Instruction Manual VT-3 132 - 174 MHz 3-3
3
3.6.2 VT-3/150 A mplifier Adjustment
The Amplifier alignment consists of two adjustment procedures; (i) a general set up (section
3.6.2.1) procedure which sets up the proper bias conditions for the RF transistors and (ii) the RF
threshold adjustments which set up the desired alarm threshold levels as well as the RF output
power. The general alignment procedure is required following major repair operations, changes in
RF input levels or large changes in operating frequency (greater than ± 1.0 MHz).
The RF output and alarm threshold level adjustments are more easily accessible so that fine
adjustments can be made in the field. Depending on user requirements, the RF alarm threshold
levels should be checked whenever a significant change in operating frequency (± 0.5 MHz) is
made. As the antenna VSWR alarm is dependent on the output power alarm, the output power
alarm should always be set first. The order of adjustment should be:
1) Set the desired output power alarm level (section 3.6.2.2).
2) Set the output power (section 3.6.2.3).
3) Set the desired Antenna VSWR alarm level (section 3.6.2.4).
4) Set the desired overload condition level (section 3.6.2.5).
Details for the alignment steps are outlined below.
3.6.2.1 General Set-Up
All of the setup steps detailed below are performed at the factory as part of the initial
Transmitter alignment. A general realignment of the Amplifier Module will be required
under the following conditions:
(i) the nominal RF input power applied to the amplifier is changed from that
which the amplifier was intially set up for, and
(ii) components, particularly the RF transistors Q1 and Q2, are replaced
during a repair operation.
1. Connect the transmitter's antenna output connector to the type N input of the radio
communications test set through a short section of low loss 50 Ω coaxial cable.
2. Turn all four (4) of the adjustment pot entiometers (R7, R21, R36, and R37) fully
counterclockwise.
3. Turn on the power to the transmitter.