Page 4-12 Vector-LP Radio Beacon Transmitter Technical Instruction Manual
Issue 1.1 Section 4 Testing and Adjustment
(k) Verify the reflected power cutback is
below 30 W (for VR250) or 15 W (for
VR125). Press RF Off.
(l) Remove the shorting jumper from the
transmitter’s RF output. Reconnect the
dummy load to the transmitter’s RF
output.
4.3.6.7 SETTING THE RF MONITOR
LEVEL
(a) Increase the transmitter’s RF output to
full power.
(b) Connect a 50 : load to the RF
MONITOR BNC connector (J8 of
remote interface PWB A3, accessible
at the back of the front panel. Monitor
the voltage with an oscilloscope.
(c) Adjust the remote interface PWB’s
MONITOR LEVEL potentiometer
(R163) until the voltage is 1.0 V RMS.
4.3.6.8 CALIBRATING THE AC FAILURE
ALARM
(a) With the transmitter connected to a
suitably rated dummy load, press RF
On and increase to rated power.
(b) Measure the dc voltage at TP13 on the
control/display PWB (A1). Calculate
and record the low ac voltage
reference:
For VR250:
Low AC ref = 170 x TP13/Ac Voltage
For VR125:
Low AC ref = 90 x TP13/Ac Voltage
(c) Monitor the dc voltage at TP19 on the
control/display PWB.
(d) In the Set Thresholds menu (see
3.6.12 and 3.6.12.2), scroll to the Low
AC Voltage threshold. Press Modify.
(e) Adjust the Low AC Voltage value until
the voltage at TP19 matches the low
ac voltage reference calculated in step
(b). Press Done.
(f) Turn off the transmitter’s ac power. If
testing is required, connect a variac
between the ac power source and the
transmitter. Turn on the ac power.
(g) Use the variac to decrease the ac
voltage until the System Diagram’s AC
Mains LED. The Diagnostic Display
should display both Shutback A and
Low AC. Record the ac voltage.
(h) Press RF Off.
(i) Turn off the ac power and disconnect
the variac. Reconnect ac to the
transmitter.
4.3.6.9 ADJUSTING FOR RF DRIVE
SYMMETRY
Measure the symmetry of the RF drive
applied from the active RF synthesizer
PWB to the RF power modules as follows:
(a) Select the side for the symmetry
adjustment.
(b) Press RF On and set the transmitter’s
RF output power to 0 W.
(c) Connect an oscilloscope between TP2
(TP1 for side B) and ground on the
exciter interface PWB (A2).
(d) The oscilloscope should indicate a
symmetrical (50% duty cycle) square
wave with a nominal amplitude of 5 V
peak-to-peak.
(e) Adjust the appropriate RF synthesizer
PWB’s SYMMETRY potentiometer
(R32) to obtain a 50 r 1 % duty cycle.
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