Vector-LP Radio Beacon Transmitter Technical Instruction Manual
Page
5-11
Section 5 Maintenance and Troubleshooting
Issue 1.1
5.7 FET CHECKING/
REPLACEMENT
The transmitter contains many FETs. The
FETs most likely to fail are in RF power
modules A and B. For failures caused in
an RF power module, remove the module
(see 5.6.1), check the devices listed below
and, if necessary, replace them.
NOTE
The station spares kit, if purchased,
contains Nautel parts referenced below.
x
FETs Q1 and Q2 (Nautel Part # QR54)
of switch mode power supply PWB A2
(see Figures MD-10 and MD-12)
x
power MOSFETs Q1 through Q4
(Nautel Part # QAP49) of power
amplifier A3 (see Figures MD-10 and
MD-13)
x
power MOSFET Q1 (Nautel Part #
QAP49) of modulator A5 (see Figures
MD-10 and MD-15; same device as in
power amplifier A3)
(a) Unsolder the gate pins of the FET
under test and all parallel devices.
Refer to the assembly’s electrical
schematic in Section 9 to determine
the parallel devices, if applicable.
Refer to the assembly’s detail drawing
in Section 10 to verify pin-out.
NOTE
To turn on a FET under test, apply a dc
voltage between its gate and source
terminals. Some digital multimeters have
enough dc voltage (9 V battery
recommended) on their test leads when
set to the ‘diode’ or ‘resistance’ positions.
If not, obtain another dc voltage source.
(b) Turn off the FET by shorting the gate
and source. Using a digital multimeter,
momentarily place the negative lead
on the gate pin and positive lead on
the source pin for each FET in the
circuit.
(c) Measure the drain-source resistance
with the positive lead on the drain and
the negative lead on the source. The
digital multimeter should indicate an
open circuit.
(d) Measure the drain-source resistance
with the negative lead on the drain
and the positive lead on the source.
The digital multimeter should indicate
a diode pedestal.
(e) If the requirements of steps (c) and (d)
are not met, replace the device as
detailed in steps (i) through (m).
(f) Turn on the device under test by
placing the positive lead on the gate
and the negative lead on the source.
(g) Measure the drain-source resistance
with the positive lead on the drain and
the negative lead on the source. The
digital multimeter reading should
indicate a short circuit.
(h) If the requirement of step (g) is met,
the FET is operational. Turn it off as
detailed in step (b) and proceed to the
next suspect device. If the
requirement of step (g) is not met,
replace the device as detailed in steps
(i) through (m).
(i) Unsolder the drain and source pins of
the defective device.
(j) Remove the hardware securing the
device in place and remove the
defective device. Clean the
associated heat sink surface near the
input PWB.
(k) Apply a thin, even coat of thermal
compound on the bottom of the new
device.
NOTE
FETs are static sensitive. Handle
replacement devices in a manner that
protects the FETs from static.
