500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page 6-3
15 October 2004
- Disconnect three screws/washers securing
assembly to associated terminal board.
- Disconnect
four
screws/washers
securing
assembly to the mounting rails.
-
Carefully lift away power amplifier assembly.
(c) On the suspect power amplifier assembly,
unsolder and disconnect wiring from gate
terminal of power MOSFET's Q1 through Q4.
(d) Ensure power MOSFETs to be tested are turned
off, by momentarily connecting a shorting
jumper between the gate and source terminals of
each device.
(e) Measure source/drain resistance, of power
MOSFET being tested, in both directions.
(f) Resistance measurements in step (e) shall be an
open circuit in the reverse bias direction and a
diode pedestal in the forward bias direction. If
requirements of this step are met, proceed to
step (h).
NOTE
If multimeter in use does not have diode measuring
capability, the resistance of the forward bias direction
will be an open circuit.
(g) Disconnect wiring and component leads from
the source and drain terminals of MOSFET's
that do not meet the requirements of step (f) and
repeat (d) through (f).
(h) Turn on power MOSFET being tested by
momentarily applying a dc voltage (4.0 to 9.0 V
dc) between its gate (+) and source (-) terminals.
(i) Measure source/drain resistance, of power
MOSFET(s) turned on in step (h), in both
(forward and reverse) directions.
(j) Resistance measurements in step (i) shall be a
short circuit in both (forward and reverse)
directions.
(k) Turn off MOSFET(s) being tested by
momentarily connecting a shorting jumper
between its gate and source terminals.
(l) Measure drain/ground resistance of power
MOSFETs Q1 and Q3 in both (forward and
reverse) directions.
(m) Resistance measurement in step (l) shall be high
impedance (open circuit) in one direction and a
diode pedestal in the other direction.
(n) If requirements of steps (f), (j) and (m) are met,
the power MOSFET being tested and it's
associated insulating washer may be assumed to
be serviceable.
(o) If requirements of steps (f) and (j) are not met,
the power MOSFET being tested may be
assumed to be defective. If requirements of step
(m) are not met, the associated insulating
washer may be assumed to be defective.
Replace the defective power MOSFET as
detailed in paragraph 6.8.1.
NOTE
It is recommended that power MOSFET Q1 in the
associated modulator assembly be tested in
conjunction with the suspect power amplifier
assembly. Refer to paragraph 6.7.2.
(p) On completion of testing/repair:
-
Install power amplifier assembly in modulator/
power amplifier module, ensuring it is oriented
as depicted in Figure MD-7, using screws/
washers removed in step (b). Connect
appropriate coaxial cable connector to J1.
6.7.2 CHECK OF POWER MOSFET IN
MODULATOR ASSEMBLY:
Check power
MOSFET Q1 in a modulator assembly as follows:
(a) If associated power amplifier assembly has been
removed for testing, it is not necessary to
remove modulator assembly to be tested.
(b) If associated power amplifier is installed,
remove modulator assembly as follows. (Refer
to figure MD-6 to identify a specific modulator
assembly and to figure MD-10 for its assembly
detail).
- Disconnect three screws/washers securing
assembly to associated terminal board.
