RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 6-2
15 September 2003
ISOLATION OF DEFECTIVE POWER
MOSFETS
6.5
Isolate defective power MOSFET devices
in the NAA18 modulator/power amplifier, using a
multimeter that is capable of measuring the
forward/reverse resistance of a diode, as follows:
NOTE
The following procedures require the power
MOSFET under test to be turned on by the
application of a DC voltage (minimum of 4.0 volts
DC to a maximum of 9.0 volts DC) between its gate
and source. Some digital multimeters have
sufficient DC voltage on their test leads when they
are set to 'diode' or 'resistance' test positions. If the
digital multimeter to be used falls in this category, it
may be used as the voltage source. If it does not, a
DC voltage source that is between 4.0 and 9.0 volts
DC must be obtained.
(a)
Unsolder and disconnect wiring going to gates
of power MOSFETs Q1 through Q5.
(b)
Verify the power MOSFET to be tested is
turned off, by momentarily connecting a jumper
between its gate and ground.
(c)
Measure source/drain resistance, of power
MOSFET being tested, in both directions.
(d)
Resistance measurements in step (c) shall be an
open circuit in the reverse bias direction and a
diode pedestal in the forward bias direction.
(e)
If requirements in step (d) are not met, unsolder
and disconnect wiring from the source and
drain terminals of MOSFETs that are suspected
of being defective and repeat steps (b) through
(d).
NOTE
Power MOSFET Q1 must be removed from the
circuit to disconnect its source terminal. Do not
disconnect it unless it is defective.
(f)
Turn on the power MOSFET to be tested by
momentarily applying a DC voltage (4.0 to 9.0
volts DC) between its gate (+) and source (-)
leads.
(g)
Measure source/drain resistance, of power
MOSFET being tested, in both directions.
(h)
Resistance measurements in step (g) shall be a
short circuit in forward and reverse directions.
(i)
Turn off power MOSFET to be tested by
momentarily connecting a jumper between its
gate and ground.
(j)
Measure resistance between the drain (case), of
power MOSFETs Q2 through Q5 and the heat
sink (ground).
(k)
Resistance measurements in step (j) shall be a
high impedance (open circuit).
(l)
Measure the resistance between the drain (case)
of power MOSFET Q1 and the heat sink
(ground) in both directions.
(m) Resistance measurement in step (l) shall be a
high impedance (open circuit) in one direction
and a diode pedestal in the other direction.
(n)
If requirements of steps (d), (h), (k) and (m) are
met, the power MOSFET being tested and its
associated insulating washer may be assumed
to be serviceable.
(o)
If requirements of steps (d) and (h) are not met,
the power MOSFET being tested may be
assumed to be defective. If requirements of
steps (k) and (m) are not met, the associated
insulating washer may be assumed to be
defective. Replace the defective power
MOSFET as detailed in paragraph 6.6.
POWER MOSFET REPLACEMENT
6.6
Replace defective power MOSFETs as
follows:
(a)
Remove attaching hardware from heat sink and
remove heat sink to expose attaching hardware
of power MOSFETS Q1 through Q5.
