NX100 Troubleshooting Manual
Responding to alarms
Page 1-34
Issue 3.2 2016-02-08
Modulator FET replacement
1. Remove five M3 screws that secure the modulator/power amplifier PWB to the chassis.
2. Remove 16 M4 screws (four for each MOSFET) that secure the PWB to the MOSFETs.
3. Desolder the gate, drain and source connections that secure the defective MOSFET to the
PWB (see
Figure 1.4 on page 1-31
). Also desolder the two connections that secure the
defective MOSFET’s heatsink to the PWB. Remove the heat sink and MOSFET from the
PWB.
4.
Remove the heatsink clip that holds the MOSFET on its heat sink
. Remove and discard the
defective MOSFET.
5. Clean the surface of the heat sink and make sure its is free of debris.
6. Apply a thin film of thermal compound to the heatsink.
7. Obtain a replacement MOSFET (Nautel Part # QR75) from the spares kit, if purchased, and
install it on the heat sink using the alignment post on the heat sink as an installation aid.
Reinstall the heat sink clip removed in
Step 4
.
8. Replace any other defective MOSFETs and then re-install the heatsink on the PWB, first by
soldering the two heatsink connections and then by soldering the MOSFET’s gate, drain and
source leads.
9. Reinstall the PWB to the chassis by reversing the instructions in steps 1 and 2. Torque the 16
power amplifier MOSFET screws (four for each MOSFET) to a maximum of 10 in-lbs
(1.1 N-m) .
10. Return the power module to service (see
“Installing an RF power module” on page 1-26
).
CAUTION:
The heat sinks of modulator MOSFETs are coated with a film of thermal
compound. Use care to ensure the film does not become contaminated with
foreign particles.
When installing a replacement MOSFET, visually inspect the mating surfaces of
the MOSFET and its heat sink. Ensure the heat sink surface is coated with a thin
film of thermal compound. Ensure foreign particles that may affect thermal
transfer are not embedded in the compound.