Nautel NX15, Troubleshooting Manual

Page: 57 / 274

NX25/NX15 Troubleshooting Manual Responding to alarms
Issue 5.0 2016-05-06 Page 1-49
Resistance measurements
Complete the following resistance measurements for each suspect RF power module. See Figure 1.4
on page 1-50 to identify the power MOSFETs on the RF power module.
1. Remove fuse F1 from its holder and measure its resistance using a digital multimeter. A
blown fuse will measure an open circuit. If the fuse is OK, return it to its holder.
2. For each power amplifier MOSFET (Q7 through Q10) and each modulator MOSFET (Q11,
Q12 and Q13), use a digital multimeter to make the following resistance measurements. Note
that Q7 through Q10 have screw-head terminals and Q11 through Q13 have solder pads (see
Figure 1.4 on page 1-50 ):
• Check for 1,000

between the gate and source.
• Check for an open circuit between the gate and drain.
3. If either measurement in Step 2 is not satisfactory, replace the affected power amplifier
MOSFET (see “Power Amplifier FET replacement” ) or modulator MOSFET (see
“Modulator FET replacement” on page 1-53 ).
4. If both measurements in Step 2 are satisfactory, replace the entire RF power module (see
“Removing and reinstalling RF power modules” on page 1-45 ).
Diode checks
Complete the following diode checks for each suspect RF power module. See Figure 1.4 on page 1-50
to identify the free-wheel diode on the RF power module.
1. Use a digital multimeter (on its diode setting) to check free-wheel diode CR8, noting the
anode (A) and cathode (K) markings (see Figure 1.4 on page 1-50 ):
2. If the diode is not satisfactory, replace the it by desoldering its surface-mount leads and case
from the PWB. Locate a replacement diode (Nautel Part # QM54) in the spares kit, if
purchased, and solder it to the PWB, noting correct orientation.
3. If the diode is satisfactory, proceed to MOSFET replacement, as necessary, or replace the
entire RF power module (see “Removing and reinstalling RF power modules” on page 1-45 ).