Nautel XR12, Troubleshooting Manual, Page: 38 / 283

XR12 Troubleshooting Manual Responding to alarms
Page 1-18 Issue 3.0 2009-07-28
3. Use an oscilloscope to measure TP2 of the RF drive buffer PWB. The signal should be a 15
volt peak-to-peak square wave at the carrier frequency. If the signal is present, suspect the RF
drive buffer PWB.
4. Measure the dc voltage at TP1 of the RF drive buffer PWB. If the voltage is less than 0.6 V,
suspect the control/display PWB or transistor Q1 on the RF drive buffer PWB.
Cutback level (1-8)
Should there be three shutbacks within five seconds, the transmitter will enter a power reduction
mode called a cutback.
There are eight levels of cutbacks, the last being a reduction to no forward power whatsoever. At any
given cutback level, there is a predefined time limit that must expire before the cutback level is
reduced to the previous level. If there are no further cutbacks, this process continues until Level 0
(normal) is reached.
The cutback recovery process can be overridden by adjusting the power (up or down), or by pressing
Reset
on the
Status
screen.
A
Cutback Level (1-8)
status indicates that several shutback alarms (at least three shutbacks in five
seconds) have occurred. A cutback alarm may be triggered by a fault in the transmitter, large VSWR,
or a fault in the control/display PWB's microprocessor or alarm circuitry. Troubleshoot a cutback
alarm as follows:
1. Read the RF output power level on the
RF KILOWATTS
meter on the transmitter's front
panel.
2. Press/release the
Control - Power Increase
switch to clear the cutback alarm and to restore
the preset power level. If the fault persists, the output power will automatically cut back
again. Observe the
Events Log
and note which alarm is causing shutback. Refer to the
appropriate troubleshooting paragraph in this section.
Ext. PDM inhibit
The external PDM inhibit is wired to the remote interface PWB.
An
Ext. PDM Inhibit
alarm indicates that an external PDM inhibit command is present. The alarm
could be caused by an short circuit in the external wiring path to the remote control PWB, a fault in
the switching circuitry on the remote interface PWB, or a fault in the alarm circuitry on the control/
display PWB. Troubleshoot an
Ext. PDM Inhibit
alarm as follows: