Nautel XR12, Troubleshooting Manual, Page: 52 / 283

XR12 Troubleshooting Manual Responding to alarms
Page 1-32 Issue 3.0 2009-07-28
check” on page 1-36 to attempt tuning of the RF drive. If the
RF Drive
LED remains on
after tuning, suspect the RF drive control PWB.
8. Temporarily latch all of the protection/alarm circuits in their reset state by connecting a
jumper between ground (TP3 or TP4) and TP2 on the RF drive control PWB.
9. All of the alarm LEDs, including the
Block
(
A
,
B
,
C
and
D
) alarm LEDs, shall turn off.
Modulator checks
Measure the filtered PA volts output of each modulator, which is controlled by the on-time of the
PDM input. The PA volts for each RF power block should be a percentage of the B+ (48 V) being
applied to the associated modulator assembly. The output voltage of all four modulators should be
within 1.5 V of each other. If one voltage measures low or high, suspect the associated modulator.
1. Measure the dc voltage between terminal
H
of each PA input/output PWB and ground,
using a digital multimeter.
2. The voltage level depends on the PDM level being generated by the transmitter. If all
measurements are satisfactory, proceed to “RF amplifier checks” on page 1-33 .
3. If any voltage measurement in Step 1 is not satisfactory, assume the modulator assembly for
the RF power block associated with the unsatisfactory measurement is defective. Replace the
suspect modulator assembly (see
“Modulator assembly replacement” on page 1-47 ).
NOTE:
If the test circuits B+ supply fails when the temporary jumper is installed in Step 8 , suspect a
failed power amplifier (NAA51A). Thermistor RT11 on the distribution PWB will limit the
current available to the test circuit resulting in the 48 V going to near 0 V.
Repeat the checks in “PA output to ground impedance” on page 1-29 to determine if there are
any shorted PAs.
NOTE:
Sufficient output power must be provided to allow the PDM signals, generated by the
transmitter, to drive the module under test. Use an oscilloscope to measure the duty cycle on
TP17 of the interphase PDM driver PWB. Increase power until the duty cycle is above 30%.
The PDM input is a square wave with nominally equal on and off times. If the on time is 50%
of each cycle, the PA volts would be 24 V (50% of the 48 V test B+ voltage).