Troubleshooting Assistance for:

One or two PA FAIL alarms on one or more of the NASM1A modulators, reduced output power.
TX FAULT alarm, all three PA FAIL alarms on the modulator, no output power.
No alarm indications, but power jumping around, heavy distortion, or intermittent operation.
Reduced output with no alarm indications on NASM1A modulators.
Meters are reading erratic & RF Sensitive.


One or two PA FAIL alarms on one or more of the NASM1A modulators, reduced output power.

Probable Defect:  Power Amplifier (NAA11) failure.

Future Preventative Maintenance: Notes:
Lightning is the most common cause of power amp failures.  Disconnecting the transmitter from the antenna system when not in use will solve some problems, in addition, good lightning protection is critical. The NAA11 power amplifier can also exhibit intermittent failures, where turning the transmitter off and on again clears the alarm.  This is often an indication of a weak FET.  As well, on lower frequency (<800 kHz) transmitters, the failure indication may be a false alarm, for which there is a modification for the PA fail detector in the modulator.

TX FAULT alarm, all three PA FAIL alarms on the modulator, no output power.

Probable Defect:  NASM1A modulator module, or the associated NAS13 rectifier/regulator.

Future Preventative Maintenance: Notes:
Modulator failure can be caused either by power amplifiers shorting, or by power surges.  Rectifier/regulator failures are almost always caused by power surges.  AC line protection is the solution. To check whether the problem is with the NASM1A or the NAS13, with the transmitter running, switch the NAS13 off, wait for the DC volts to discharge, then switch the NAS13 on again.  If the DC volts charge back to the full value (nominally -72Vdc), then the problem is with the modulator (NASM1A), otherwise, there is something wrong with the NAS13.  Most likely cause is blown 50A fuses, possible shorted thyristors, however, the NAS13’s also have thermal circuit breakers (S2).

No alarm indications, but power jumping around, heavy distortion, or intermittent operation.

Probable Defect:  Jones plugs or LOCAL/REMOTE switch.

Future Preventative Maintenance: Notes:
Over time, the Cinch-Jones connectors used on all plug-in modules will develop a clear, varnish-like oxidation.  Annual cleaning with a high quality cleaner degreaser, such as Cramolin Red or Stabilant-22, will eliminate this situation. On the rare occasion, the female sockets in the transmitter chassis will also require cleaning.  This can be done with a fine grit burnishing tool.  If problems persist, contact Nautel for assistance.

Reduced output with no alarm indications on NASM1A modulators.

Probable Defect:   AMPFET 5 – NAFP7 current probe,
AMPFET10 – NAFP7, NAH24 combiner or output filter.

Future Preventative Maintenance: Notes:
NAFP7 failures are usually lightning related, and limited to IC U1 on the pwb.  Combiner failures most frequently occur when a PA is removed while the transmitter is operating, and filter failures are most often associated with loose hardware or poor ventilation (overheating). A filter or combiner failure can usually be located by a visual inspection (arcing/burned insulation).  Failures of the current probe require checking the cutback line into the monitor module – this line should be high (+15vdc) under normal circumstances.

Meters are reading erratic & RF Sensitive.

Future Preventative Maintenance: Notes:
Wipe the face of the meter with a cloth containing a small amount of dish detergent or antistatic spray.

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