Troubleshooting Assistance for:

TX FAULT alarm, one or two PA FAIL alarms on the NASM1A modulator, 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.
SWR alarms and cutbacks with no indication or reflected power.
Meters are reading erratic & RF Sensitive.


TX FAULT alarm, one or two PA FAIL alarms on the NASM1A modulator, 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.  The NAA11’s are in pairs,eg. an “A” indication on the modulator could indicate a failure of either or both of the two leftmost PA’s.

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

Probable Defect:  NASM1A modulator module, or NAS15 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 NAS15, with the transmitter running, switch the NAS15 off, wait for the MOD I/P volts to discharge, then switch the NAS15 on again.  If the MOD I/P volts charge back to the full value (nominally -72Vdc), then the problem is with the modulator (NASM1A), otherwise, there is something wrong with the NAS15.  Most likely cause is blown 40A fuses, possible shorted thyristors.

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/contact enhancer, 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.

SWR alarms and cutbacks with no indication or reflected power.

Probable Defect:  Narrowbanding of antenna system.

Future Preventative Maintenance: Notes:
Unfortunately, there is no preventative maintenance, other than regular impedance sweeps of the system. Temporary solutions are:  reducing output power, or reducing high frequency modulation.

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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