Troubleshooting Assistance for:

TX FAULT alarm, no output, no MOD O/P VOLTS or CURRENT.
TX FAULT alarm, no output, no MOD O/P VOLTS or CURRENT, with RF Drive Alarm.
VSWR, RF DRIVE, or MOD DRIVE alarms, with transmitter operating normally.
No output, TX FAULT alarm, no MOD IP Volts.
TX FAULT alarm, with transmitter operating normally.
Meters are reading erratic & RF Sensitive.


TX FAULT alarm, no output, no MOD O/P VOLTS or CURRENT.

Probable Defect:  NAA13 Modulator/Power Amplifier.

Future Preventative Maintenance: Notes:
Lightning protection is of critical importance to the P400, due to its single amplifier.  Any lightning protection measures that can be taken, within reason, should be done.  If assistance with lightning protection is desired, contact Nautel’s Customer Service Department for help with transmitter site review. If FETs Q1 and/or Q2 on the PA are defective, then check the associated driver transistors (A1Q1, A1Q2, and A1Q3) as well.  In addition, A3U1 may be defective.  If this IC is not available, it can be removed and pins 2 and 3 of the socket jumpered together as a temporary solution.

TX FAULT alarm, no output, no MOD O/P VOLTS or CURRENT, with RF Drive Alarm.

Probable Defect:  NAA13 Modulator/Power Amplifier, plus Q5 and Q6 of rf driver pwb (A1).

Future Preventative Maintenance: Notes:
Lightning protection is of critical importance to the P400, due to its single amplifier.  Any lightning protection measures that can be taken, within reason, should be done.  If assistance with lightning protection is desired, contact Nautel’s Customer Service Department for help with transmitter site review. Whenever the PA is repaired or replaced, it is critical that the parallel rf drive inductor, L1, be tuned to match the new FETs to the output of the rf driver.  This procedure is found in the handbook for later versions of the P400, or is available from Nautel.  In addition, this procedure requires a current probe, also available from Nautel, if there is not one already at the transmitter site.

VSWR, RF DRIVE, or MOD DRIVE alarms, with transmitter operating normally.

Probable Defect:  U8 and/or U10 of modulator driver pwb (A2).

Future Preventative Maintenance: Notes:
This situation is always associated with power surges or lightning activity.  Again, increased lightning protection is the best defense. The circuits that are driven by U8 and U10 are for monitoring purposes (local and remote alarms) only.  While the situation should be corrected as quickly as possible, there is no harm to the transmitter if it is operating under these conditions.

No output, TX FAULT alarm, no MOD IP Volts.

Probable Defect:  PA Power Supply (A8).

Future Preventative Maintenance: Notes:
AC line protectors, or surge suppressors are the best defense, as this problem is almost certainly caused by surges or transients on the AC power line. Whenever the output of the PA power supply exceeds a nominal 95Vdc, a crowbar circuit (Q1) in the power supply triggers.  Repeated triggering of this circuit will cause failure of either the Q1 FET or the U1 rectifier assembly.

TX FAULT alarm, with transmitter operating normally.

Probable Defect:  No problems, need to adjust fault threshold.

Future Preventative Maintenance: Notes:
No preventative maintenance is required. Fault tolerance pots are located on the interface pwb.

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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