Troubleshooting Assistance for:

Current meter on one or more power modules at 1/3 or 2/3 normal power.
Transmitter at 1/4 power or no output, no alarms.
Output severely reduced, transmitter shutting back erratically.
Power block (group of four modules) not operational. Could be as many as all three blocks in one rack.
Burned wires on relay control, repeatedly blowing 40A fuses on relay panel or 2.5A fuses on interface panel.
Meters are reading erratic & RF Sensitive.


Current meter on one or more power modules at 1/3 or 2/3 normal power.

Probable Defect:  Modulator failure in one of the three PA’s contained in the affected power module.

Future Preventative Maintenance: Notes:
Modulator failures are most commonly linked to power surges or continued operation at high ambient temperatures.  Increased voltage regulation and regular fan filter cleaning will usually help. It is possible to determine which of the three PA’s is defective while the power module is still installed in the transmitter.  Insert the negative lead of a dc voltmeter through the grill on the front of the module, and measure the voltage from chassis ground to the case of the modulator FET nearest the front for each of the three PA’s.  Operating PA’s show a voltage equal to that on the module test point (nominally 29Vdc), the defective assembly will measure 0Vdc.

Transmitter at 1/4 power or no output, no alarms.

Probable Defect:  Either one or both of the main B- 20A fuses is blown, or one or both power supply rectifier assemblies are defective.

Future Preventative Maintenance: Notes:
Rectifier assembly failures are associated either with loose hardware or high operating temperatures.  Annual hardware checks, and regular cleaning of fan filters is the best prevention. The 20A fuses are located on the inside rear wall of the control cabinet, behind and slightly below exciter B.  AC mains should be switched off before checking/replacing.  For transmitters with poor site ventilation, an upgrade is available which increases airflow over the rectifier stacks, reducing operating temperature by 10-15°C (18-27°F).

Output severely reduced, transmitter shutting back erratically.

Probable Defect:  Failure of one or more components in the rf output filter.

Future Preventative Maintenance: Notes:
Similar to rectifier assemblies, failures in the rf filter are most commonly associated with high temperatures or loose hardware.  Annual hardware checks, and regular cleaning of the fan filters is the best prevention. The rf filter must be kept free of dust, in addition to the requirement for adequate ventilation, so regular cleaning of fan filters is a necessity.

Power block (group of four modules) not operational. Could be as many as all three blocks in one rack.

Probable Defect:  Interface panel relay (K1-K4) defective.

Future Preventative Maintenance: Notes:
This is usually a statistical failure for which there is no preventative maintenance. There is a protection diode on the relay coil (mounted on the relay socket, on the rear of the interface panel), which can short if subjected to a transient.  This can be tested by pulling the relay and measuring across the socket.

Burned wires on relay control, repeatedly blowing 40A fuses on relay panel or 2.5A fuses on interface panel.

Probable Defect:  On older style relay control panels (NAX25/1, /1A, and /1B), this is a sign of arcing of the relay contacts.

Future Preventative Maintenance: Notes:
Regular burnishing of contacts for open face relays is recommended.  If the relay panel is of the newer type, with a sealed relay (NAX25/1C), contact Nautel to confirm that the relay driver circuits are up to date. Quite frequently, for transmitters which are retrofitted with the NAX25/1C, the relay panels are shipped less the control pwb, to reduce cost.  However, on older style pwb’s, the relay drive transistor and a bias resistor must be changed to positively switch the new relays.

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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