Ed Murlatt & Associates, Inc.
RF Services for the Broadcast Industry
Maintaining Soft Start Circuits
The information below discusses work performed in 480 Volt circuits, and is intended for qualified engineers.  Primary power should be removed from the transmitter prior to servicing.  As a safety precaution, one should not work around high voltage circuits alone or when tired.

I've had a run of soft start failures in the last month.  It pays to keep an eye on the soft start circuits.  Periodic checks of lug tightness should be supplemented with monitoring connection temperatures.  I use an infrared thermometer to check the temperatures on the jacket of the wires as close to the connection as possible.  If one of the phases is significantly hotter than the others, its a good indication of a pending failure. 

Some years back the transmitter company currently known as Comark recommended checking lug tightness in their soft start circuit on a regular basis.  That is a good idea on all transmitters, not just Comark.  However, it became apparent this procedure alone is not sufficient.  The addition of thermal monitoring should aid in early detection of failing contactors.  While I carry a Raytek Raynger ST60 Pro, I have seen Fluke infrared thermometers that do a very good job in the $100 range.  I would steer clear of bargain basement models.

The contactor below was removed from a Comark (nee Thales) transmitter in which the lug torque has been checked every six months by yours truly.  The second stage contactor did not engage properly, causing one of the surge resistors to overheat.  In an abundace of caution, the station opted to replace the surge resistors (all three come as an assembly), and both contactors.  I recommended they check the lugs one week after repairs, and they needed to tighten most of the connections.  Since the repairs, they have also been monitoring the temperature of the connections.   

    Overheated Surge Resistor              Overheated Contacts                     Overheated Contacts                       Repaired Circuit 
Overheated Surge Resistor Overheated Contactor Overheated Contactor II Repaired Soft Start Circuit
Shortly after the incident above, I was called to check an Axcera transmitter that was experiencing primary cavity arcs.  That turned out to be an issue with the IOT.  While I was conditioning the new IOT (see a related article about upgrading the ion pump supply here»), we had a step start failure.  A careful inspection of the contactors in the step start circuit showed an overheated wire on one terminal of contactor K3.  Upon further inspection of that contactor, we discovered the contacts had been overheated.  The contactor and all three wires feeding it were replaced.  We had to go to a local electrician to have the crimp lugs affixed to the welding cable, and the contactor was ordered through a local electrical supply house.  The time delay relay was also replaced, as it was taking too long to command the second stage contactor.

                               Overheated Crimp Connector                                                           Overheated Contactor
Overheated Cable Lug Overheated Contacts