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  and 60+ years of broadcast engineering 
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Technical Tips from Mark W. Persons
 BE FX-30 FM Exciter


Here is a Broadcast Electronics FX-30 FM Exciter on the bench undergoing testing.  Equipment employed in the diagnosis and repair of exciters includes an RF wattmeter, frequency counter, modulation monitor, RF spectrum analyzer, oscilloscope, audio voltmeter, audio oscillator and audio distortion analyzer.


Here is a portion the AFC/PLL (Automatic Frequency Control/Phase Locked Loop) circuit board assembly from a Broadcast Electronics FX-30 FM Exciter.  The original Vectron 390-0001 TCXO (Temperature Compensated Crystal Oscillator) went off frequency and could not be brought back with the trimmer capacitor.  It is a 10 MHz module that runs on 5 VDC.  The oscillator is in the upper left of this photo and the trimmer is adjusted through the upper left hold in the oscillator.  In December, 2011, Broadcast Electronics wanted $418 for a replacement TCXO.
 


An excellent $62 replacement part was purchased from Digi-Key.  Their part number is 744-1050-ND.  It is a Crystek CXOH20-BP-10.000.  This new oscillator has twice the frequency stability of the original oscillator and is half the size.  However, it was not a drop-in replacement.  A new hole needed to be drilled in the circuit card and three of the wire leads needed to be soldered to the circuit card on the top side. 

It all worked out very well and the repair returned the exciter to its original factory specifications.
 


One place you can get into trouble in the field is when you set the frequency of the exciter.  The labeling is not as specific as you might like.  To help others who might be retuning this exciter in the future, you should added markings to help them understand.  In this example of 98.1 MHz, the 8 and 1 switches in the10 MHz window need to be pulled to the "off" position to get 90 MHz.  Then 8 only in the 1 MHz window to get an additional 8 MHz.  Double circled in red is the notation that 10 KHz needs to be added to the selected frequency to achieve the desired frequency output from the exciter.  Instead of putting 1 off in the 100 KHz window, all switches need to be left on.  Then 8 and 1, for a total of 9 need to be turned to the off position in the 10 KHz window.  That means the dip switches are set to 10 KHz below the desired FM channel. 

It all makes sense once you understand it, but can be confusing to anyone doing it the first time.  Also, best to check the exciter output with a frequency counter before putting it in service.  One or more of the dip switches may be set incorrectly.    
 


Whenever shipping one of these exciters, it is extremely important to install the "shipping screws" on both ends of the modulated oscillator module.  The module stands on foam rubber and can rip away during shipment causing damage.  One of the shipping screws is circled in red in this photo.  It is a 6-32 machine screw.  They can be removed when the exciter goes in service to get better than 70 dB audio signal to noise or can be left in especially if the exciter is not mounted in a transmitter with vibration that could raise the audio noise floor a bit because components in the modulated oscillator can be microphonic.  

 (Story from December 30, 2011)     

All of the electrolytic capacitors in this exciter should be replaced if they have not been replaced in the last 15 years.  It makes good sense when the exciter is in the shop for any kind of repair or retuning.

If you are looking for a replacement 2N6200/B40-28 Transistor for the RF output stage of the exciter, they are no longer available.  A wonderful drop-in replacement is the NTE360 Transistor.  Transistor failures are often the result of an arc-over in the transmitter tube stage that the exciter feeds.  To protect the exciter against hundreds or thousands of volts of voltage spikes, a shorted quarter wave stub filter should be installed.    

The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   ham W0MH      

Questions?  Email Mark Persons:  teki@mwpersons.com       

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