Armstrong 100 watt FM Exciter.  Approximately 20 years in service.  Built by RVR in Italy, these units are re-badged by Armstrong, Bext and probably others.  They’re generally very good, until the day they aren’t.    Today is that day. 


These are heavy units due to that massive toroidal transformer.  To it’s left is the power supply/control board.  Underneath is the variable switching supply that provides power to the RF amplifier.  To the right  with the large heat sinks is the RF amplifier assembly.  Lower left is the microprocessor control and display board. 


On the left side are the audio input board, Mono/MPX board and the PLL/VCO board.  


This unit has gone into “external mute” and will not turn on.  There is nothing attached to the external mute connector, so the issue is likely in the Power supply/Control board. 


 Here is the control board.  A 7407 is used to buffer several fault signals. The”external mute” signal is one of them.  It has failed, along with a small resistor.  Replaced both, using a higher wattage resistor and leaving plenty of air space around it.   That turned the mute circuit off, but we still have no output power. 


 This is the switching supply that provides 0-22 volts to power the RF amplifier. It is not doing that. 

The electrolytic capacitors were so bad the supply shut down.  Often when capacitors fail the device will run until it’s turned off then never re-start.  After replacing the capacitors it came right back to life. 


This time we’ve got power! 

The exciter ran right up to 100 watts, then backed down to 50W for 24 hours burn-in.  If it passes that without issue we’ll clean up the interior and the exterior, set the frequency and audio settings back from the defaults and return it to service.