We love working on FM Exciters!

Somewhere under that dirt is a Harris THE-1.   Late 80/s early 90’s these were the replacement for  the MS/MX series. 

Not much different other than the addition of a 50W power amp. 

 

Problems: Random AFC unlocks,  failed multimeter, power output unstable.   

We’ll start by disassembling the entire unit and cleaning each component.  

 

Not surprisingly most caps on the AFC board were dried out.   These units also use a ZIF (zero insertion force) edge connector that requires a lever be locked to clamp the edge connector into the socket. 

Over time these become intermittent and cause a host of issues. 

 

VCO module was the same.  Aging capacitors and a oxidized edge connector. 

 

Power supply/regulator board needed all new caps.  I also replaced the voltage regulators as operating them with heavy ripple over an extended period of time does not help their lifespan.  And they’re cheap. 

 

 

Reassembled unit except for the front panel.   The multimeter on this unit had failed and Harris no longer stocks the part.  A bit of digging turned up the OEM manufacturer:

Modutec, P/N 2S-DUA-100 100uA.  Allied electronics stocks them at $78.  Just swap in the harris meter face. 

 

 

And it’s back to making RF.  A quick audio test into a Belar monitor set and ready for burn-in. 

55W forward into a dummy load for 48 hours before being returned to service. 


     

And more exciters! 

 

     

These are easily my favorite Exciters to rebuild, the Broadcast Electronics FX Series.  30(left) and 50(right) 

Not much difference between them other than power output and cosmetic changes. 

 

Then..there’s this.  The FMO module on some units is potted in something that just won’t budge, even when heated.  There are four electrolytic caps in there that age and should be replaced. This one still operates correctly, thankfully. 

It takes several hours to carefully cut and scoop out enough of that potting material to free up those caps.  I use a dremel tool to drill tiny holes around the cap, then an artist’s razor pen to cut them out. I’d rather not. 

This one did NOT still operate correctly: 

 

Another FX series FMO module, this time with the caps blown, spewing electrolyte on the PC traces.  This one took some work to clean up and repair the traces.   

BE does still have these FMO modules available new.  About $700, making it worth the time to repair. 

 

   

Power supply regulator on the left wall, AFC module, FMO and far left- RF power amp.  These exciters take only a few minutes to disassemble, making repairs fairly easy. 

There is a DC Pass transistor on the rear of the RF amplifier that tends to fail.  I replace it, along with all capacitors and the  cooling blower just as preventative measures. None of these units are young anymore.  But they still perform very well. 

 

 

One final upgrade is to replace these jumper headers with DIP switches.  It’s a simple matter of unplugging the jumper blocks and plugging in DIP switch blocks.  It makes the units easily frequency agile, at least once you figure out the odd coding scheme.  Don’t forget to add 10Khz…