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 24 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. 

 

  The FMO module on some units is potted.  There are four electrolytic caps in there that might need replacement. This one still operates correctly, thankfully. It takes several hours to access those potted caps.

 

This one did NOT still operate correctly: 

 

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

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. 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 once you figure out the odd coding scheme.  Don’t forget to add 10Khz…

 


Another day, another FX30 Exciter! 

This one had all sorts of bad behaviors, including a rumble on-air. 

 

First stop is a new Fan. These Papst fans are pretty well built, but eventually the bearings wear. 

 

Main filter cap is also long past it’s useful life.  The new ones are much smaller and have better temp ratings. 

 

Pass transistor needed replacing.  Shorted.  Looks like it’s not the first time.  Fortunately the final transistor is still good.  The 2N6200  is no longer available and the NTE 360 we used  as a replacement is also discontinued.   

Upper left is an optional output low pass filter, meaning this exciter can be used as a low-power transmitter. 

 

This one still has the adjustment tool!  Most have been swiped long ago. 

 

Second problem- the meter movement had fallen apart causing the needle to stick.  These have not been available from BE for quite a while..but there is a workable replacement that is reasonably priced.  Just swap the meter face with a little trimming. 

 

And here’s the Rumble.  The FMO is sitting directly on the metal chassis.  It was picking up vibration from the old noisy fan.  Some models have rubber isolators for the FMO, others have a foam sheet underneath. Both require you remove the shipping screws before use for best noise.  Please put them back before shipping. 

 

This one needed the rubber isolators.  BE no longer stocks these either.  Fortunately I have some on hand. 

 

They snap in the bottom of the FMO, isolating it from the chassis. 

 

Power supply regulator still has the factory caps.  They had failed to the point the regulator no longer functioned. 

 

Same situation with the control board.  Failed capacitors. 

 

The AFC/ Frequency control board had the worst of them. Quite a few were bulged and leaking causing damage to the PC board.  This took a bit of scrubbing and repair.   I will also change all four  dip switch packs as they get flaky over time.  

 

Here is one of many capacitors that bulged and vented.   Heat and age…

 

Bolt everything back together and off it goes! 

 

32 watts into the dummy load  with 100% modulation applied for 24 hours, a quick check into the Belar test set….. and back to it’s owner. 

 

 

BE FX50

 

If there were awards for best overall analog FM exciter, this one will win a ribbon. Broadcast Electronics FX-50.  They still sound great even by today’s standards.   

 

 

We’ll start by stripping this one down to the chassis and cleaning each part.  Years of mouse stuff needs to be removed. 

 

The metering on this one did not latch in.  Found a bad IC, fortunately nothing obsolete. Yet. 

 

Cleaned and ready for new capacitors. 

 

This one had a non -potted FMO. That makes cleaning and recapping fairly simple. 

 

That is dirt mixed with bearing dust.  The fan was locked up. 

 

And replaced. 

 

Refurbished voltage regulator board. 

 

This one still had the shipping screws and tag in the FMO.  The foam underneath had rotted.  Closed-cell foam sheets make an acceptable replacement. 

 

Back together, metering working again and making power into a dummy load for 24 hours..then back to the customer.