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Technical Tips from Mark W. Persons

Harris THE-1 FM Exciter Modifications

This is a Harris THE-1 FM Broadcast Exciter.  Manufactured starting in the late 1980's, production ran through the 1990's as best I can tell.  We modified this one with oval holes in the rack ears for easy rack mounting.    
One of the modifications I like to make is to put the RF power output control on the front of the exciter so it is easy to adjust.  The original control is on the RF power amplifier module.  To adjust it, you need to slide the exciter out of the rack or transmitter and use a jewelers' screwdriver to make the change.  In my opinion, this is more hassle that it is worth.       
In the center of the photo is the new control.  It is a 10,000 ohm linear 2-watt control.  It did not need to be rated at 2 watts, but it was a convenient size for panel mounting.  Wiring from the new control to the original control location is done with Belden 8451 or 9451 shielded cable.  I recommend this because the exciter will probably be in a high RF location.

The blue trim-pot on the left is a replacement for the original modulation meter calibration control, which was damaged by someone trying to adjust it with a screwdriver, which was too large for the job.   


This is the original RF power amplifier module from a Harris THE-1 FM Exciter.  It was designed to provide 55 watts of RF between 87.5 and 108.0 MHz.  Unfortunately, a some of them have failed in the field, including this one.  To make matters worse, the manufacturer of the power MOSFET semiconductors is out of business and there are no direct substitutes that can replace them.           
It occurred to me that the RF amplifier could be replaced by an amplifier of another manufacturer.  I chose an FM70 Pallet Amplifier Module from Broadcast Concepts.  You see it as a green card at the lower left of the output module.  The FM70 amplifier is about $140 and was designed to output as much as 70 watts of RF power.  In this application and with the RF drive available, this modified exciter will produce 30 watts of RF power at the high end of the FM broadcast band or 40 watts on the lower end of the band.  The really good news is that parts are readily available to repair this new amplifier if the need arises.

I found the low-pass filter on the Harris module had poor performance, which may have led to the demise of the original semiconductors.  More specifically, the return loss, as measured at the input to the directional coupler, was about 8 dB at most points on the FM band.  Retuning it resulted in 20 dB or better return loss and relatively low loss in the filter.  The RF amplifier is much happier looking into this load.

Now that I figured out how to do this one, I could do the same repair on other THE-1 Exciters.            

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

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