January 2017
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Harris FM25-K

Old blue, I like to call them, the Harris 1980’s model transmitters with black faces, white cabinets and blue trim. I have yet to find one that I really like, the FM 25-K is, well okay. Sort of like that 200,000 mile jeep that works, most of the time, and it’s paid for.
This particular FM-25K transmitter is located at WIZN in Charlotte, VT.

Harris FM25-K transmitter

This transmitter was new in 1987.  It had a bad day yesterday, deciding to throw a temper tantrum and trip the HV power supply breaker.  Fortunately, the station has a back up transmitter.  When we arrived, we found the HV power supply feed through insulator at E1 arced over and broken.  Again, fortunately this station’s management believes in stocking spare parts and a replacement was on hand.

Harris FM25K HV power supply feed through insulator

This is part of the RF filter for the HV power supply. This happened once before, about two months ago. The replacement insulator then was used, so that might be a factor. Two months ago, both capacitors in the Pi filter and the HV power supply cable (RG-8 coax) was replaced all the way back to the rectifier stacks.

The FM25-K can produce spontaneous high frequency oscillations if not tuned properly.  We looked at transmitter output with a Rode Schwartz spectrum analyzer and found it to be clean.  Exactly why it blew out another feed through insulator is a bit of a mystery.  Since the first replacement was a used part, we surmise that it may have been cracked.  If this replacement insulator arcs, there needs to be a full investigation.

As I said in the beginning, I have found these transmitters to be okay, not the best, not the worst.  Most of the problems I have encountered with the K series FM transmitters had to do with the controller cards.  There are two, one analog and one digital.  That’s what Harris calls them anyway.  Like the SX transmitter, and the MW transmitter to a certain extent, the control circuits are way over complicated and full of +/- 5 volt CMOS logic.   Having that type of control logic connected to a radio tower (e.g. lightning rod) is asking for trouble.


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