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Thanks, Ermi, for the link and for doing the math.
What follows is some rambling memories of my campus carrier current adventures (from the ’60s) which is primarily for entertainment and to describe what can go wrong.
Some of you might remember my mentioning in other posts that I was involved with the carrier current operation on the university campus when I was a student. We did a lot of things right but at times things went wrong. Mostly, the five or so transmitters around campus were built, maintained, and operated by ham radio folks from the electrical engineering student pool, but with the post Sputnik emphasis on engineering rigor the students found it almost impossible to continue in this role and only I was left after a while and that lasted about six months.
The studios were located at a conservatory of music about 1/2 mile off campus and everything was audio linked by leased phone lines. The transmitters all were loaded into the power system in each dorm and the field strength just a few tens of feet from the dorms was not enough for the signal to be usable on a portable radio even though the transmitters were rated from 5 to 25 watts input. The signal for a line operated radio in the dorm was really good though.
Here’s what went wrong. I was in a co-op program, three months at school, three months away working. I was on the engineering staff and knew the faculty advisor and the “jocks” so when one of our transmitters went wacky while I was on work section, listeners heard the university’s name on the ID and called the full power licensed FM campus station to complain about their signal being heard all over the AM band in one of the dorms. The FM staton called the advisor who gave them my name and from campus records they found and called me at my work residence some 60 miles away. Uh Oh…now what do I do?
Since it was a weekend, I drove to campus to investigate and found the 25 watt tube transmitter which I had built and tested to be engaging in Barkenhausen oscillation. Not good! Never happened before. So I retuned the transmitter and the loading and all was well, but I still did not know what went wrong. I did notice that the tuning was off quite a bit and it took a lot of adjusting to get it right. Was someone else messing with the transmitter?
Yep. The station’s “chief engineer”, a music performance major with no ham or electronic background, had performed “routine maintenance” on the unit. We later had a staff meeting and reorganized a bit with a new “chief engineer”….me!
Things went pretty smoothly thenceforth until the day that I was either fired or quit…probably both. I had gone to the studio to find out why all the audio feeds to campus were distorted and had been that way for weeks. The leased phone lines were connected to a patch panel in a rack in the studio and at the top of the rack was the “audio distribution amplifier”, a tube type Heathkit Mono HiFi amplifier. Inspection revealed that the glass envelope on one of the push pull output tubes was broken. I unpatched the feed and removed the amplifier so I could clean it and replace the tube. Had I known then what I know now about politics, I should have at least told the faculty advisor what I was doing, but I didn’t and he walked in and went ballistic when he found out that we were “off the air”.
Another five minutes and it would have been fixed but due to the verbal abuse I walked (or was fired) and I don’t know what happened after that.
I have been told that a few years later the FCC shut them down for a year because someone had taken the 25 watter and used it to feed an antenna strung between the light poles at the campus stadium. Rumor has it that they were heard at the FCC monitoring post some 40 miles away. It is surprising that someone smart enough to make this work was dumb enough to try it. As far as I know, the station is operating to this day. When the conservatory was moved onto main campus there was a major infusion of money for the performing arts and the station benefitted with new equipment. They have a website up so I presume all is well with them.
Thanks for letting me recall fond memories and I hope it wasn’t too boring for you. If you read this far then I guess it wasn’t.