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Using All of Your Senses to Troubleshoot

By Archie Stulc

I used to work as a radio engineer for a local radio station in Minneapolis. When I was a neophyte I experienced the cautionary tale from hell. It happened when the department's head guy – our contract engineer – had just gone on vacation. And, just as he had joked about before leaving, of course, the transmitter went down.

I got to the transmitter site only to be met by the general manager, the operations manager and a salesman for the radio station that lived nearby. Each one took turns asking me if I had done the things that I just did in the first few minutes of being there.

All Senses To get some advice and help, I called one area engineer and he basically said "hmmm... good luck." I called the transmitter manufacturer's engineering hotline and was told "it sounds like something in the start up sequence." No, really? That was all the help they could offer.

I called another engineer who actually came out to see if he could help. We both looked over the box, the schematics, checked voltages and pressed all the buttons again. No luck. Then he got called away... his transmitter was on fire! It was a bad day for radio in the Twin Cities.

Around the fourth hour of being there with the downed transmitter, I was finally alone and able to think step-by-step about what to do. I studied the schematic stage by stage and tested each stage in order. Everything seemed to be OK except for the "snap-thunk" sound just before the transmitter failed to start up again.

Professional-grade transmitters have safety interlock switches on each access door, presumably to keep engineers from turning into ash piles while on the job. It is strictly forbidden by the Federal Communications Commission (FCC) to operate the transmitter with any of those switches bypassed. But, every engineer does it... at least for testing.

So there I was with all of the panels open and the interlock system "bypassed like a Christmas tree" (if you saw Star Trek II: Wrath of Khan, you know what I mean). I started using more of my senses to try to figure out what I was missing. I’m looking, smelling and listening for any clue as to which component was causing the problem. I pressed the button and heard "snap-flash-thunk."

Flash?

I pressed the button again and heard the "snap-flash-thunk" sound again. It sounded like it was coming from down in a corner. In that corner, the only thing that could make the thunk and vibration sounds that I had heard was a large contactor. I watched the contactor while I pressed the button again. "Snap-flash-thunk" – found it!

There was a 49 cent diode across the contactor coil for back EMF suppression that wasn't even on the schematic! It had burnt out and was arcing at start up. Fifteen minutes later, and after six hours of being off the air, we were back on.

That day I learned to use all of my senses, including my "sixth-sense" suspicion that the transmitter manufacturer didn't know everything.
Archie Stulc lives in Flagstaff, Arizona and is a retired radio engineer. His interests include computers, fixing things around the house, his cats and Dr. Who. Archie has been a Jameco customer since 1992.