My first job out of college was with the Canadian government, working at a major international airport maintaining the radar and communications systems. It was a relaxing job. Not much happened that needed immediate attention, and since we had quite a few technicians on staff, we were able to take our time with our work.
One day I was doing some preventive maintenance on the ground-to-air radio systems for Air Traffic Control with one of the senior techs. Things were going well until I started work on the transmitters. All of the transmitters had a high level of AC hum on them. That’s not great on a critical communications system.
I mentioned this to the senior tech, and he said he was already fully aware of it. For several years, they had been trying unsuccessfully to get this problem resolved. The communications system was designed and installed by a highly regarded communications manufacturer, and the problem had been occurring since the installation. I asked if I could give it a shot.
First, I examined the audio lines that came down from the control tower, which was six floors up. If the problem was coming from the control tower, I could leave the lower racks alone, and focus on the tower. I grabbed a meter, and pulled the audio line for one channel. I checked for any audio on the cable and it was clean. The problem had to be at the lower end.
The strange thing at this point was that the audio lines were RG-59, 75-ohm coaxial cable, normally used for video. They had BNC connectors for the terminations. I would have thought that a 600-ohm balanced line would have been required to reduce noise. Apparently, the engineers at the communications company that did the installation thought coaxial would work. In some instances, it probably would.
When I went to put the cable I had removed back onto its connector, I inadvertently hit the chassis with my hand before the shell of the coax was connected. I received a very large and unexpected jolt, and my meter told me that there was 90V AC on the shield. No wonder there was so much AC hum on the circuit!
I dug around the shop, and found a 600-ohm isolation transformer. I inserted it between the tower audio feed and the equipment rack, then had Air Traffic Control make a test transmission while I monitored the transmitted audio. The hum was gone.
I discussed what I had found with my supervisor, and got the go-ahead to install isolation transformers in each of the transmit audio lines. The total cost was minimal, $105, much less than the cost the government had paid in trying to fix the system over several years.
This entry was submitted by Clint Millett and edited by Rob Spiegel.
Clint Millett has worked in Japan, Siberia, Benin, Canada, and the US, commissioning, supporting, and installing equipment, primarily in the oil and gas industry. He lives in Calgary, Alberta, with his wife and stepson.
Tell us your experience in solving a knotty engineering problem. Send to Rob Spiegel for Sherlock Ohms.
Click here to access the Sherlock archives.