Sunday, August 10, 2008

Tracking down the elusive electrical noise in my home

I have finally found the annoying “mystery noise” I’ve been hearing on HF (mostly 75 meters) today. I first begin to hear the noise after I moved my Yaseu 857D from my outdoor (not air conditioned) ham shack to the comforts of a spare room in my home. While it is great to have the space (and air conditioning) I immediately began to hear a crescendo of noises that sounded like “birdies” or raspy “carrier like” noises. In one case the noise appeared every 100 KHz pretty much from the broadcast band to 10 meters!

I first had to determine if the noises were radiated signals or were, instead, internal “birdies” sometimes heard from receivers of poor design The way I did this was to switch the 857 to dummy load and verify I could still hear the noise – I still heard it even though it was somewhat weak.

The next step was to determine if it was an internal “birdie” from the 857 or a radiated signal. By disconnecting the HF antenna all of the noise disappeared so I was pretty sure it was not a birdie.

At this point I became convinced the source of the noise was something electrical inside my home. I powered up the 857 from a couple of AGM batteries and tuned the VFO to a spot where I could hear the noise quite loud. I then went to my power switchbox and tripped the main breaker.

What do you know – the noise went away! In fact, I’ve never heard the band so quiet!

After I turned the main breaker back on it was just a matter of tripping the individual breakers until I found the offending circuit. Turns out the noisy circuit was one feeding one of those “touch lamps” – the kind where you touch the metal lamp base and the bulb steps thru three levels of brightness. Those things are awful noise generators! I unplugged the lamp, put the breaker back on and listened to verify the noise was gone. Thankfully it was. But, then I heard yet a different noise which I quickly tracked down to a horizontal sweep signal from the TV upstairs.

That was easy to fix -- just turn off the TV.

But wait – I was hearing yet another raspy noise on the frequency. This one was a little harder to find but it was definitely man made inasmuch as it appeared on even frequency multiples across the band. I suspected the source to be a switching power supply, perhaps my UPS or maybe even the laptop computer itself. It took a while to pin it down but when I unplugged the laptop charger the noise went away – and resumed when the charger was again connected to the computer. It didn’t matter which end was unplugged, the AC supply end or the DC cable at the computer.

So, now I am finally happy. I have found the offending noise and those I can’t eliminate at least I know what to turn off should I want to listen to the same spot on the dial where these local oscillations are occurring.

In most cases it would not even matter but this morning the touch lamp was just tearing up the Georgia Cracker Net on 3.995 MHz and that is what got me started. I would have probably just written it off as a distant carrier on the band but because I was able to listen to my friend Lyndy’s Brannon’s station via his streaming audio setup I knew it was not affecting the entire net – It had to be something local.

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