Chuffin QRM…

It came in the night

It was a quiet November morning. The baby was asleep after just being fed and no one else was around to interrupt me. The idea time for a spot of radio.

I made myself a cup of tea and sat down in the operating position. 05:04, I should be able to get a hour before the rest of the house starts to wake up.

I powered up the ic-7200 and spun the dial to 40 metres. There was a surprise waiting for me.

S9+60dB of QRM. A continuous “chuffing” sound like a steam train running full speed across the band. I spun the dial again and found the noise covered from 3MHz all the way up to 14MHz starting from S3 and peaking S9+60dB at 7MHz then dropping back down. Typical. 40 metres is my favorite band.

I tuned up to 10 metres and sipped my tea in the hope that the QRM was temporary and would stop soon. 6 hours later when I checked back on 40 metres, it was gone. 40 metres was usable again!

My joy was short lived. Over the next few days the noise made an appearance several times for hours on end with intermittent gaps. I started making a log in the hope of finding a pattern but it was random, until 2 days ago. The noise started and remained constant all day through the night and was still there in the morning. It was time for action.

My war on QRM

My Eton G3 receiver was my first weapon of choice in my new war on QRM. With a fresh set of batteries installed, I tuned the set to 7MHz where the strongest interference appeared and went for a walk. I could hear the QRM getting louder as I walked to the side of the house. Out into the side courtyard and the noise grew stronger. I was convinced that the noise was coming from the next door neighbour who had recently gone away for a few weeks. I started jumping to conclusions. Have they put a car battery on trickle charge? Is their alarm system faulty? Yes, that must be it. They put their alarm on when they are out or asleep which would account for the randomness and they are away which would account for the constant noise over the past few days.

I caught myself jumping to conclusions and stepped back. Was I sure it was coming from next door? Could I identify when could be the cause from the sound of the noise? Well the first question was easy to answer. I headed to the main fuse board and flicked off the main breaker. Seconds later I thought about my wife who was busy working away on the PC in the office and expected to hear some verbal abuse heading my way. Nothing, she must have finished just in time for me to kill the power. That was lucky! Unfortunately, the QRM still remained.

Convinced that the noise was coming from next door, I decided that I needed to escalate this conflict and call in the heavy artillery. I managed to make an audio recording of the interference and went looking for some answers.

Many heads are better than one

I needed help, specialist help which came in the from of the exelent UKQRM group on Yahoo. I posted details of my problem and what I had done so far with the sound file and asked if anyone recognised what it could be. The answer soon came back, “it resembles a faulty SMPS that is in standby mode” and “Or a television switched mode PSU giving problems when the set is switched to standby” were 2 of the suggestions. It was beginning to look like my “faulty alarm” conclusion may have been correct.

The advice was to approach the neighbours on their return so I started thinking about ways to broach the subject. I decided to sleep on it.

A new dawn

I woke in the morning to find my wife in the office. I turned on the radio hoping for silence but the noise was still in full swing. The neighbours were away for 8 weeks and I didn’t look forward to the prospect of no lower HF for another 2 months. I was in a state of semi depression. “Why did you turn the power off?”. The question came from my right. I explained that I was suffering from interference and the steps I had taken to track down the source. “I’m sorry if you lost your work when the PC went off” I said. I was almost knocked over with her response. “It didn’t go off”.

How could I have forgotten? Almost a year ago I installed a small UPS around the back of the office PC and another in the kitchen to power the internet modem and WiFi. When I turned off the power, the UPS kicked in and kept the PC and modem running. I had a glimmer of hope.

Armed with my trusty G3, I shut down the office PC and removed the UPS. The noise was still there. One last hope. I went to the kitchen (which is the closest room to next door) and shut down the UPS. Silence!

The source

I unpluged the 4 way power strip from the UPS and powered it back on. Still no noise. The UPS was clean. I proceeded to plug in each power adaptor for the various equipment one at a time. Still no noise. Everything was powered up and the QRM had gone. I was confused. Maybe next door’s alarm system had given up just as I unplugged the UPS?

I went back to my main radio and spend 20 minutes enjoying 40M. Suddenly, in a surprise attack, the noise came back but this time I was prepared. Back into the kitchen to check the UPS. I paused for a moment and thought about the best way to find the source. Instead of turning everything off and plugging them back in I decided to start unplugging things one by one. First, the WiFi which made no difference. Next, the BT Infinity modem. No change. Next, the wired router. Silence. The G3 went quiet and I had found my source.

A quick search through the junk box and I soon found a new PSU. I plugged it in and waited. 2 hours later and the noise is still banished. The war was won.

Noisy PSU

My lesson

I learnt an important lesson during the course of this saga. No matter how sure you are, check and check again. Sometimes it takes someone who is a step away from the problem to find a solution.

Thanks to the UKQRM group for pointing me in the right direction and to my wife for finding the solution.

2 Replies to “Chuffin QRM…”

  1. Hi Matt,
    Reminds me of a similar QRM problem here at home that bugged me for a while – short story is that the 12V LED MR16 lamps in the bathroom ceiling must have some sort of regulator or control cct, and between the 5 of them can put out a fair broadband signal!

  2. Thanks for your comment Dave.

    I’m always being surprised by odd sources of noise. We have some touch controlled lamps in the house which caused a few problems. I had to open them up and add a ferrite bead and small RF choke to get them to behave!

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