160 meters RFI caused by low voltage bookcase lighting

Last week I encountered some RFI on 160 meters which only appeared at night, but the on and off times were not consistent. On Friday the on time was approximately 9:45pm (Friday) and the off time was approximately 7:35am (Saturday), and today (Monday) the on time was 1:29am and the off time was 6:35am. When the interference first starts it drifts upward in frequency a considerable amount before becoming somewhat stable (maybe a half hour to have minimal drift). The signal is present at intervals of approximately 41 khz which sounds like a switch mode power supply related issue.

Recording of interference on youtube : http://www.youtube.com/watch?v=ijTXCMibcaQ&feature=youtu.be

A spectrum plot of the audio output on an AM receiver (DX-440) shows considerable 120 hertz component (twice that of the 60 hertz line frequency) as shown in the spectrum plot below.

Click on image below to enlarge

Initial direction of the interference signal was estimated to be 10 Degrees from WD8DSB using 3 fixed point fed pennants (pointing 40 deg, 160 deg, and 300 deg), and using portable direction finding equipment it turned out to be approximately 20 degrees at a distance of 430 feet (to the utility feed on the side of the garage of the offending home).

Portable direction finding equipment :

DX440 receiver

W1FB adjustable preamp

Tuned 4 turn 13" diameter shielded loop (tuned to approximately 1.82 mhz)

Finding the problem

On Saturday (Sept 27, 2014) I contacted the homeowner and told him about the interference, and that on first pass I thought it was coming from his home. He was very cooperative and willing to help figure out the source, and we exchanged cell phone numbers which would allow us to text each other, and he also gave me permission to wander his property as needed.

This morning (Monday Sept 29, 2014) the noise was again present so I decided to camp out in front of the suspect home to see if its outside CFL lights were the cause of the interference. To my surprise the interference went off at 6:35am while the outside CFL lights were still on (I also noticed some lights came on in the house at approximately the same time). I then sent a text message to the homeowner letting him know that the interference went away at 6:35am today and therefore it did not appear related to his outdoor lights. I then headed off to work but within a couple of minutes the homeowner responded via text and asked "Did it go on again?". I therefore went back to his house, and sure enough the interference was back on and I sent him a text saying "Yes it is on right now and I am at your house". Then the interference went off, and he sent a message saying "Is it off?", and I sent a text message saying "Yes yes yes it is off" He then sent me a text message saying the problem is "Low voltage lights in a bookcase". We will now investigate his low voltage lighting system. More info to follow.