DVD Recorder Keeps Changing Time

[TANAKASAN]

Here's a weird one.

I have a Samsung DVD Recorder that is also fitted with a hard drive. Once in a while the time and date changes from my local time to some random setting, for example this morning at 7am it insisted that it was 04:35 on August 18th 2015. The recorder has already been to Samsung and they replaced free of charge the PCB that contains the CPU and time circuits but the fault remains. Now the mystery:

1) The time change always occurs at night some time between midnight and 07:00 CET

2) There have been no brownouts or interruptions in the mains supply, I know this because the computers reset if the power gets iffy.

3) If I fit a 1 MHz low pass filter on the mains lead the fault does not occur (this is the radio connection).

4) A careful connection of my oscilloscope/spectrum analyzer to the mains supply shows only tiny amounts of RF

5) Operating 100W on all HF bands has no effect on the clock or the picture and sound, I do not have a TVI problem. In any event this is happening when I am asleep and not on the air.

So, what could be coming in along my mains supply that would reset the clock on a DVR to random settings?

Tanakasan

[KA5N]

Ethereal drift.

Allen

[TANAKASAN]

Got you puzzled huh?

I set up a spectrum analyzer on peak hold mode and left it running every night until I experienced a clock shifting event. At exactly 3am there was a 10v signal on the AC line at 125.1 KHz and this repeated at 3:30am (link a spectrum analyzer to a PC and you can do all sorts of tricks).

So, what would use 125.1 KHz for signaling along a mains line?

Now that I know what's causing this I can build a 125 KHz notch filter but it's still a puzzle.

Tanakasan

[N4CQR]

So, what would use 125.1 KHz for signaling along a mains line? Tanakasan

Electric Utility meter maintenance or signaling perhaps?

Here the utility offers a control box where they can cycle off AC units in the summer for a few minutes to ease the electrical load. I am told this is done at a low, low frequency due to distance and and interference.