Distorted residential power, only at night? 3

[st50maint]

For many weeks I have had this problem with my power. I hear a slight buzz from my audio equipment, only at night. Hooking up an oscilloscope shows this waveform.

?
At dusk the waveform becomes dirty. At dawn it becomes clean. Everyday like clockwork.
Some more clues:
1. The problem is only on one of the 120 volt legs. The other leg is always relatively clean.
2. I thought that street lights might be a problem because of the timing. However I observed them on a few occasions and they did not come on or off exactly with the start or end of the distortion.
3. This is a residential only neighborhood with 14 houses and 3 streetlights on one transformer as far as I can tell.

Any thoughts on what is causing this and a possible solution?

 

[marks1080]

Can your scope do a FFT? You can prove harmonic distortion (or disprove it) yourself.

 

[st50maint]

No, my Tektronix RM504 can not do FFT. However, I did hook up an audio distortion analyzer to the AC. It reads about 3.5 percent THD all the time. I assume this means that the problem is not harmonically related to the 60 Hz.

 

[Skogsgurra]

An audio distorsion analyzer isn't the best choice for AC power. But the number seems to be in the right community or even ballpark. The THD doesn't say much, I have had installations with 14 % THD and no problems. What makes your equipment buzz is that higher frequency transient at the end of the flat top.

You never told us if the buzz comes from the speakers or from your amplifier. Which one is it?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[st50maint]

The buzz is not heard from the speakers. It is heard faintly from the electronics, noticeable only when the speakers are silent.

 

[Skogsgurra]

Does the electronics have a good old-fashioned power transformer in the PSU - or an SMPS?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[st50maint]

Oddly enough the unit with the buzz is an AC power regenerator that feeds AC to the tube amplifiers. It is a PS Audio- Power Plant Premier. I suspect that it does have some sort of SMPS.

 

[st50maint]

Got home from work yesterday and found a tag on the door from the power company. At 1 PM they had checked my power and found no problems. I had called last week and again explained that the problem was only at night yet they still come during the day.