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Looking for advice on mechanical hum


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3 hours ago, sandyk said:

 

 Probably a little difficult for the average member to do though..

 Even the steel top/lid of the case which is a little too close to the top of the transformer can cause this.

The hum affects three different components, all with transformers. When it's all three, there's got to be more than a mechanical resonance. Now if the hum had a different pitch and changed there maybe a clue as to what, but it hasn't.

 

For all three to hum and buzz, there are a few possibilities, without looking at the waveform, one can only make a smart guess. DC offset kits are just as dangerous to apply than the cause itself, so not recommended. Remove the source.

 

Can be anything from a lamp dimmer, flourescent, heater controller, or a really large SMPS, but not RF.

 

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@extracampine the meter should be capable of 300V or better, it is a digital type?

For UK sockets the orientation of the earth , neutral and active are:

top earth, clockwise next is active, clockwise next is neutral. 

 

Use the black probe and plug it into the earth. Take the red probe and connect to active.

this should read 230V. 

 

Move the red probe and connect it to neutral. This should read very low, less than 2V, mV is even better.

 

take both probes out and measure across active and neutral, the value should be the same as active to earth, 230V.

 

This is measured at the wall. Some outlets have a shutter that will prevent you sticking probes in and you won’t be able to make contact. In that case, a flying lead with bare ends needs to be fitted to a set of terminals to measure, a standard IEC 320 kettle lead would work as they have the inscription on the plug to tell you which is active, neutral and earth.

 

there is no need ever to touch the metal ends of the probes, always keep fingers at the other end of the probe!!!!

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5 minutes ago, sandyk said:

 

I don't have access to gear like this that Garry mentioned, but if it happened to me I would try to get a rough idea by looking at the mains waveform on the secondary windings of a transformer that wasn't connected to anything else, with my old C.R.O.

To distort the mains waveform so badly it would normally need to come from some heavy current drawing device such as Peter mentioned  previously.

 If you are able to verify that it is from some external source, your electricity supplier should be able to investigate further.

That's also a good option and safer. The transformer doesn't need to be anything special even the secondary can be anything small like 6V 12V.

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Tip 2: Measuring an AC Signal with a DC Offset

An ac-coupled true RMS measurement is desirable in situations where you are measuring small ac signals in the presence of large dc offsets. For example, this situation is common when measuring ac ripple present on dc power supplies. There are situations, however, where you might want to know the ac+dc true RMS value. You can determine this value through a manual calculation, shown below, add the square of the dc measurement and the square of ac measurement and take the square root. You should perform the dc measurement using at least 10 power line cycles of integration to reject the ac signal.

True RMS ac + dc =SQRT( ac^2 + dc^2)

Even if your multimeter contains a built-in function to measure the True RMS ac+dc value. You may want to manually determine the value using a dc measurement, an ac only measurement and apply the formula. The manual method will yield a better result, particularly when the dc and ac values are of different magnitudes.

 

DC Offset on AC is a crossover distortion when the sine wave passes through zero. To measure use a scope. What to do? Lets get the DAC to the workplace and measure some volts to start with. Jan 8th deadline!

 

 

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1 hour ago, extracampine said:

OK One and a half - I looked back at the post you mentioned. Here are my findings:

 

Black probe to earth, red probe to active: 246V

Black probe to earth, red probe to neutral: nothing registered on my multimeter - 0.00V (however the lowest "setting" for AC V is 200V)

Black probe to neutral, red probe to active: 246-247V

 

You initially said that it should read 230V. Does this therefore seem quite high?

 

The measurements tell me that the plug is oriented correctly. I don't believe the 0.00V reading, the meter is not capable of auto ranging and I'm not going to suggest lowering the meter scale in case you place the leads on the wrong terminals and carbonise the meter and your fingers.

 

I didn't know where you were located, not a mind reader ya know, UK is nominally 240V, 247V is only 2%, 10% on top of 240V is the limit, but transformers have a limit that's not that high. They tolerate it, but complain.  

 

This DC offset conversation is pure speculation. Unless you can organise someone who has a Powerscope and actually takes a measurement of the waveform and an FFT, there will be guesswork to what the remedy is . Any sharp rises in voltage, or aberrations on the waveform can be readily determined. If you have persistent headaches, the doctor is consulted and by process of elimination and their experience, often the remedy works, same situation here.  

 

Your local power company can be contacted, the ones that provide the stuff in the first place, they have engineers that specifically determine root causes of power problems, mention to them that the equipment doesn't hum in other places, but only at home. Perhaps the problem is on the lines in the street, then they would be interested. No need for a long term logger to be installed, a spot measurement is more than enough. 

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  • 2 months later...

@extracampine, glad this problem is put to rest and the manufacturer found a quiet transformer. In all good engineering practice, a larger than necessary transformer was spec'd for the DAC, and transformers being what they are like to have a minimum load to keep them quiet. The construction though is the main issue, two transformers built on the same day can have different tolerances, but it's up to the designer to factor in those tolerances. 

 

In some respects SMPS are very quiet acoustically, except perhaps for a low level high audio frequency pitch from the switching transistors. That's about their only advantage!

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