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I am trying to repair the power supply of the sewing machine. This card I think, which is in SMPS structure, is an LCC Resonant feeding card. I have repaired this board and I get the output voltages healthy. When I plug in the 220 AC sockets from the card's input points, there is a buzzing sound from the small transformer. When I searched a bit on the internet, I learned that they called it "coil whine". The system works well, but this noise gets even more severe, especially when the card's output pins are open. I am affraid of exploding this power board.

I added a video (the video link) to better explain what I wrote above.

in the first case:

card's outputs are open and energized. There is a noise,

in the second case

When I put a magnet on the transformer, the sound is cut off. I saw this in a video. However, this is decresing the output voltage 24v to 12 volt. I think this transformer goes to the mosfet's Gate.

In the third case,

I plugged a fan into the 24 V outlet and again the noise was gone and the system works healthy. What is the reason for this noise and how can i fix it?

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    \$\begingroup\$ Sounds to me like the little transformer is "pulse skipping" when at no-load. SMPS supplies usually require a minimum load to avoid this. Does it still do this when installed in the sewing machine? It is probably fine. \$\endgroup\$ – rdtsc Apr 13 '20 at 17:00
  • \$\begingroup\$ well we have same boards, but there is no noise like this board. \$\endgroup\$ – mehmet Apr 13 '20 at 20:00
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It doesn't have to be the transformer producing the noise, it could also be a capacitor (like the big red one next to that transformer).
Using a magnet or a metallic item to check the noise on a transformer (especially on a high frequency transformer) is neither recommended nor helpful, because it could affect the electro-magnetic fields in it and change the operating mode of the switching power supply, which might change its operating frequency or the electrical pulse shapes through the circuit, making the noise disappear for the moment and give you a false impression that the transformer is making the noise.

You should use a sturdy piece of plastic and press (not too hard to break something) on the transformer (different part of it) to see if the noise changes/reduces/stops.
If it's the transformer, you should push some hard thick paper in between the coils and the core to dampen the vibrations.

If it's not the transformer, you should try pressing other components like the big film capacitor(s). The electrolytic capacitors are less likely to produce whine because their paper insulator inside is a natural dampener.
You could also press the capacitor or other components with an eraser as that could dampen its vibrations if it is the one making the noise.

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  • \$\begingroup\$ I've pressed but noise did not change. but I will replace the big red red capacitor and then try again tomorrow \$\endgroup\$ – mehmet Apr 13 '20 at 20:05
  • \$\begingroup\$ Just try pushing against the transformer or its coils, or in between its coils and sides of the core. You could also press the capacitor with a an eraser as that could dampen its vibrations if it is the culprit. \$\endgroup\$ – Edin Fifić Apr 13 '20 at 20:07
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    \$\begingroup\$ yes you are right after changing red capacitor, the noise goes away. thnks \$\endgroup\$ – mehmet Apr 14 '20 at 9:01
  • \$\begingroup\$ I am glad I helped you solve it. Best wishes from Bosnia! \$\endgroup\$ – Edin Fifić Apr 14 '20 at 13:22
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It's really quite a bad idea to put a powerful magnet on those transformers, especially the larger one, you can destroy the power supply. I don't see any SMD transformers there, the smaller one looks like a drive transformer.

The noise might be coming from the coil or from nearby capacitors. You can try painting the coil with some insulating lacquer, or just put the board in a box. Ideally the transformers are vacuum impregnated during the manufacturing process.

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