question

@Finihen ...transformers can be very annoying when they start to sing on their own. I'm not sure if you are seeking advice based on experience with this particular transformer or transformers in general. I cannot comment on any experience with this particular transformer.

The isolation transformer has 2 windings, a primary and a secondary. Each winding has approximately the same amount of turns. If they are the same turns the voltage in to the primary equals the voltage out of the secondary. If the turns ratio is different the voltages change in the same ratio.

The two windings are magnetically coupled. To make the transfer of energy efficient the transfer of magnetic energy transfers with the help of a magnetically low reluctance core. The core is made of special material designed to transfer that magnetic energy from one winding to the other. The core is laminated. It is laminated to reduced the ability for current to flow in the core. These currents are called eddy currents. Some current does flow in the core but it is contained individually to within the lamination and not across the laminations. When magnetic flux cuts the laminations in sympathy with the AC waveform, current is induced into the lamination just as it is induced into the transformer winding. This causes heat but also exerts a little mechanical force as the current induced also produces its own magnetic flux. Each magnetic flux interacts and that causes "lamination rattle". The laminations are typically insulated with a substance like shellac. If the laminations become loose or the windings become loose the hum can be annoying.

The trick is to stop the lamination or winding movement. In the old days people used to immerse a rattling transformer into a solution of shellac and allow it to completely dry to stop lamination rattle.

I know it took us a long time to get here but that could be the cause of the noise.

Good luck with it.

Thank you for your very clarifying answer!

The device has been connected to the mains for a total of about 2 months. If that’s enough to break an expensive device into an almost unusable state - too bad.

But back to the business. What would be a slightly more modern equivalent of shellac, or is it still getting somewhere? I understand it is made from the resin secreted by fleas and alcohol according to an ”ancient” recipe. Could I use slightly heated epoxy resin instead? It becomes more fluid when heated and penetrates better into small crevices. Or two component clear varnish maybe?

@Finihen yes… I think resin or varnish or something like that. I think you can still buy shellac in dried flake form that you mix with methylated spirits. It may just be a couple of bolts holding it together loose but I think you will soon see where this overall sounds is coming from. I would not coat it in anything too thick or perhaps the heat dissipation properties may suffer.

Once again, thank You for Your advice. After reading some adiitional articles about ”lamination rattle” I found out being totally wrong in thinking that two component varnish or epoxy would be suitable for this job. The idea of Yours is the way to continue. I’ll inform if it was a successfull with my unit. But don’t hold Your breath on waiting, I’ll still have a ton of jobs to do With the Yacht.

Finally got time to tear down the transformer into pieces. I suppose the noise came from some loose parts inside as after tightening all the bolts and adding some LocTite between all the parts the noise is gone. There was one long bolt a bit loose holding the transformer core and windings in place. It’s very silent now and does not react in anyway to load changes. So I’m happy with it!

BTW 1: the transformer core/windings had some aluminium/plastic (?) wrapping around it so I did not try to immerse it into shellack/alcohol solution.

BTW 2: Shellack flakes were easy to get from eBay.de with a very attracting price (-80%) compared to local suppliers (Finland). Maybe I just had some luck!