How would you damp the hum from this transformer?

I have a Sony TAN-15F amplifier, which sounds beautiful, but is rendered unusable by a transformer that hums loudly enough to be very irritating when music is not playing. The seller told me that it was quiet before it was shipped to me, so I decided to take this as an opportunity to learn something about transformers. How hard can it be, right? ;)

So far I've managed to open the metal housing of the transformer, which was a pain as it was a folded metal box coated in lacquer - and held together by the lacquer, too.
The grey metal band wrapped around the core appears to be stuck in place with yet more lacquer, which limits my options for further disassembly.

I have identified that, if I squeeze hard all the way across the left and right sides of the sheet metal band that crosses over the copper strip, I can considerably reduce the hum to a level that should mean it wouldn't be audible (or at least not irritatingly so) after boxing back up.

Since this transformer isn't of traditional construction (see the pictures) I don't have an option to just "tighten everything up" in the normal way. There is a gap between the grey metal band and the copper loop - if a suitable approach would be to pack that with something, what's a suitable material? Or does anyone have other ideas for possible solutions?
 

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Replacing or rewinding an old transformer just because it is humming is a waste, difficult also.
Read the thread here, many methods were suggested.
Varnish, insulating varnish, Loctite Green were among the suggestions.
I used to do tube light chokes on load, connected, the hum acted as a vibratory feeder, allowed insulating varnish to penetrate where it was needed, and the warm temperature helped flow.
Dramatic improvement, and durable.
Varnish is cheap, not hard to get, and you will find it hard to get a shielded transformer of that quality, the Japanese stuff was in a different class.
And do check if the power supply capacitors are in need of changing.
 
Was the noise a buzz or 60 hz hum? There is a chance the transformer mounts were bent during shipping which can cause more of a buzz or rattle instead of hum.
One thing to try before varnish or other modifications is suspend the transformer on rubber or packing material and see if the noise goes away.
Mounting on rubber feet can sometimes cure the noise problem.l
 
Thanks all for the input and all the questions. I’ll see if I can reply to all the points:

* The buzz is a mechanical vibration that makes a 100Hz noise (2x50Hz, because I’m in Europe). I know it’s mechanical because I can feel it, and by holding a stick against the various parts of the transformer I can identify roughly where it’s coming from.

* Yes, the mounting is a bit basic - the metal transformer screws on to the metal box of the amp. I know that the hum is mechanical and from within the transformer because it hums even when suspended by the wires. I can feel where it is. I think adding some rubber is a good finishing touch that will help further if I don’t manage to quieten it enough by other means. At the same time, I respect that Sony’s designers knew what they were doing - it will have been silent when it left the factory, the design should be good enough.

* I did see the mention of Loctite Green in another thread; I prefer the varnish approach as I believe I’d have to take the transformer apart further to get to the vibrating parts and get the Loctite into the right places. I’ve identified an insulating varnish designed for transformers that I can use.

* Regarding the filter caps, I will test these and/or test the transformer with no load. If we are to trust the seller, the hum appeared due to something that happened during shipment. Is sending an amplifier by courier (where it was likely subjected to sub-zero temperatures and may have been bumped around a bit; it was very well packed though) more likely to damage the filter caps or the 40-year-old varnish? And would old filter caps cause a mechanical vibration in the transformer? My suspicion is that replacing the filter caps would be an additional task, not the only thing to do to eliminate the noise. Possibly difficult to find, as they are big but not tall.

* Replacing the transformer is something I really don’t want to do. As NareshBrd says, this is a fine piece of kit. I’d like to keep it original, both to preserve its value and because I can tell it’s really well made. Never mind that a toroid wouldn’t fit and finding a suitable EI transformer with the right voltage would be a challenge. Also, I want to use this as an opportunity to learn. I could have taken it to a repair shop or just sent the thing back, but that would be boring!
 
The hard mass of insulation could crack under mechanical shock, but that level also has a good chance of breaking a winding wire, resulting in an open winding, failure.
Use the thinnest varnish you can get, the thread referred to has a method of removing the excess, using a pan and newspaper.
Thin as in high flow...

Read it, he has done it many times.
 
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It has to be said, the construction of this transformer doesn't make the drying process easy! Two days later, there are still parts buried deep inside it that haven't set yet, so the buzz is still there for now. I'll leave it a week in a warm place and see how it performs after that. I'm using Royalac R129, which is an air-drying varnish designed for transformers.
 
Ah, I thought that instruction was for while the transformer was dunked in the varnish. I warmed it up before submerging it, left it in the varnish switched on for about 15 minutes or so then another couple of hours switched off, then took it out to drain. If it doesn't work this time then I'll try again, but I'm hopeful. Thanks for the advice!
 
Run it on load while submerged in varnish, the load can be an ordinary car lamp, about 30 to 50 % of the rated value of the transformer.
You will see bubbles come out, and hear a reduction in hum.
Switch off afterwards, while draining excess.
Use a small amount of the proper thinner if needed.
Varnish takes a long time to dry, and use a slow setting type, so it is able to reach all the gaps.
The shield is soldered at the fold, if you are experienced, break the joint with a fine chisel, you can join it later.
That will allow easier flow into the structure, after you remove it. It is a folded piece of sheet metal, open it carefully, so that you can use it later, don't dent it.
Fit it back later after you are done with the repair, how you fix it is up to you.
Do not use a heat source, the bobbins are plastic, will distort.
 
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Thanks again. After the first attempt dried out completely I got a considerable reduction in hum, but there is still some noise from the input side of the transformer; I will follow your method to the letter over the weekend and give the transformer a good week to dry out to give it the best chance of success.
 
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you need to determine what the type of noise you have.

1. is it making a noise all the time or only sometimes? Is it worse at certain times of the day? Its important to understand this in order to find the root cause.
2. is it because of DC on the mains or is it because of loose windings or core laminations (different problems with different cures)
3. is the amp for 110/120 V or is it for 100V (Japanese voltage). If the latter, you may have core saturation problems - again, different cure required.
 
It's making a noise from the moment it's switched on, and it's a 220V European model. I think I can rule out the core laminations, because the vibration is coming more from the wiring part of the transformer. Given that one dunk in the varnish has muted the hum considerably, I'm hopeful that another dunk with a more thorough method will improve things further. Of course, then I have to reassemble it into its box and back into the amp in such a way as not to introduce any other sources of mains hum! But that is a problem I can look forward to further down the line.