Jon's advice is very good and may do the job. If not, overload of a transformer can make it buzz. You can then try to disconnect the secondary wires and see if the buzzing remains.
Please check the existing transformer before worrying about where to find a replacement transformer. The core laminations may get loose or even some windings and that can be repaired. They may make noise if overloaded or driven into saturation. But in 50 years, I cannot recall I have seen a single transformer really burn off.
Let's hope it is not the first I will know of.
Let's hope it is not the first I will know of.
See if the buzz is consistent at all times of day and night, and it might also be worth checking the mains voltage isn't to high.
Transformers don't usually suddenly have this issue and are usually just more 'prone' to it when conditions are right.
What would an open circuit diode in the bridge cause I wonder.
Transformers don't usually suddenly have this issue and are usually just more 'prone' to it when conditions are right.
What would an open circuit diode in the bridge cause I wonder.
Maybe you have a new neighbor. A lady equipped with a Chinese Hairdryer which she loves to operate at half speed (with single direction rectifier diode). Half speed, as in half power, but loading only one half of the sine wave. Hence unevenly loading the mains. Hence: creating a DC offset. It could be a hairdryer nearby, but more probable, it could be some big industrial machine that generates a similar problem.
Have you considered using a DC blocker?
A very simple circuit. If you are not savvy with electrical stuff, let someone make one for you, who is.
BEWARE: Although simple, this circuit is intended to be used on the LIVE side of MAINS. It hence needs to be thoroughly insulated and housed in an insulation chassis, or better yet, in a grounded chassis.
Transformers HATE to see a DC offset component from the mains. Even the smallest one.
The DC blocker will allow to stop the DC offset from getting into your transformer, magnetizing it and raising havoc in terms of its less than optimal operation.
Please take special notice: The rectifier bridge is wired so that the "+" and the "-" are connected with each other. This basically boils down to a set of four diodes: two in series in one direction, and two in series in the other direction.
Please take special notice: The pairs of series connected electrolytic capacitors are "facing each other". The "plus" of the one is facing the "plus" of the other. This is purposefully so: this way you create a bipolar capacitor out of the pair of two.
It is important that the capacitors are capable of operating in "both directions", because they are operating with AC. If you do not connect them in such "bi-polar" manner, they will fail / explode.
Please take special notice: The rectifier bridge is an OVER-SIZED BEAST. In the example, I use 35 Ampere, 1000V. 50A or 100A would be better. Why is it over-sized to such an extent? Because it is not permitted to fail. It is not allowed to fail. It can NOT fail. It needs to withstand a short / a critical event in your audio device, and not evaporate. It needs to blow your fuses in your fuses board, without failing. It NEEDS to be able to survive such a critical event.
The circuit works like this: Any DC offset (less than 1,5V) will be blocked by the electrolytic capacitors (connected pair-wise in a bipolar fashion).
AC will repeatedly charge and discharge the capacitors, hence "flow through them" to your load / audio device.
Any DC offset of less than 1,5V will simply be stopped by these capacitors.
Alas, if the DC offset is bigger than 1,5V, then the safety net kicks in: the diodes that bypass the capacitors. The capacitors shall never see anything bigger than 1,5 Volts on their terminals.
So why the 63V DC of these capacitors? You may ask.
They have to be for 63V, because they have to be big. Big as in big can, acting as a heat sink. Big capacitors will handle high ripple current much better. They will get warm, so it is important that they have the "bulk" and the surface to radiate the heat out of them. You need High Ripple Current capacitors for these positions.
Please check with your local authority your local electrical code regulations, is it legally allowed to use such a device in your specific geography. There are places, where this could be considered as unlawful.
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Is a church mouse quiet?
It tells us that it is not a transformer failure as such. Transformers cannot repair themselves.
It may have been a temporary DC component on the power net (primary) that drove the transformer into saturation during one half-wave.
Perhaps more likely is that you may have an intermittent failure of an electronic component, perhaps in the power supply circuit. Such an intermittent failure is likely to be dependent on temperature. Intermittent failures are very annoying to look for because they tend to show up when you want a nice evening with some music and when you have the test-setup ready, there is no problem.
It tells us that it is not a transformer failure as such. Transformers cannot repair themselves.
It may have been a temporary DC component on the power net (primary) that drove the transformer into saturation during one half-wave.
Perhaps more likely is that you may have an intermittent failure of an electronic component, perhaps in the power supply circuit. Such an intermittent failure is likely to be dependent on temperature. Intermittent failures are very annoying to look for because they tend to show up when you want a nice evening with some music and when you have the test-setup ready, there is no problem.
It does tend to suggest that less than perfect mains quality is a probable cause. Initially you should just keep a record and see if the issues really do occur at certain times of day. I would also note the mains voltage at these times.
Mains DC and Transformers
The saying is because mice were thought to be quiet, and a church-going one would be especially respectful of the service(!).
Actually we now know mice vocalize and sing, but ultrasonically, so you need a bat-detector to hear them.
Yesterday a mouse popped out of a ceiling at the US White House. Mouse sound was not measured, but apparently the reporters in the room were very excited.
"The rodent fell onto the lap of NBC correspondent Peter Alexander about 10:45 a.m., eventually seeking refuge amid a tangle of wires behind a shelf.
Some reporters ran for cover, while others sought to corner the mouse and capture it. The rodent sneaked under the door into the main hall of the press area before it eventually..."
https://www.washingtonpost.com/dc-md-va/2019/10/01/mouse-fell-ceiling-white-house/
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