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Buzzing PSU - Apple LED Cinema Display 27" (2010)

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MRbeginner1

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Hello!

My first post here.

I have a problem with my Apple LED Cinema Display 27" from 2010. A while back it started buzzing quite loudly and I could not understand why. At first I thought it was the speakers but after speaking to some electronics enthusiasts on a swedish electronics forum we could conclude that it was the power supply unit that was making the sound.

They suggested that I should replace the capacitors on the psu because those are often the problem. I got the psu out of the screen and I gave it to a local tv repair man who replaced all the capacitors. I put the psu back into the screen and it was fine again! I was very glad!

BUT, about a week after this the buzzing started to come back again. At first very softly but now it is quite loud again. Somehow these capacitors where now broken again.

The buzzing gets worse when I have used the screen for a couple of hours and especially if I have the mag safe (power cord from the screen to my mac book pro) connected. The higher the brightness setting the louder the buzz.

Here is a video of the buzzing sound:

https://www.youtube.com/watch?v=7sPd6vfR3dY

Here are pics of the PSU before the capacitors were replaced.

https://imgur.com/a/2zxdJ#3

I managed to hear that the buzzing is the loudest from the two orange components on these pictures:

https://imgur.com/a/2zxdJ#9

https://imgur.com/a/2zxdJ#19

The three big black capacitors laying down on the card were not replaced:

https://imgur.com/a/2zxdJ#10

I really do not know what to do now and my friends on the swedish forum are unsure also. I would really like to fix it somehow.

Does anyone here have any ideas? I would be very grateful for any help!

Thank you!

//Jakob
 
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betwixt

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I can't download the links (problem at my end) but usually if it is still working the capacitors are not at fault. The symptoms suggest it is a magnetic problem caused by movement in one of the inductors, either a transformer or a choke. As the current through them pulses it generates a varying magnetic field. In a 'good' component there is nothing mobile in close proximity to the field so nothing moves but sometimes through aging or poor construction, something starts to vibrate as the field pulls on it. Higher brightness usually means more current and in turn more vibration. This kind of problem is also influenced by expansion of the component as it warms up in normal use which alters how much it can vibrate.

If you can power it up while the power supply is exposed, USING A WELL INSULATED NON-METALIC STICK, try putting a little pressure on each of the inductors and see if it changes the buzz. If you find one is responsible, you can either replace it with a new one or a 'dirty' fix is to mix some epoxy resin and drip it over the wire so it locks the turns together and to the core inside. Don't go mad with entombing it in a thick layer of resin as it can prevent air circulation and make it overheat.

The inductors usually responsible are thick copper wires wound around a ferrite ring.

Brian.
 

MRbeginner1

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Hello!

Thank you so much for a very well written reply. You explained this perfectly so even I understand :)

I will take your advice and try this out shortly. I will post the results here.

Best, Jakob
 

MRbeginner1

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Hello again!

Last week I opened up the screen took the PSU out and glued the copper transformers. I put the glue over and around the transformers and let it set. I then put the screen back together and started it up. It was silent. For three days. Now it has started again and it is as bad as before.

Now I and stuck again and I would really appreciate further advice. Should I have the transformers replaced with new ones? Should I just replace the whole PSU? Any other tips/thoughts?

Thank you!
 

betwixt

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Looks like the diagnosis was correct but the cure wasn't!

Before thinking about new transformers, which may not be available anyway, did you do the 'poke it' test to see which was responsible? You may have glued the windings together but still have them rattling against the core. If you can find the the transformer responsible there may be other ways to silence it.

Brian.
 

MRbeginner1

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Alright.

The problem is that it is very hard to get it to buzz when the screen is taken apart and the LED-screen isn't connected to the PSU because the amount of power needed is minimum. But I will try and make it buzz somehow and perform the test. There is no fun in giving up right? :) I will post back here when I have done this.

Thankyou!
 

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