Linear Power Supply, buzzing. Defective? Repairable?

[Rainer Dynszis]

Hi,

I just purchased a GW Instek PSM-3004 for less than half the list price. It is a linear power supply that should be comparatively high quality.

Unfortunately, it emanates a buzzing sound even when outputs are disabled, and under load it gets even louder.

I uploaded a video here: https://sendvid.com/vhfmdsjs

Is this PSU defective? Can anyone guess what might be wrong with it?

Thanks for reading 

[GerryR]

Probably the power transformer core laminations are loose and are humming to the line frequency.  If that is the cause, it really won't hurt anything; it will just be annoying.  I'm not sure how it is done today, but the laminations, at one time, were varnished when assembled and sometimes they break loose.  It doesn't sound really bad as some that I have heard, and the older you get the less you will be able to hear it. 

[Rainer Dynszis]

Probably the power transformer core laminations are loose and are humming to the line frequency.

Oh dear. If your guess is correct, then there's nothing that can be done about it, yes?

[GerryR]

Depending on how the transformer is made, you can sometimes wrap the core with electrical tape to keep the laminations from vibrating.  I have seen where some people re-varnish or paint the edges of the core laminations and achieve the same thing.  A coating of epoxy will instaed of varnish / paint would work, as well.

It might be advisable to contact GW Instek and let them know that the transformer is humming (if you determine that is what is actually making the noise), and they might send you a new transformer. 

[wraper]

It might be advisable to contact GW Instek and let them know that the transformer is humming (if you determine that is what is actually making the noise), and they might send you a new transformer. 

They won't send you anything. Will suggest to seek help from distributor you bought it from.

[Rainer Dynszis]

Depending on how the transformer is made, you can sometimes wrap the core with electrical tape to keep the laminations from vibrating.  I have seen where some people re-varnish or paint the edges of the core laminations and achieve the same thing.  A coating of epoxy will instaed of varnish / paint would work, as well.

Thanks for the reply. This sounds laborious and requires more forbearance than can be reasonably expected when purchasing a new item, even if it's discounted. And we don't know whether it really is the transformer, and not some other defect :-(

Therefore...

Will suggest to seek help from distributor you bought it from.

... that's probably what I should do.

Thanks again to both of you.

[Kleinstein]

The sound is quite loud and no just the normal 100 Hz hum.  There could be an additional defect, like a bad diode in the rectifier. Uneven loading the transformer with an DC part can increase the transformer hum. The normal magnetostrictive hum should not change much with load.

[xavier60]

My Agilent U8002A psu was doing the same sort of thing when I first received it. The buzzing was from its DC choke in the pre-regulator section rather than from its toroidal mains transformer.

[AVGresponding]

If it's a diode in the bridge that's out, it's easy enough to check.

Just load it up somewhere near the spec limits, and see if the output sags.

Keep it brief and there shouldn't be any additional damage from this.

[mc1822]

If you bought it new then I would contact the seller about the defect and ask to return it and replace it with another unit. You paid your money, it's not your responsibility  to diagnose or repair it! 

[paulca]

Have you tried hitting it?  Generally when things buzz a good WHACK stops it.

The same applied for whining, but you'll end up divorced.

[dnwheeler]

If you're having trouble locating the source, hold a small dowel or plastic rod against your ear (against the bone, not IN your ear), then touch the other end against each component to see which one is vibrating the loudest.

[RoGeorge]

If you're having trouble locating the source, hold a small dowel or plastic rod against your ear (against the bone, not IN your ear), then touch the other end against each component to see which one is vibrating the loudest.

That's a neat trick, thank you!   

[andy3055]

If you're having trouble locating the source, hold a small dowel or plastic rod against your ear (against the bone, not IN your ear), then touch the other end against each component to see which one is vibrating the loudest.

This is very true. This is the common method used to find if a bearing is bad on a motor. I can say that the number of times I may have done this is so many. You can easily hear the scraping noise inside a bearing!

If the transformer is making the noise, mark the wires and remove it from the chassis and take it to a place where they do motor winding and ask them to varnish and bake it. It should be a very cheap thing to do. If you know a place where you could get insulating varnish, you could do it at home. Just don't put it in an oven! Once you have it out, put it in a metal container and hang a high wattage bulb besides it so that it gets real hot. Then, remove the bulb and pour in the varnish and hang the bulb again and let it get real hot. There is no damage to the winding at all. Leave it till the varnish is dry. Let it cool down and re-install.

[Jwillis]

I have an old battery charger that started making noise like that. Don't leave your battery charger in the rain. Anyway .all I did  brush off the rust and sprayed a good heavy coat of paint all over the transformer. Good as new . I'll get another 20 years out of it. It doesn't take much gap in a lamination to make a lot of noise . Just a good couple coats of thinned varnish and one coat of straight varnish  will probably do the trick.