Transformers, capacitors, buzz, and explanation?

[CkRtech]

Hey all. Long time (LONG time) lurker. First time poster.

I have a plasma TV that seems to make a buzz when nothing is being displayed on the screen. Increasing buzz volume with brighter picture (more power), etc, is understandable - but a buzz with a black screen?

Why would this happen when there is nothing on the screen? Could it be because the boards requiring power (scan boards, etc) from the power boards aren't pulling anything, and the power sitting (undrawn) in the power board is causing a transformer to buzz?

I have replaced the power sub-board from which I have heard the buzz. The buzz still exists, but it now sounds slightly different in sound. Here is a sample of what it sounds like: http://www.youtube.com/watch?v=Pg9guk4PqeY

Is this sound buzz from a transformer, the capacitors above it, or something else?

The buzz stops as soon as something is shown on the screen - even if it is just the word "mute." So long as the screen needs to display something, that buzz doesn't exist.

It seems like my only other options are to either replace the main power board or find some sort of material to try an muffle the buzz. There is a pot on the board near the transformer, but I do not know what it adjusts.

The manufacturer says it is working as designed, so I am on my own. I appreciate any input you guys may have. Everyone I talk to seems to just shrug and accept it.

 

[Juddog]

The buzz sound generally falls along the same 60hz that power lines operate at (Alternating Current), or a multiple of that.

 

[Juddog]

To clarify, the mechanism is a wire wrapped around a ferrite core; normally this is dipped in varnish or something else along those lines to stop the buzzing, but if that gets worn out you will hear the buzzing.

Audio engineers will sometimes open up the part and coat the transformer wires with a light layer of silicone or something along those lines to silence it; if you're not good with electronics though I wouldn't suggest doing this with your plasma TV, as you could literally shock yourself if you don't know the proper safety precautions.

 

[CkRtech]

I still have the back off of the TV at the moment after having replaced the power supply sub board. If there is anything to do to dampen the sound from what I believe to be the transformer shown in the youtube video while I am "in" the TV, I would like to give it a go. Not sure about opening the transformer itself, though. I believe I saw one instance where a guy coated the outside of a transformer with silicone in attempt to reduce the noise.

I also still have the previous board available. Seems like the buzz is coming from the same transformer on each board - or at least from around the same area.

 

[ScottMac]

Then you'd look to the board / component group that the PS feeds.

Does anything smell funny? Like maybe there's an arc happening somewhere behind teh scenes / out of sight?

 

[CkRtech]

I haven't noticed a smell. I have been back there while the TV was on (just like when I shot that video), and haven't noticed a smell during normal operation while at a safe distance.

I am embarrassed to admit that I have, perhaps, left out a crucial bit of information. The original power supply went out about 30 days after purchase (March 2012). Panasonic replaced the board at that time. I noticed the buzzing some time after the replacement was put in. I do not recall having the buzz prior to this. Don't know if this is relevant, but it is certainly worth noting. The only part in the power path that was not replaced was the line conditioner at entry.

 

[sm625]

Find out exactly which component is making the noise, then coat it with silicone.

 

[CkRtech]

Sounds like a plan. I am fairly certain it is the transformer, but I will try to isolate the component before coating it. Any particular brand/type of silicone you like to use or can recommend?

 

[CkRtech]

Powered up, got it into the replicated state and gave the transformer a little push along the top edge with the eraser end of a plastic mechanical pencil. While pushing, the volume of buzz would increase ever so slightly. Gently pushing on heat sinks nearby yielded no change. I am about 95% certain it is the transformer at this point. There are no additional components underneath it on the opposite, solder side of the PCB. It doesn't take much pressure at all to alter the volume of the buzz. The pencil has a bit of a vibration in it to. As soon as something is displayed on the screen, the buzzing as well as the vibration of the pencil sitting on the transformer housing completely stops.

Does it sound like silicone treatment on this transformer is the way to go?

 

[bryanl]

For coils with widely spaced turns of wire, silicone sealer is probably the best choice, but for transformers, varnish may be better because it's thinner and can better soak deep between the wires. There are also some very runny sealants made for car windshields, one crystal clearn and made by 3M, another is translucent and very thin silicone sealer by Loctite/Permatex.

 

[CkRtech]

Thanks. Sorry for being so naive, but are you inferring that I de-solder the transformer, open it, and apply the silicone or varnish to the coiled wires, or am I looking at something closer to masking the components around the transformer and spraying or coating the heck out of it? :\

My board work has generally been resistors, capacitors, voltage regulars, and transistors. Transformers are uncharted waters that I am willing to chart.

 

[bryanl]

Thanks. Sorry for being so naive, but are you inferring that I de-solder the transformer, open it, and apply the silicone or varnish to the coiled wires, or am I looking at something closer to masking the components around the transformer and spraying or coating the heck out of it?

From the appearance of your transformer, I doubt anything can soak into its coils, unless the whole transformer is submerged in varnish in a vacuum chamber. I would leave the transformer in place and lightly wedge pieces of wood (tongue depressers, popcycle sticks) between the yellow part and the metal frame around it, to see if that silences the noise.

 

[Blain]

Do you see any bulging or leaking capacitors?

 

[CkRtech]

Thanks, bryanl. I'll have to give that a try. Is the epoxy-like gray goo between the yellow part and the metal frame supposed to reduce the vibration (and not doing a good job)?

Hi Blain, thanks for weighing in. Actually, no. The capacitors appear to be in good shape (no pun intended). Boards are clean (both of them). Nothing leaking.

 

[bryanl]

Is the epoxy-like gray goo between the yellow part and the metal frame supposed to reduce the vibration (and not doing a good job)?

The grey goo is supposed to reduce vibration, but it's silicone sealer, not epoxy glue.

 

[dkozloski]

Remove the transformer from the CB. Take it to your local electric motor rewinding shop. Have them submerge it in their varnish tank and then bake it. They would probably do this for little or nothing when they process a motor rewind job. You'll have to prune off the excess varnish by hand before reinstalling on the CB but this should be simple to do.