Anyone ever hear of a little Black Magic Box? (Grounding issues).

[PMM3RD]

I have a major problem with 120 cycle hum going through my cable. Something is just not grounded right either in my cable or in my house. I do have dimmer switches which seem to adversly effect this. This does happen on all my TVs, its just 100x more noticable on my Plasma. My TV installers say its the cable companys fault, Comcast says its not their fault, etc... It happens when I watch DVDs or anything over Comcast. I have had this problem for a year now!
:mecry:

My friend gave me a little black box with 1 coax input and one coax output to connect from my cable into this box and then out to my STB. It cuts out some of this hum significantly but not totally.

Can anyone give me the scoop on this and any suggestions on what else I can do? The box is a very small, black, rectangular cube with the word MAGIC across the front in big gold letters. Any help/comments will be appreciated as always!

[mjones73]

Sounds like you have a ground loop issue, you might want to get a qualified electrician out to your house to inspect your wiring...

[lizzyh]

You should be able to get a ground isolator from comcast for free.When i had cable I had a similar problem and the isolator cured it. The problem is you have a different ground potential on cable than your home does. That would be a lot cheaper than the Magic Box which costs $150.

[Ratman]

The first test is to disconnect the cable (coax) from your TV/AV gear. If the problem subsides, call Comcast and have them resolve the grounding problem.

But keep in mind that dimmers and/or fluorecent lighting can introduce 'noise'.

But if removing the cable connections to your gear resolves the "60Hz" interference, it's in them to resolve at no cost to you.

[Kal Rubinson]

I also used a Magic Box Splitter from Mondial (now Klipsch) to avoid hum from a ground loop. However, after 5 years of faithful service, I started to get pops in the DMX/Cable Audio and the tech replaced the Magic Box with their generic splitter to cure the pops. Of course, the hum returned.

At my prodding, he moved the cable ground from one side of the house to the other and next to the main power ground. Voila! No hum and no need for a box to block it.

My advice: Cure it; don't cover it with a band-aid.

Ka

[Splicer]

PMM3RD,
From what I read in your post, the 120 hum is also occuring when you play your DVD? If this is the case, the cable co is NOT at fault. You have a problem in your homes grounding and for the sake of safety, you should have a QUALIFIED elctrician out to investigate before some one gets a shock of their life.

You say it happens on all of your TVs? Is cable on all the TVs? The magic box would only help one TV and not the others.

Another possibility (I know you don't want to hear this but it must be said) your plasma 'could' be the source of the problem. When unplugged, does the 120 hum still exist on the other sets?

If the problem only occurs during cable operation, and not with DVDs, video games etc..., then call the cable co and have them move the ground next to powers ground OR to the other side of the house away from power, depending on where it is currently.

Hope this helps.

[Ratman]

PMM3RD,
From what I read in your post, the 120 hum is also occuring when you play your DVD? If this is the case, the cable co is NOT at fault. .

I humbly disagree...
As long long as the poor ground potential is connected, to the eletrical appliances, a ground loop can/will occur. Even if not 'powered up' or not being used.

A) most posts that I've read with this symptom can be related to poor/improper grounding by cable instalation (installers).
B) The 'hum' or 'hum bars' is caused by the 60Hz frequency of poor grounding. Not 120 Volts. Although the U.S. eletrical standard 120VAC/60Hz. Don't confuse the two.

[Eddy]

The little black box is not a "splitter." It is most likely an isolation transformer, which is useful in decreasing ground loop hum. One potential solution is to use a power supply for your TV and sound system which also incorporates an isolation transformer (much bigger and heavier than the little black box) but these are quite expensive.

[Splicer]

There are 2 types of hum..60Hz and 120Hz. My bad, I should have said 120 cycle hum instead of just "120". Thanks for pointing out my discrepancy.

A 60 cycle hum will cause 1 horizontal line that moves from the bottom to the top of the screen. A 120 cycle hum has 2 bars that move up the picture.

I also agree that many times it is a poor installation, though in this case I doubt it. No doubt I could be wrong as I am not there to fix the problem but I still stand behind my original post. I have seen and fixed many service calls for this problem.

[Ratman]

Whatever...

You may be correct. But why discourage the simple test that I suggested?
Takes about 15 seconds to eliminate the cable as the source.

[Kal Rubinson]

The little black box is not a "splitter." It is most likely an isolation transformer, which is useful in decreasing ground loop hum. One potential solution is to use a power supply for your TV and sound system which also incorporates an isolation transformer (much bigger and heavier than the little black box) but these are quite expensive.

The MAGIC box comes with or without the splitter. Using the PS with an isolation transformer may work but, again, it is not a real cure.

Kal

[PMM3RD]

Thank you as always for all your responses.

Ratman: I am going to try your suggestion when I get home tonite and see what happens. The hum is in the TV even when I have the breaker to the dimmers shut off, its just not as bad. Its funny because as you turn the dimmers slowly on or off, the hum lines in the TV go crazy up and down in relation to the dimmers going on and off. I am not sure if I have 120 or 60 hum b/c sometimes there is one line, other days there is 2 lines. I guess it depends on what type of mood its in.

Splicer: Yes I have cable connected to my other 3 TVs, they are older sets and I do not have STBs for them. I rarely see the hum in those sets, only sometimes. People have told me you wont see it in these older sets like you do in the more expensive plasma (it figues, right).

I'll let you guys know what I find out. THanks!!

[Ratman]

If the dimmers are on the same electrical circuit, you might want to try plugging all of the equipment into a different circuit, just for kicks.

Also... if those dimmers are a few years old, maybe replacements/upgrades would be 'quieter'.

Just some thoughts...

[PMM3RD]

Sorry it has taken me a while to get back to everyone, but I finally got around to testing out my cable.

When watching a DVD, if I totally unplug the cable coax from my HD STB box the hum lines 100% go away which leads me to believe that my cable is grounded wrong somehow. To further strengthen my arguement, I left all my nearby dimmer switches on and still no hum as I unplugged the cable wire from the STB. Thanks to Ratman and Kal also for showing that the MAGIC box is strictly a cover up and not a cure.

My question now is, "What do I tell Comcast???" If they send some kid out here to sniff around they are just wasting my time. Should I ask for a supervisor to come out to my house. Anyone have any suggestions as how to approach them with my dilema???

[Ratman]

Call Comcast and request for a tech to come to check or properly ground your cable feed. Explain that you have visible 'hum bar' interference on your TV. Also, explain how you tested.

Glad to see that my suggestion worked out.

[Ratman]

A 60 cycle hum will cause 1 horizontal line that moves from the bottom to the top of the screen. A 120 cycle hum has 2 bars that move up the picture..

Here's a little more insight (FWIW) in regard 120hz hum.

http://www.dslreports.com/forum/remark,8950688~mode=flat

[loadams]

A little more reading

http://www.epanorama.net/documents/groundloop/

( Check all cables. Make sure your power cables are not "binded together".)