Axiom Home Page Forums General Discussion Technical Questions Ground Loop Hum/PartsExpress Part Not Working

So after searching some threads on ground loop hum and identifying the source of my hum (the cable connection), I ordered from Parts Express Part Number 180-075.

Now I have basic cable, no box, and the cable runs directly from the wall into my 10 year old tv. I've attached the part to both the wall/cable connection and the TV/cable connection.

I am getting serious vertical bands on the NBC, Fox channels and some other trippy action on a few other channels. Other channels are not affected.

I emailed Parts express explaining my setup and asked if the part may have been defective or if I ordered the right part for my situation. They replied by stating that they are shipping out a replacement part. I received this today and it has not resolved the issue.

Have any of you had any issues with this part not working, or something tells me that it may have to do with my cable line?

Any thoughts. I'd rather not have to go the Jensen route and I plan on contacting PE again, but I'd like to run this by the experts!

Thanks again!

Easy peasy... you're not looking at 60Hz hum, your cable signal's too hot.

If you just moved into the house (or if you're just taking a close look at your cable for the first time) what's probably happened is some previous owner had the cable come in to a 8 way unpowered splitter so they could watch The Price is Right everywhere in the house including the john... each split of your CATV is about a 4dB loss with a good passive unit... that means you're 12dB down at each of the 8 taps before you even add cable line loss... they called up the CATV provider and complained their picture was snowy, cable guy comes out, realizes their signal's too low at the TV drops and cooks it up 15dB. Next guy comes along, takes the crappy splitter out, replaces it with a better splitter, or one with less taps, and suddenly the signal's so strong you're getting crosstalk from neighbouring channels (appearing as a rolling picture over the program or various other manifestations)... so how do you fix it... call the cable provider? You have a picture, they've done their job, they're not going to tweak every armchair TV guy's signal on their route...

You need an attenuator (we call them pads) P/E has them in 3dB, 6dB and 12dB varieties. How much pad do you need? Who knows... and the equipment to test RF signal strength isn't worth the bucks unless you're a cable company... so buy one of each, they're $2 a crack... and try them in order... they'll give you up to 21dB of attenuation in 3dB increments. If you guessed too high a pad, the picture will be snowy or washed out, like bad antenna reception, if you guessed too low, they problem you have now will still be there.

My $0.02 based on your description, anyway. If the picture is actually more like snow, you're probably too low instead. You can try an RF amplifier but it's best to not amplify a weak signal, at that point, call the cable company and get them to give you more juice at the pole instead.

For future reference, 60Hz hum is usually a HORIZONTAL band that moves through the picture... anything else is usually signal or interference related.

Bren R.

Thanks Ben!

I'll start the process but I have a couple of final questions/comments.

I presume that it doesn't really matter where I put this piece, i.e., before/after the other parts express piece.

Also, the other part definitely does remove the hum from my sub when installed.

And one final thing, my picture is fine when I'm not using the P/E part. Does this alter anything you've suggested?

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Thanks Ben!

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It's Bren... Brenden... I'm Irish-Canadian.

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I presume that it doesn't really matter where I put this piece, i.e., before/after the other parts express piece.

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No, you can put pads before or after the isolation transformer.

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Also, the other part definitely does remove the hum from my sub when installed.

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Which is a start.

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And one final thing, my picture is fine when I'm not using the P/E part. Does this alter anything you've suggested?

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Yes, yes it does. I think I missed the first part of your describing the issues. Didn't realize this was a fix for hum through a sub... thought you were using the iso xformer to battle the picture problems.

That adds a bunch of other layers to the issue... not sure if you've tried any other methods to remove the sub hum before this...

- a "cheater" plug on the sub... one of the ones that cuts the ground out of the plug. (use as a diagnostic tool, do NOT use it as a permanent fix - it's dangerous, the only path to ground from the sub then is the shielding on the sub cable then - and it's not rated for mains voltages, in case of a shock, it may melt and leave YOU as the only path to ground)
- "break ground" to the sub from the receiver. Most people use a basic 75ohm "video" type cable to the sub, which connects both the centre pin and the outer ring of the RCA - the outer ring doesn't have to be connected, if you use or build a cable that doesn't connect the shield at the sub end, that allows the sub and the receiver to run at their different ground planes without hum. So you'd have the conductor in the wire attached to the centre pin on both RCAs, and the shielding/drain wire attached to ONLY the RCA ring at the receiver. This still allows all interference captured by the shielding to drain to a ground (at the receiver) but keeps the ground planes separate for each piece of electronics. Technically, you should do this for all analog audio cables (but NOT video - it requires the signal return)
- try plugging the sub into the same power bar as the HT to put it on the same ground plane as the receiver.
- is your CATV even grounded as it enters the house? It should be grounded in ONE place, no more, no less... use one of these, bring the cable in one side, and out the other, and hook up a nice big beefy piece of 12ga. copper (again - big conductor in case mains voltage hits it - you want it to carry that voltage to ground, not melt like a fusible link) to the screw terminal and clamp it to a cold water pipe (if you have metal pipes - legally this also has to be kept separate from the mains grounding, so don't try to tie it into anything of Edison's) - if your CATV isn't held to ground like this, the place it's grounded to will be probably hundreds or thousands of feet away, which would mean your ground differential between the cable and your home wiring could potentially be very large.

Hope one of these helps.

Bren R.

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legally this also has to be kept separate from the mains grounding, so don't try to tie it into anything of Edison's

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Is this a regional regulation? I have read a few articles that say you should have the cable grounded in the same place as your houses earth ground. That is how mine is set up, it is normal here in Il. for the cable and telephone co. to ground to the same location as Com Ed.

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Is this a regional regulation? I have read a few articles that say you should have the cable grounded in the same place as your houses earth ground. That is how mine is set up, it is normal here in Il. for the cable and telephone co. to ground to the same location as Com Ed.

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It's best to have them as close as possible without actually bonding them. CATV and telecom grounding at the same place is illegal here (under the CEC - at least of MY understanding at my last reading about a year ago) and I found a site a while back that said the same applied in the US - written by an electrician and showing some seriously dangerous and illegal grounding methods... the telecom side is covered in TIA's document J-STD-607, not sure offhand where CATV standards are.

Just two sides at battle here... you want your CATV on as close a ground plane as possible as the mains (to reduce ground differential), but without being connected to reduce the chance of mains power being fed into your CATV.

Bren R.

I stand corrected... further research looks like it's legal and suggested in US locales, at least in commercial buildings, to bond the telecom ground as often as practical to both structural steel and electrical panel grounds. In a large data centre, this makes sense, since with miles of cable stretched, it would reduce the distance needed to travel to find ground reference... but in a residential situation, and in the context of CATV, I'm wondering what the theory is... my brain hurts.

Bren R.