Ground loop problem

Calibos

Just got UPC installed and seem to have a ground loop problem. During house renovations I got the electrician to run a cable (WF100) to Node 1 in the attic to the UPC supply box outside. This cable left coiled until we organised UPC to make the connection. We have twin runs of (WF100) to 7 rooms terminating at Node 1 in the attic too.

The UPC install guys who came to install and setup our 3 STB's used that single cable we had provisioned to make the connection outside and put a 3 way splitter on it inside the attic. A single WF100 run from each of the 3 rooms where we wanted STB's was connected to the splitter.

At each STB room we have those big wedge shapped UPC connection boxes with a UPC provided coax cable connected to the STB's. The STB's are connected to the TV's via Scart.

On two of the TV's we have a background hum on the audio and on the remaining TV we don't seem to have an audio hum but instead have a faint vertical band that rolls across the screen from left to right.

Please tell me there is a simple solution or things to try, to rectify this

rurs

My neighbour recently changed from UPC analogue to the basic, non-PVR UPC digital. Same problem, noticeable hum in the background. This is with a Sony LCD. So it mightn't be anything specific to your cabling.
It does sound like a ground loop, maybe both devices are double insulated, and don't have an earth?
Have you tried adding another device, maybe an old VCR, connected via Scart to the UPC box? This might add a proper earth. I'm only guessing really, I'm no expert on these matters.

watty

You can make or buy a "braid breaker"

Simplest.

score around the coax cable twice about 2mm or 1/10th" apart.

Remove the outer, foil and braid so the inner insulation is visble.

At one side of gap, score very lightly back about 1" or 2.5cm from gap and remove only the outer plastic. Leave Braid and foil.

Tape the cut at other side of gap to insulate any exposed braid or foil.

Now wind a strip of aluminium cooking foil tight with one end tied on exposed connection and the other side overlap insulated coax by about 6" or 15cm.

Mains hum and DC now can't flow in coax braid, but the Foil to Coax insulated outer will pass the RF (TV).

Connect Foil end to the UPC Setbox or TV and the insulated end to the wall socket.

Calibos

watty wrote: »

You can make or buy a "braid breaker"

Simplest.

score around the coax cable twice about 2mm or 1/10th" apart.

Remove the outer, foil and braid so the inner insulation is visble.

At one side of gap, score very lightly back about 1" or 2.5cm from gap and remove only the outer plastic. Leave Braid and foil.

Tape the cut at other side of gap to insulate any exposed braid or foil.

Now wind a strip of aluminium cooking foil tight with one end tied on exposed connection and the other side overlap insulated coax by about 6" or 15cm.

Mains hum and DC now can't flow in coax braid, but the Foil to Coax insulated outer will pass the RF (TV).

Connect Foil end to the UPC Setbox or TV and the insulated end to the wall socket.

I'm having a bit of trouble following that Watty mate. Can I clarify a few things.

This definately needs to be done 3 times (once at each STB location) to the coax cable between the UPC wall box/plate and the STB as opposed to doing it only once to the Coax coming in to the attic from outside before its split by the 3 way splitter?

If done 3 times. Should I do it to the thinner white molded UPC supplied coax cables or make my own modded leads to go from wallplate to STB's. (I have spare WF100 coax lying around)

I think I understand the first few steps. I assume I need to make this 2mm wide cut at least 6inches back from the end of the coax lead.

Its the follwing steps I can't visualise

Now wind a strip of aluminium cooking foil tight with one end tied on exposed connection and the other side overlap insulated coax by about 6" or 15cm.

Mains hum and DC now can't flow in coax braid, but the Foil to Coax insulated outer will pass the RF (TV).

Connect Foil end to the UPC Setbox or TV and the insulated end to the wall socket

Can you clarify?

Cheers

watty

Try it once between splitter and outside

If that doesn't help, then make 3 x patch cords for each wall box.

         xx----------------------------- Foil
==========**---======================================== Coax 
==========**---========================================  wall end
          xx----------------------------- Foil
      foil
      short
      to braid	  
	  
	  
 ---==
 ---== Coax and braid cut back and insulated

 
===**--
===**-- coax with exposed braid
 

The cut braid means no mains hum can flow

The Foil wrapped around coax cable with one end connecting to braid only on one side of cut allows RF to connect across insulation to cut braid. A capacitor.

Use PVC tape, sellotape or Tie Wrap to tighten foil around the coax.

Calibos

Well I made up a lead but unfortunately it didn't solve the problems on any of the TV's.

I assume I did it right as I wouldn't get a picture if I had done it wrong. Is that correct??

Still getting a hum on two of the Plasmas. Just noticed that when pulling up the On screen Guide or changing channels and the STB displays the info bar on the bottom the pitch and volume of the humming/buzzing changes slightly. I don't know if that is a clue to anything?

On the third plasma that does not exhibit a hum, I still have that band that stretches fropm top to bottom that rolls across the screen from left to right.

Really perplexed at what this could be if its not Mains hum?

watty

You have all three cords between TVs and wall sockets done?

You have nothing else connected to any UPC wall box?

You now have no electrical connection between outers of each end of patch cords? (Foil only connects direct to one end)

If you have a Meter put it on AC and with cable unplugged measure between outer of aerial socket on TV and UPC Wall outlet?

Your next step is to have UPC and an electrician at the same time. It sounds very bad.

The rolling bar is strange. Those roll up or down very slowly if it's mains. Side to Side is 15KHz approximately.

Antenna

Watty,

Your suggestion has a low-frequency block on the co-ax outer, but the co-ax inner left as it was before?

This is enough to still cause a hum-loop depending on the design of the aerial input of the TVs.

Try getting an ISOLATED wall socket? and pass connection to the TVs through that. these would block any mains hum loop on both inner and outer of coax.

An Isolated wallplate will have capacitors visible on circuit board at the back.

(BTW don't use isolated wallplates in other situations where there is no loop problem BTW, as there is slight extra signal loss, as well as being incompatible with Sky-Eyes, masthead downleads etc!)

watty

Depends on TV tuner if inner is via a capacitor or not. Most tuners it is.

The Commercial braid breakers do have a capacitor on centre wire.

What you say Antenna is perfectly true.