Noise down cable screen!

[Monkfish]

I just checked my NTL cable modem and was surprised to find the screen on the incoming cable was connected directly to the screen on the USB connection. Surely this means that the NTL network is linking the ground line (case) of every PC on the system.

This is fine for those pieces of equipment that are also commoned to the PC ground, but I have a set of powered speakers that pick up a terrible buzzing from the NTL cable screen.

The problem is, I don't know if this is normal, or whether some other user on my local network has a problem with their PC ground. Some people can't even wire a 13-amp plug correctly!! I came across one system that had the neutral and earth connections interchanged!!

I would have thought the cable input would be opto-isolated from the PC connection.

I was thinking of ringing NTL Technical help, but then I thought... Naa :rolleyes:

Any thoughts?

[altis]

The small white box on your wall contains an isolator so there is no DC connection between your earth and the NTL cable. This is a legal requirement for all cable systems.

[ProfPete]

No DC will get through, but 50Hz AC will jump across just like the RF. Well, not so much jump as influence. 240Volts of mains going through the isolator wouldn't put 240volts the other side, maybe a 1volt 50Hz wave would get on it though...

As to neutral and earth being swapped around, it will work. Most houses have PME earthing, where the earth wire is connected to the neutral wire at the meter anyway. The purpose of the earth wire is if the live comes in contact with the case, it will short circuit, causing the fuse to blow. As long as the case is connected to SOMETHING that creates a circuit with the fused wire, then the fuse will blow on contact.

If you have RCD circuit breakers (MCBs work in the same way as fuses) though and have neutral and earth swapped around, they will trip immediately, due to the high imbalance between live and neutral. A mate of mine once spent 3 hours trying to work out what was going on as his new RCD kept tripping. Turned out that one of the ring mains in his house had the neutral and earth cable swapped around in ALL the sockets on that ring :Yikes: - and had been like that for 20 years or more since (he'd only lived in the house a few years). Re-wiring the sockets then stopped the tripping. But it had worked fine for 20 years or more.

The earth wire (and hence neutral due to PME) here is approximately 70 volts above true ground. Makes a few things buzz nastily.

[Monkfish]

The small white box on your wall contains an isolator so there is no DC connection between your earth and the NTL cable. This is a legal requirement for all cable systems.

Do you mean the NTL junction box? I think they only installed an attenuator in there. I don't think it's an isolator. I might be wrong, but I think the little in-line gizmo has a metal construction like the one on the back of my modem (I had a strong signal), so that wouldn't isolate the screen.

If the coax cable screen was isolated from the network, why do I get the buzzing noise on my speakers when I attach the coax screen to the modem?

Hmmm, I'm confused.

[Acropolis]

the isloators come in many differnt shapes and sizes.

the newer modern ones are rectangular and look like a splitter, some of the older ones look like a barrel. it may be that there is no isloator in the white box on the wall and it has been put in the omni box on the outside of the house, which was common practice when cable first came out, or it may even be, although i serisously doubt it, in the cab in the street, although this isnt a very good idea, i have seen it a few times.

[Monkfish]

the isloators come in many differnt shapes and sizes.

the newer modern ones are rectangular and look like a splitter, some of the older ones look like a barrel. it may be that there is no isloator in the white box on the wall and it has been put in the omni box on the outside of the house, which was common practice when cable first came out, or it may even be, although i serisously doubt it, in the cab in the street, although this isnt a very good idea, i have seen it a few times.

I'll have a look in my NTL box tomorrow, but unless they do a combined attenuator/isolator, then it'll be in the cab in the street if they added one. I suspect they forgot!

[Monkfish]

Well, I just checked my connection and the only kit between my PC ground and the cable screen going to the street cabinet are a couple of attenuators (8db in my NTL junction box and a 4db in the back of my modem).

I'll have to call out an engineer to see if they fitted an isolator in the street cabinet.

[Monkfish]

I just put a scope between my PC ground line (case) and the screen of the NTL cable (disconnected), and it shows a 50Hz 4V p-p noisy signal with 2V spikes. I didn't measure the signal power (source impedance) but would guess it was higher than for simple cross-talk pickup. In fact even just connecting the negative lead of my scope to the NTL screen was enough to produce the buzzing on my powered speakers, and that was with the PC and modem turned off!

[Escapee]

I just put a scope between my PC ground line (case) and the screen of the NTL cable (disconnected), and it shows a 50Hz 4V p-p noisy signal with 2V spikes. I didn't measure the signal power (source impedance) but would guess it was higher than for simple cross-talk pickup. In fact even just connecting the negative lead of my scope to the NTL screen was enough to produce the buzzing on my powered speakers, and that was with the PC and modem turned off!

I guess there could be a lot of earth loops, the PC case is often a problem in itself. I know from experience of watching someone else (fortunately) carrying out a PAT test on a PC and blowing the Graphics card. It may be worth checking the point you are connecting your scope to on the PC is a good earth point, and not one of the plug in card plates. (I appreciate you probably know that already)

It would also be worth considering the point you metioned about making the measurement with the ntl cabel disconnected, I know from amateur radio experience that a HF diploe can have hundreds of volts across it when theres a thunder storm about, I used to often connect my old Avo across and be suprised. I'm just thinking what a good aerial all those customer drops could be if the cabinet is not properly earthed. I worked in the South Wales ntl franchise area, and the local electricity supplier was fussy about earths. There was a copper plate under the cabinets, earth spikes and sometimes they still had problems passing them. I understand some electricity suppliers were not so fussy and did a visual inspection, perhaps your area is not that well earthed at the cabinet.

Remember the amplifiers are run from 65V AC fed down the coax with the RF, so there will also be a certain amount getting past the line injection choke/cap filters. I have even come across distribution amplifiers with fuses in the output port feeding the customer taps, with some types of customer tap banks the RF splitter/transformer on the input would burn out, but I think some would probably keep themselves warm for years!

A bit like the 18 volts being fed through a 6dB resistive attenuator I found in a cable TV headend one day, I found it when I burned my fingers on it. :D

[Ignition]

Remember the amplifiers are run from 65V AC fed down the coax with the RF, so there will also be a certain amount getting past the line injection choke/cap filters.

Oh God no, did you have to mention that :p:

*waits for the 'ntl electrocuted me and all I was doing was playing in a big green cabinet I 'accidentally' found' law suit :D

[Escapee]

Oh God no, did you have to mention that :p:

*waits for the 'ntl electrocuted me and all I was doing was playing in a big green cabinet I 'accidentally' found' law suit :D

I remember going to change one of the 65V power supplies at a cabinet one day!

There is a 3 core mains lead for the mains input and a seperate earth bonding wire, the power supply was just thrown on the shelf (typical ntl) and I had a little belt and used a few bad words when I lifted the still powered PSU to make room for the replacement. Unfortunately the contractors had mixed up the Earth and Neutral connections on the mains socket fitted to the backboard of the cabinet. It was very fortunate that I had not removed the extra earth wire from the unit, or the shock could of been much worse.
I powered the unit down, removed the extra earth wire and had a nice brightly lit neon screwdriver when I touched the psu case with it powered up again.

See, ntl were looking at ways of getting rid of network tchnicians in those days. :D

[Monkfish]

You're doing wonders for my confidence in NTL. :erm:


I now wonder what they've done in our street cabinet?

Anyway, the screen is obviously not isolated in the cabinet, otherwise I'd only get aerial pickup and not this 50Hz earth loop problem.

Listening to your horror stories my 4V ac interference seems tame.

I wonder whether wrapping ptfe plumbing tape around the coax thread on the back of my modem would act as a highpass filter; i.e. pass through the cable signal but open circuit the 50Hz? A coax is a waveguide after all.

Maybe I'll just get an engineer out.

[Escapee]

You're doing wonders for my confidence in NTL. :erm:


I now wonder what they've done in our street cabinet?

Anyway, the screen is obviously not isolated in the cabinet, otherwise I'd only get aerial pickup and not this 50Hz earth loop problem.

Listening to your horror stories my 4V ac interference seems tame.

I wonder whether wrapping ptfe plumbing tape around the coax thread on the back of my modem would act as a highpass filter; i.e. pass through the cable signal but open circuit the 50Hz? A coax is a waveguide after all.

Maybe I'll just get an engineer out.

Yes, you are correct the screen is not isolated in the cabinet.

The screen would be connected directly to one of the F-sockets on the customer tap bank, this would be grounded by its connection to the amplifier and the amplifier and tap banl should have their own earth wire wired to the earth spike.

Wrapping ptfe tape around the thread would mean loss of signal, the higher frequencies would jump across but the lower ones would be severely attenuated, the return path Upstream would be non-existent.

I dont think isolating would cure the volts you are seeing on the screen, I think most of it is probably due to different earth potentials and pickup. If grounding the screen causes it to dissapear, I would tend to think its too high in impedence to cause any problem. Also remember that whilst it is easy to earth a 50Hz signal (mains power) Its very difficult with RF. The RF earths need to be very short to avoid pickup on the screen, if the screen is multiples of a wavelength, (bound to be with such a wide freq range) it will still act as a good antenna. A coax drop cable say 20 meters long earthed at the cabinet end will still be a very effective HF antenna, it may look like a short to earth less the cable resistance at 50HZ but to RF it's another matter.

[Womble]

I have had this problem in the past (a long long time ago) with analogue. Our cure was to change the Isolator in the Drop Box / House (IF there is one!). If that didn't work, we would fit an in-line isolator just behind the TV.

My advice is book a service call, be mellow and explain the problem and my suggested solution to the service guy, then make him/her tea/coffee!!

[altis]

You could always try a ground loop isolator in the audio lines...

http://cpc.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=AV08632

http://www.maplin.co.uk/Module.aspx?ModuleNo=33172

[Monkfish]

Well, I can't get in contact with anyone at NTL to get an engineer sent out. High tech problems aren't catered for by telephone call services and there's no one with any technical knowledge to understand the problem and get an engineer sent.

Anyway, I used some clear tape on the thread to isolate the screen and that's done the trick. It also hasn't affected my connection quality. This is probably because very few people are tapped out from our street cabinet. My signal is so strong I didn't even need to remove the 12db of attenuation the engineer connected in-line. I was worried about my up-line bandwidth however, but that too seems fine.

As I suspected, the frequency of the broadband signal is so high that attenuating the lower frequencies made little difference. It did, however, remove the interference on my speakers, which would suggest the source impedance of this interference was in fact quite high (or maybe just the 50Hz component was audible and that's all I needed to block).

I can understand why it might be law for the screen to be isolated. Earth-loop/mismatch problems can be potentially dangerous. I wonder if isolation is an area where NTL is skimping?
I might have another attempt phoning NTL and simply ask for an engineer to be sent out to fit an isolator (under the KISS principle :dunce: ).

No, I haven't got the TV service, but I will remember about those earth-loop audio isolators from Maplins. Thanks.

[Stu038]

If that didn't work, we would fit an in-line isolator just behind the TV.

When FM hook ups were common, it wasn't unusual to get feed back into the HiFi from the TV.

You don't mention whether you've got the TV service Monkfish and if so you've probably already tried it but if not just pull the RF connection from the back of your TV and see if your spikes disappear.

[Ignition]

If you have TV service complain about seeing lines going down the screen and ruining your picture, hopefully that's in the troubleshooting guides as AC hum and will get an engineer out to figure out why 50Hz AC is getting into your TV tube.

Obviously once the chap gets there then have a chat with him re: the real problem.

[Monkfish]

I can't wait for an engineer to tell me he hasn't got one with him, so I've bought the item atop the following product list for £10 and will fit it myself. Geez, what a palaver!

http://www.mediachannels.co.uk/isolaterscatvuhf.htm

[Monkfish]

Okay, that little box did the trick. I have to say however that my little tape bodge seemed to work just as well. ;)

I just noticed that they put the price up by £2.70 after I bought mine. :p: