Weird cable 'noise' problem solved!

michaelab

Active Member
My cable reception is pretty good but I ran into a problem recently. When I hooked it up to my stereo amp (Arcam Alpha 6 plus), however indirectly, I got a terrible mains hum through my speakers, even when the volume on the amp was at 0 and/or the input selector was not on 'video'. The hum wasn't evident on the TV speakers at all.

First I had cable->TV->VCR (scart)->amp (phono) (my VCR tuner didn't work anymore so I used the TV as a tuner and then recorded via SCART).

Now I have Cable->DVDR->TV(scart) and amp(phono).

Both of those setups had the terrible mains hum if I so much as touched a phono lead from the DVDR or VCR to my amp.

I just couldn't figure it out but it was obviously coming from the cable somehow. In the end I wondered whether the cable maybe wasn't earthed properly so I earthed the cable at the outlet box (to a conveniently close mains socket) and that solved the problem!

Where is the table supposed to be earthed? I guess it's possible that something has gone awry at the junction box in the street. Anyway, I'm well pleased that the problem is now solved.

Michael.
 
J

jim.rae

Guest
Glad you have solved the problem, but I would have earthed the amp first - it's not unknown for spurious signals to cause amps to hum - magnetic turntables used to be famous for it.

However, I am slightly concerned that you mighty be earthing the entire cable network in your street - especially if there is a dodgy lamp post near by...

Might be worth mentioning to your cable company to have it checked.
 

michaelab

Active Member
Well I'm pretty sure the amp. is earthed. It has a 3 pin mains connection and the a steel case which would slightly concern me if it wasn't earthed! It also has a little earth connector on the back "for earthing turntables" according to the manual.

At first I earthed the cable via this connector on the amp. and that worked aswell (so presumably the amp. earth is working). I ended up using earthing directly to the mains earth just because it was neater.

I'll mention the problem to my cable company but the chance of getting someone who knows what they're talking about coming to the house is pretty remote :(

Michael.
 

AMc

Distinguished Member
I'm interested in what you've done as I get noticable mains hum and I'm at a loss to cure it.
What part exactly did you connect to your mains earth?
The cable itself, the inlet box, the cable splitter or the cable set top box?

Any clarification appreciated!
 

michaelab

Active Member
OK, the cable that comes into the flat is a normal coaxial cable. It goes through the wall upto a wall outlet in my living room. If I take the cover off the wall outlet there's a metal junction box inside. It allows two coax cables to connect to it, the incoming from the street and an outgoing (in my case to continue the cable internally to a second outlet in the bedroom).

The box also has two external sockets which show through the plastic cover and are the same except that they cater for different types of cable plug.

Now, the inner wire of the incoming coax cable connects to a little copper clip which then goes to the inner wire of the sockets. The outer, braided, part of the coax cable is just pressed against the whole metal casing. It's this metal casing (effectively the braided outer wire of the coax cable) which I earthed to my mains earth.

The metal casing had a handy little screw which wasn't used for anything else and looked all the world to me as though it was designed to connect an earth wire! So I connected a wire to it and fed it out under a little gap I made in the plastic wall cover. There's a mains socket about 1 inch away from my cable outlet so I made a similar gap in the plastic wall covering of the mains socket and then attached the other end of the wire to a conveniently free screw on the earth terminal. Hey presto - no noise! (and the wiring is almost unnoticeable).

If I were you I would first just get a piece of wire and try using it to earth your cable (outer coax braid) to see if that fixes your problem before trying a permanent solution. Are you sure it's your cable causing the problem? In my case I knew it was because I could get rid of the hum by disconnecting my cable from the wall.

WARNING: Switch off your mains at the main fuse box before opening up any mains sockets and messing around with them!! I know you probably know this but just in case :)

Michael.
 
J

jim.rae

Guest
I would just repeat the warning - for God's sake don't start earthing things unless you know what you are doing...
 

AMc

Distinguished Member
Thanks for the extra information.

I need to spend a bit more time isolating the source of the problem before I try this. I've read a lot about ground loops and various other sources of noise and figured I was stuck with it as the remedies were too extreme to implement. My first port of call will be to disconnect
the cable from the wall and see what happens.

Like most of us I have a load of different components linked together in a fairly small space and despite my best efforts to keep mains, signal and speaker wires apart they do have to cross near each other. Add to that our cleaner who moves it all around once a week (despite being asked not to) and you've got a multitude of places to start a Hmmmmmmm.

Don't worry I won't be doing any work on earthing or mains without turning off at the consumer unit!

Thanks again.
 

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