Shocking TV Aerial cable

[GrantF]

I recently decided to add a TV card to my PC and to use one of these popular TV aerial amplifiers to boost the signal down to the PC and the main TV.

When the amplifier arrived, to test its effectiveness, I hooked it all up using the existing TV aerial cable at the TV end, and subsequently the picture dissapeared as if the aerial was disconnected completely, as well as the light on the unit dimming, as if to illustrate a fault. So I sent the unit it back to the seller citing a damaged unit.

In the meantime I went to the loft and cut the incoming aerial feed to the main TV and split it using a Y connector to feed both TV and PC thereafter. I was surprised when when connecting the main TV aerial to the Y connector, that I was getting very light electric shocks, similar to static, however after connecting both the PC and TV up both TV's have a picture fine. I repeated the connect\disconnect of the main TV aerial and got shocks all the time.

A replacement unit subsequently arrived I swapped out the Y connector for the amplifier, and connected the PC aerial up only and everything was fine. I then connected the main TV aerial again, and the same thing happened again, mild shocks and total picture loss, with the indicator light dimming.

Its currently sitting boosting the PC's signal only, and doing fine....., but why am I likely to be getting a shock from when connecting the main TV aerial to either the Y connector or the amplifier.

The telephone extension wire runs alongside the TV aerial up the wall chase to the loft, but I would have thought that would cause this......


Thoughts please.... ?

Ta
Grant

Oh and the house is only 6 months old

 

[kendor]

there may be a masthead amp being powered by a supply that uses the coax as supply as well as signal if the power had been disconnected ( tv distribution amps tend to isolate the coax and therefore any power being supplied along it ) this may account for loss of picture originally and the unit may not have been faulty. the shocks suggest that volts are present and therefore there is a power supply unit situated somewhere.

 

[ban-all-sheds]

Have you measured the voltage(s) present on the aerial cable, with an analogue meter?

People sometimes use the co-ax cable to carry power for a masthead amp, but I guess you'd know if you had one of those....

 

[GrantF]

Spoke to the aerial installation company, as the house didnt come with and aerial, only the internal cabling, and they say its just a std 'High Gain' aerial.

 

[GrantF]

Have you measured the voltage(s) present on the aerial cable, with an analogue meter?

People sometimes use the co-ax cable to carry power for a masthead amp, but I guess you'd know if you had one of those....

Nope, would I be right in presuming thats something the aerial installation engineers use to guage the strength of the signal, no??

 

[kendor]

Spoke to the aerial installation company, as the house didnt come with and aerial, only the internal cabling, and they say its just a std 'High Gain' aerial.

if true then that throws a spanner into the works, are you sure they gave correct information? can you find a box anywhere which the coaxes connect to that is plugged or wired to the mains? this would likely be the power supply mentioned.

 

[plugwash]

i've had similar minor shocked

my guess is they are caused by capacitors in filter networks in class 2 equipment

 

[GrantF]

Spoke to the aerial installation company, as the house didnt come with and aerial, only the internal cabling, and they say its just a std 'High Gain' aerial.

if true then that throws a spanner into the works, are you sure they gave correct information? can you find a box anywhere which the coaxes connect to that is plugged or wired to the mains? this would likely be the power supply mentioned.

Nope, there is no mains power supply in the loft aerial prior to me adding sockets, and the aerial just comes through the wall and directly into the loft from the outside........


....im now thinking that it must be getting its 'current' from the TV end or in the wall chase, but not from the BT telephone cable right? as its far too low a current?

 

[mapj1]

If there is a TV connected to the same aerial, see of the shocks stop when it is unplugged (switched to standby will not do!)
In older sets the chassis of the TV is connected to the mains via a bridge rectifier, and although the antenna coax jacket is supposed to be isolated from the chassis by some high voltage caps, it is not unknown for these to break down, and as nothing actually stops working when this happens, the fault goes un-noticed. Even in modern sets with an isolated SMPS, the capacitance to L and N is roughly equal, and as most sets are without an earth connection the voltage on the antenna port will float to about 120V, albeit at a high enough impedance to deliver a painful 'nip' rather than a fatal belt.
The cure is to replace the isolation caps in the TV, or if you dont fancy opening it up then to make up a lead with 100pf caps in series in both the braid and the centre conductor. (try a google search for 'briad breaker' circuits known to radio hams for some recipies)
Please check this as a matter of urgency - as it is possible your TV antenna could be live if the set has this fault.

 

[ban-all-sheds]

Nope, would I be right in presuming thats something the aerial installation engineers use to guage the strength of the signal, no??

No - just a regular analogue multimeter, e.g.

 

[GrantF]

Please check this as a matter of urgency - as it is possible your TV antenna could be live if the set has this fault.

I will get another TV set and use it to see if the conditions still exists and exhibits the same fault.

Cheers

 

[mapj1]

No need to wait to find out - just unplug the telly and see if the shocks cease - If they do, then treat the telly with great care, until you can confirm it is OK.

 

[GrantF]

Gotcha....will do and let you know

 

[ban-all-sheds]

PLEASE find a way to test for the presence of a voltage on the aerial cable other than seeing if it gives you an electric shock!

 

[Big_Spark]

Have you got overhead powerlines nearby?

I have come across instances where voltages are induced in aerials by the movement of HT conductors in the wind, of course the magnetic field moves with the conductor and this sweeps across the aerial, inducing a voltage. Usually this is very small, nor more that a few millivolts, but I have seen instances of voltages up to several tens of volts being induced.

Further, if there is any equipment in the area that could cause the induction of voltages in the aerial by other methods then this should be looked into, mobile telephone masts nearby have been known to have this effect is close enough.

I appreciate that both of these potential causes are rare and probaly rather unlikely to be the cause, but nothing should be ruled out until the actual cause is discovered.

Remeber, if all other causes have been ruled out, then no matter how unlikely one may be, if it is the only one left then it must be the cause.