AC volts seen between box coax screen and house earth
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Could someone with an AC voltmeter do a test for me please? Check between a nearby house mains earth point, on a wall or extension block socket and the outer, screen, part of the aerial coax input connector on the back of the Humax box.
When switched on I am seeing about 120 volts AC. When switched off it drops down to about 10 volts AC
With a non earthed fully isolated input I am surprised to see this and it is causing problems here. The voltage is finding its way back up the coax outer, into a non earthed distribution amplifier and back down to a computer PCI TV card which is earthed internally. Sparks when the coax is connected here!
Yes, I have unplugged both coax leads and scart lead out to the TV and yes the aerial amp DC supply has been disabled!
There are possibly other reasons for this but I would like to know if my Humax is OK before investigating elsewhere.
Thanks
When switched on I am seeing about 120 volts AC. When switched off it drops down to about 10 volts AC
With a non earthed fully isolated input I am surprised to see this and it is causing problems here. The voltage is finding its way back up the coax outer, into a non earthed distribution amplifier and back down to a computer PCI TV card which is earthed internally. Sparks when the coax is connected here!
Yes, I have unplugged both coax leads and scart lead out to the TV and yes the aerial amp DC supply has been disabled!
There are possibly other reasons for this but I would like to know if my Humax is OK before investigating elsewhere.
Thanks
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I am not at all surprised as there is inevitably some capacitive coupling through the switch mode power supply transfomer, and it is probably a weird waveform at either mains or switching (or both) frequency but I don't have a 'scope to see this. On mine it is obviously a very low energy coupling as the voltage vanishes on plugging in the phonos to my hifi amp, which is earthed, and I don't get any sparking nor any additional noise on the amp output. The previous poster suggested mains interference capacitors causing this effect but as the Humax has no earth connection and is of double insulated construction I doubt that these are present.
Assuming no other fault perhaps your sparking is caused, despite the very low currents involved, by the inductance of the cabling between the Humax, the dist. amp. and the PC, particularly if there are high frequency components present. It does sound like a very long cable run and there may be inductive elements in circuit in the dist. amp. I would try 'tickling' the isolated Humax with a wire connected to a local earth. If it doesn't spark then I would not worry, if it does then I might worry more and investigate further to try to get an idea of the source impedance and frequency.
Assuming that this just 'normal' internal coupling and that no other serious fault exists then if you disconnect at the Humax or PCI often and are worried about the sparking then I would arrange for the Humax to be permanently earthed, either like mine through another bit of AV kit, or with a wire from a case screw to a local earth.
I get this on most of my AV kit. The TV seems to give the most output.
I also first noticed it when pluggin in a PC Card - I think it is because the PC case is earthed (unlike most av stuff), and as you are plugging in the aerial feed, you are normally touching the metal of the connector, and then touch the pc case before plugging it in, and get a light tingle.
At first I was blaming a new vcr - until I separated everythign, and realised it was still there.
Adrian is absolutely right, this is due to filter caps between the mains and chassis. The problem gets worse the more kit you have connected together by aerial leads, because the filter caps of all devices are in paralell, multiplying the effect.
All the feedback is appreciated and I will sort out local earthing. We do have several tuners and TV sets interconnected via TV coax from the splitter amp. Tested earth voltages at each unit and it varies a lot so clamping everything down will help.
From my past work with professional aerial rigging all parts of these installed systems were bonded together and VERY well earthed. It is horrifying to think of the millions of domestic TV masts poking up above roof level and unearthed, just coax going straight down to the equipment then nowhere! No doubt this will prompt comments from others on the subject!