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TV Audio/Video Filter Help

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Newbie level 2
Jun 8, 2011
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Please help.

I live in the UK. I have moved into a flat in a tenement. I have a coaxial cable entering my living room. I'm not sure what it's source (i assume it's an ariel somewhere on the roof) is but outside the flat it comes from a splitter which seems to send a cable into each flat. It was connected to a small circut just inside my flat which is broken. The circut consists of three 470pf capacitors. The coax core goes through one 470pf capacitor while the shield ground goes through two 470pf capacitors in parallel. Without this device I am getting very poor freeview signal. Does anybody know what this device is and if my signal will improve if I replace it?

I would appreciate any help anybody can offer.


The circuit is an LF isolator but the values suggest it may date back many years. Does the circuit have any identification on it?
If it is indeed comng from an amplifer/splitter you should be able to repair it and use it for your Freeview signal. Perhaps the simplest way of checking is to ask the occupant of one of the other flats if they have the same system and what their signal is like.


Hi Brian,

Thanks for the quick responce. Yes it looks like a very old circuit. The pcb has C366 printed on it but there is no other identifier. There is no casing. I will try n ask one of the neighbours what they use - all the flats also have cable from private operator also which i suspect most use.

P.s. are you sure you you mean LF isolator and not RF isolator. I don't know what an LF isolator is.

Will post again after speaking to neighbours.


The LF isolator lets HF signals (TV signals) throughbut blocks lower frequencies.
They were used primarily because on older TV and radio equipment it was common for one side of the antenna socket to be connected directly or through it's own isolator to one side of the incoming AC mains. Without the isolator it is conceivable that one TV connected correctly would short the Neutral mains wire to the splitter and someone else with the TV wired backwards (The TV would still work exactly the same) would connect the live wire to the splitter. The result would be a short across the mains and depending on which fuse blew first, the whole thing could either adopt neutral or live potential and be dangerous to other properties on the same feed.
The capacitors exhibit a low impedance to the high freqiencies of the TV signal but are much less conductive at the very low 50Hz of the mains.

From your description it sounds like it dates back to the pre-UHF days, most of these boxes were out of use by the mid 1970s. It might even be part of the old Rediffusion network which was quite common in cities. They received and redistributed the TV channels and some of their own content over a cabled network.


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