I came across this today via a tip off Guido‘s “seen elsewhere list”. Jonathan Miller provides a spectatularly comprehensive roundup of (a) the sheer iniquity of the telly-tax-scheme, and (b) what to do about it at indiviual level. Worth a read, and worth also spreading virally.
I personally have had my doubts about “detector vans” for some time. The Landed Underclass will like this one that’s coming…..and he’s got his own pennyworth of useful stuff here….
In fact, in the mid 1970s, I performed this experiment as follows:- (WARNING! DO NOT PERFORM it yourself without supervision by a qualified electrician or a Radio Ham – the voltages present inside the case of an old tube-type colour TV can be LETHAL – up to 35,000V, which is rather a lot !!! (and you won’t know where they are unless you already know.) TOUCH the EHT from the line-output transformer, while it is working, and you are DEAD !!! ) (UPDATE – as far as I am aware, the insides of modern flat-screen tellies are safe, except for the presence of mains (230V AC) voltage. Don’t monkey with these either or you will invalidate your warranty…)
I completely lined the inside of 405/625-line “dual-standard” colour Tele Vision Set fully (these were still available new, and also widely used) with a double layer of aluminium foil, electrically attaching it to MAINS EARTH (NOT the chassis whcih is almost always live to the mains) and screening as far as possible all round, up to the edge of the tube front.
I then made a RF “sniffer”, using a coil/capacitor netwrok approx tuned to the 45 MHz “intermediate frequency” which was used inside the receiver module in the set I was playing with. This was in efect a small radio RX whose output registered on a milliammeter instead of a loudspeaker. I was able to tune the incoming frequency through a range of about 8MHz either side of the 45 band centre, so I could also check if the sniffer was picking up any other oscillator signals or sums/differences coming from inside the set as a result of its decoding the vision signals and line/frame sync pulses.
In the room, which was about 9ft x 12 ft, there was a detectable 45 MHz RF flux, but it was rather weak. I also found small peaks at 49.33 MHz and 40.57 MHz roughly, probably from heterodyning witht he 4.43 MHz crystal which did something or other in the receiver. There was a weak 45 MHz signal outside the window but nothing else.
I could not detect the 45MHz signal out on the road. Nor incidentally any other ones from nearby houses.
I did not think of trying to detect the harmonics of the line-output frequency, which was 15.625 KHz. I expect they owuld have been weak, at any frequency above about 10 MHz.
Since the uniform characteristic of machines then was a 45 MHz IF, I expect that this would have been what “detector vans” would major on, for the lifetime of cathode-ray Tellies – none of which would have been built with full electromagnetic screening of their innards – what would be the point? (Radio-Hams suffer from interference from consumer electronics far, far more than consumers do from interference by hams!)
I was therefore unconvinced, and have remianed so ever since, of the claims of the BBC about “detection”.