Submitted By Member: Terrykc

In the early 70s I worked in the City of London on a 22 channel cable system which displayed Stock Exchange prices. The receivers were ITT/KB domestic TVs with a special varicap-diode VHF tuner which was remotely controlled by a wired 22-button unit on the desk.

As a result, the isolation between the live chassis and the earthed tuner was at IF – making it the most dangerous TV I’ve ever come across! (And the only “live chassis” TV to be fitted with a 3-core mains lead!)

A broker complained that one of their receivers was unusable first thing in the morning. The first time I saw the fault, the problem was very obviously caused by hum. Removing the IF lead produced an even, blank raster, so I replaced it and went back to the workshop for a replacement tuner assembly. By the time I returned, only a faint hum bar was present, which disappeared when the tuner was replaced.

Back at base, the unit behaved perfectly, with no sign of any hum.

The following day, the same fault was reported again but, by the time I got there, it had disappeared!

The next day I went straight there first thing and it was in fine form – the hum being far worse than I’d seen previously. I reached behind the TV and removed the input cable and thought I saw a spark. Impossible! I brought the earthed signal cable close to the earthed input socket and yes, there was definitely a spark.

It was mid winter and bitterly cold but the office was starting to warm up nicely so I removed another layer while I pondered on this situation. While I was doing so, the hum suddenly dropped to a lower level. I then noticed a girl returning to her desk after turning a 3kW fan heater down a notch …

It turned out that the ring wiring in this office had earth and neutral swapped and the fan heater was drawing power via the cable screen all the way down to the safety earth in the basement, thus inducing hum onto the RF.

Each kW increased the PD between the cable and the TV by just over half a volt! The heater was only used on very cold mornings for half an hour or so and, fortunately for me, the TV was in rather a dark corner!

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