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THE TRANSMISSION GALLERY

TALES FROM A COLD FIELD

Being fairy-stories told to the author as a young engineer
© Ray Cooper, 2005 (2nd revised edition, Jan 2006)

Shaving Seconds

This one also involves the activities of an individual: a Senior Maintenance Engineer whom we shall call C. He was a very good engineer, thoroughly knowledgeable, highly competent and all that (he had been at Alexandra Palace before the War), but hasty.

I found out about this haste myself as a young TA on the desk. With just minutes to go to the start of the evening programmes (no all-day broadcasting then) his head suddenly popped over the control desk, and with a quick "just going to change a valve..." he switched off the vision transmitter and beetled off back into the hall. He certainly was an exponent of high-speed valve changing. Within a minute he had returned, re-powering the transmitter. It creaked back onto the air just as the continuity announcer was getting into her stride.

I discovered later that he had been going on like this for years, although not always with such happy results. There was always the distant spectre of that terrible evening so many years ago.

What had happened was, he had been researching ways of shaving seconds off his personal valve-changing record time. His reasoning went like this:

After he had diagnosed a valve problem, he would of course have been standing in front of the affected unit. Seconds would therefore be wasted in walking across to the control room, announcing his intentions, and de-powering the transmitter. There must be a quicker way, and he was sure that he had found it. His eyes had alighted on the interlock handle.

You must understand what happened when this handle was wound when the transmitter was under power. The sequence of operations goes like this:

  • A switch opens, breaking the 50V DC supplies to the control circuits. All of the supply contactors fall open, removing A.C. power from the power supplies. Then:
  • Another switch opens, isolating the units to be worked on from the D.C. outputs of their power supply. Then:
  • An earthing switch closes, connecting the supply rails within the units to ground. And, finally:
  • The door-locks to the units are released.

It all seemed so easy. All he had to do was wind the interlock handle slowly, till he heard the supply contactors fall out. The desk TA would be sure to wake up at that point.

Then wind the handle like fury, get in there and change the valve, and close up again. The desk TA, by now considerably alarmed, would then re-power the transmitter.

One evening, a few minutes as usual before the programme start, and without announcing his intentions, he gave it a try.

It is reported that the lights in most of the houses in the northern half of Sutton Coldfield dipped perceptibly. Inside the station, there had been the most almighty bang. People tumbled out into the transmitter hall with their ears ringing.

It was at this point that C. discovered that the interlock handle had frozen into position, and couldn't be moved in either direction. Something appalling had evidently happened inside the interlock cubicle.

Someone was despatched to get a ladder. By the time he had done this, the unit had been completely isolated. It was then short work to climb onto the roof of the cubicle and remove the top cover (this was the only way in). When the cover was pulled back, eyewitness reports say that a rectangular column of dense black smoke rose slowly from the unit and mushroomed off the ceiling. All of the earthing switch contacts had been welded shut.

It didn't take long to diagnose the problem. The sequence of events as mentioned must have been what the manufacturer intended, and to what the system was eventually restored: but it certainly wasn't what happened that evening. It seems that the earthing switch was the first to close, onto an otherwise fully powered transmitter.

Well, it took quite a while to dismantle the mechanism, a good while to hacksaw the contacts apart and dress them up with a file, and even longer to reassemble it all so that it worked in the manner intended. In the end, the transmitters were repowered just in time to catch the closing bars of the National Anthem at the end of the evening's broadcast.

Marmite to the Rescue >

 
mb21 by Mike Brown
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