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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)

The Quieter Years

As time passed, the reliability of the old 405-line installation improved somewhat, as its dodgier aspects were weeded out and various subtle and not-so-subtle modifications performed. By the time the early 'seventies came, it was possible to turn your back on the thing for maybe hours at a time: the only fly in the ointment was, it was still a manually operated transmitter that needed a TA on the control desk to walk it. But about this time, attention was given to the possibility of automating it. This was done as a purely local project.

The big problem was of course the control system: but various keen types on site devised and produced a replacement automatic solid-state control system, using Mullard Norbit modules. These devices were hardly state-of-the-art (they were actually made from discrete components potted into an epoxy package) but had the overwhelming advantage of a colossal noise immunity - a good point in a transmitter bristling with transients. This control system would automatically power up and depower the system, and handle any overload trips on a three-shot basis. It worked well.

The only other thing that was a bit of a problem was the transmitter's black-level stability, which was poor, and needed an operator to keep it in line - previously, the desk TA would give an occasional tweak to a nice large knob on the control desk to do this.

Fortunately, at this time there were a few of the earlier UHF installations that were being re-engineered, and we were lucky enough to scrounge a complete black-level controller chassis from an old Pye Mk I site. This didn't work in quite the way needed (in it, black level was maintained by a motor-driven piston attenuator in the drive output) but all the bits were there and it was easy enough to make the motor drive the required variable resistor instead.

The monitoring functions of the old TA were usurped by another scrounged unit, a 'comparator monitor'. This also came from a re-engineered UHF site, and again needed the odd alteration to make it work on 405-lines, but work it did, and became the latest in a line of contraptions known generically as 'tin TAs'.

So by this point, the transmitter was on its own, and the desk TA redeployed to more rewarding activities. In general terms, it all worked well: there would still be bangs from time to time, and bells would still ring (though automatically, now). It was just that people now had further to run when they did ring. Kept us all fit (I used to reckon that on an average day, I would walk about five miles back and forth through the corridors of Sutton. It was not an ergonomically designed site).

The Writing on the Wall

About the mid 'seventies were probably the most reliable ones for this installation. After that, things started to worsen. The problem was, that when the vision transmitter had been built, it had been largely wired with rubber-insulated cable, and this was now starting to rot with age. There was no question of ever completely rewiring the thing, it would not have been economic and the unit would have been out of commission for months. Worst of all were the heavy-current supply cables to the power transformers: rubber-insulated, these also had lead sheathing. Unfortunately, over the years transformer oil seeped from the transformers into the junction boxes, thence into the cable insulation. This oil converted the rubber into something resembling black cherry jam - it still insulated, but if you accidentally displaced the cable during maintenance, the lead outer sheath would move but the copper conductor within it wouldn't... The result was an almighty bang when the unit was repowered, and the transmitter would probably be off for a day or so whilst the jammy cable was cut back to firmer material, and a new length spliced in. Since most of this cabling was in underfloor ducts, this could be a ticklish job, since your prime desire whilst doing it was to avoid disturbing any further cables..

Around 1980 or so, the decision was made that enough was enough. From that date on, programmes would be carried by the reserve transmitters, with the old high-power transmitter relegated to use as a standby. This meant of course that if a service transmitter failed, it would take over five minutes to power up the old fellow, and no guarantee that it would work when powered (it was the custom to run the old setup into load every few days, just to keep the damp out of the works).

Not long after, everything started to change. Both the original FM radio and the original BBC1/BBC2 UHF installations were becoming due for re-engineering. Since you cannot just wheel a new transmitter in through the door and plug it into a wall socket, some room was going to be needed. The scheme was to install a complete new FM installation into an emptied former Band I TV hall (this re-engineering was going to require a new mast, as well) and after this was up and running, to strip out the old FM installation and use the space for a new UHF TV one. The old 405-line high power transmitter, by this point easily the oldest surviving in the world, was going to have to go.

Station staff undertook all of the dismantling, with the spoils being taken away by a local scrap merchant. Many of the staff were happy to see it all go - one could see people working off old grudges against the gear as they sailed into it with hacksaws and oxy-acetylene cutters - though personally I felt a little regret - it was, after all, a historic artefact and parts of it, at least, should have been preserved. The problem was, the thing was simply too damned big for any museum to display more than a passing interest.

Turning Down the Wick

With the old high-power unit gone, there was now no standby for the medium-power units that were carrying the service. In order to give some sort of a standby, provisions were made that had a slightly farcical flavour.

Somebody produced an antique S.T.& C. CG1 Band I TV transmitter, which had been used for training purposes at Wood Norton, the BBC's Engineering and Training Department. These units were rated at a modest 500W peak white power output - a bit of a comedown after 40kW! The CG1's were produced in a great hurry so that they could be got into service for the 1953 Coronation. Legend has it that only six weeks elapsed between drawing board and finished article. Our specimen was tuned up for Band I channel 4, and the unit installed in an extra-wide corridor (an area always known in Sutton parlance as the 'Zoo', for some inscrutable reason). Initially, the thing really was plugged into a 13A wall socket, and was used only as a standby for the medium-power transmitters.

The odd thing was that, when in use and giving it's mighty output of not much more than one-hundredth of the original output power, there were few viewer complaints. Maybe there were just very few viewers left on 405 lines in those days, and maybe those viewers blamed their sets rather than the broadcasters ("Well, it's old, innit, it doesn't owe me nothink"). The public were never told that these power reductions were taking place, so saw no need to complain. Or maybe the setup really did give usable results: after all, sets of that period were greatly more sensitive than those of 1949. Perhaps a slightly more discriminating user of the output was the Band I television relay at Hereford, which re-radiated pictures from Sutton - although over 50 miles away, it continued to radiate satisfactory pictures even when Sutton was using the low-power setup.

The final ignominy followed - the low-power transmitter became the preferred one, with the medium-power one as standby. In order to do this, it was 'automated' - namely, switched on and off with a time switch. If programmes over-ran, they were simply chopped (unless somebody remembered to press the over-ride button). Again, there were no complaints. Serious thought was given to eliminating the medium power transmitters, and another antique CG1 was located and pressed into service as a standby. Once this became available, the medium-power transmitters were scrapped. The space vacated by these was not immediately required, but ultimately all entries to the space from within the building were bricked up, an exterior door provided, and the space rented to NTL, who used it for their Classic FM service.

Ultimately the 405-line service came to an end in January 1985. The CG1 transmitter, believed to have been the last 405-line transmitter to close down, was offered to and accepted by the Birmingham Science Museum in Steelhouse Lane . There it was for a while on public display, along with other broadcast items such as a pre-war Marconi-Stille magnetic recorder, and a BBC-designed and built analogue standards converter. Shortage of space eventually caused it to be withdrawn and put into storage - the museum has since moved site, and it is not known if the transmitter survives.

"In eighteen months, medium-wave broadcasting will be dead..." >

 
mb21 by Mike Brown
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