Visiting the Post Office Tower

[ppppenguin]

BBC coat of arms at Crystal Palace. Taken about 3 or 4 years ago when I saw the digital transmitters in action. At the time there were 6 of them, in 2 rows facing each other. I was asked not to take photos in the transmitter hall. Don't know why they were sensitive about it but I was their guest so was in no position to argue.

[Refugee]

They are sensitive to "flash" photography on Euro-tunnel car shuttles for some reason. You just need a sensitive enough camera to take the shot without the flash.

[Nick]

It was a looong time ago - could easily be confused by the entrance.

We were shown the coax feeder - massive, ISTR, remember the final stage being water cooled...

Also, no photography - in those days it was film photography but we were told no photos as it was forbidden on national security grounds.

Flash is often forbidden, as on channel tunnel Le Shuttle trains, as it confuses the fire detection systems which treat flashes as potential ignition indicators...

[ppppenguin]

The co-ax feeders were very impressive bits of plumbing. The outer was about 5" diamter, the inner more than an inch. I have a U link from the old analogue installation. I'll try to find a photo.

The old output stages (klystrons) were water cooled. The new transmitters use vapour cooling on IOTs https://en.wikipedia.org/wiki/Inductive_output_tube

[Nick]

The co-ax feeders were very impressive bits of plumbing. The outer was about 5" diamter, the inner more than an inch. I have a U link from the old analogue installation. I'll try to find a photo.

The old output stages (klystrons) were water cooled. The new transmitters use vapour cooling on IOTs https://en.wikipedia.org/wiki/Inductive_output_tube

This is bringing back soooooo many memories - The klystrons were in cabinets and you could see the water-cooling jacket - was it pyrex or something? - I recall the cabinet being opened and being shown the klystron in situ.

Also recall some mighty PIO capacitors (at least I assumed that they were PIO) with shorting straps on them as there was a huge amount of RF in the transmitter rooms and the caps self-charged. Also saw the "debollocking stick" - a long wooden pole with a hook and earth strap on one end that was used to check that stuff was safe.

Was told a story by the engineers that the core of that coax feeder was a solid 1 inch diameter copper bar... and that they once had a tower short due to bad weather or somesuch that caused the feeder to blow out every 1/2 wavelength... maybe they were winding us schoolkids up? Who knows! 

Also recall the duty engineer was just sitting there monitoring the outputs and that if any of the input feeds failed, up went the "Normal service will be resumed as soon as is possible" or test card etc. Probably most boring job in the world... no automation in those days.

Multiple power feeds in from differing utility providers too... can almost picture the place though I was only 17 at the time...

Another story centred on them finding an armchair on a platform halfway up the tower! Someone had broken in, undetected, over the fence (this was very pre-IRA) and climbed the tower (was there a service lift cage?) with a small armchair, then sat up there admiring the view, before disappearing! They reckoned he (assuming it was a "he") would be possibly sterile as a result...

[ppppenguin]

I have one of the earthing wands (Nick used a different expression ) from CP.

There were certainly some hefty paper caps around. Shorting links are standard practice on these, not just in RF heavy areas. I tried to acquire one but H&S objected as they contained PCBs. Or at least were believed to contain PCBs.

I don't see why the feeder would have had a solid centre. Sheer waste of material due to skin effect.

On my first couple of visits to CP in the mid 1980s I was a guest of the BBC, collecting CO6/509 625>405 converters. The ITV side of the station was unmanned, controlled from Croydon which was their main engneering base. ITV had invested a lot of money and effort in automated TX control and monitoring. See IBA Technical Review volume 6. In theory these are available online but I can't find them at present. At 30MB or so each they're too big to upload here.

[Terry]

Yes, 39.9Mb this one! 

The secret is remembering that it is on the ntl pension site!

[ppppenguin]

Thanks Terry. Google links to a site which says it has them but doesn't work.

[Nick]

I have one of the earthing wands (Nick used a different expression ) from CP.

There were certainly some hefty paper caps around. Shorting links are standard practice on these, not just in RF heavy areas. I tried to acquire one but H&S objected as they contained PCBs. Or at least were believed to contain PCBs.

I don't see why the feeder would have had a solid centre. Sheer waste of material due to skin effect.

Quite possible he just said that it was 1" in diameter and at the time I took that to mean 'solid 1"' - you are quite right that skin effect would render that pointless.

Hadn't thought about PCBs - nobody did in those days as we happily detonated electrolytic caps in the waste-paper bin

Funny place - I have vague memories of people smoking in there too... as in "smoking cigarettes", not that the engineers were smouldering (they definitely weren't attractive either, before anyone asks)

[ppppenguin]

AFAIK, electrolytics never contained PCBs. I don't think the electrolytes were very harmful or toxic. Showers of ali foil might cause intersting problems in elecrtical kit

In the bad old days loads of people smoked at work. Yuk. In one job the department manager smoked evil cigars and his office was just a partition in an open plan area. Glad my desk was about as far as possible from his and by a window.