Re: Laptop PSUs - bit OT - sorry

charles wrote:
In article <Szw9+wJdqkmIFwq3@xxxxxxxxxxxxxxxxxxx>,
I have an illuminated globe (with a metal base) bought in Germany in the 1960s, of which the mains lead (I _think_ it originally had a moulded plug) was red, grey, and black (and I _think_ red _wasn't_ live).

correct. Red was earth in Germany.


And a very bright red it was!

There was a problem in the 60s with the more affluent pop groups who bought German "Echolette" echo chambers such as used by The Shadows (actually a tape loop, rather than a chamber!)

I got a panic call one night from a local group who were trying to set their gear up in a local club and kept blowing fuses.

By the time I arrived, they'd run out of 13amp fuses and resorted to the old silver-paper-from-a-***-packet dodge. This had resulted in such a flash and a bang that it stopped them in their tracks!

There was, of course, a dead short from live to earth on their distribution board which I quickly traced to the "Echolette".

Apparently they'd lent it to a friend and got it back minus the 13amp plug. This was in the days when the 13amp plug was far from universal, of course. (I was astonished when I found that the BBC seemed to exist on nothing but 5amp 3 pin sockets!)

Whoever re-fitted the plug used his common sense but, of course, was doomed to failure! Interestingly, although they'd tried a number of fuses, it was always the 'master' fuse that blew.

I re-fitted the plug - correctly, this time, and replaced the siver paper with a proper fuse. Then everything worked ok - which I thought was a bit puzzling ...

.... until the following Sunday, when I got a call from the lead guitarist who was trying to practice at home but was having problems with his VOX AC30 - all he could get from it was a loud hum.

As was common with such amplifiers, it was built with a common earth bus bar of 16 swg tinned copper wire, connected to the chassis at one point only, adjacent to the input sockets.

In this case, the earth was in two parts! I never found the break as it ran beneath some tagboards and was completely inaccessible - all I knew was that there was one earth for the input section and another for the output stages! I connected the two together with a suitable piece of wire and everything worked fine.

What had puzzled me previously was why the mains sockets in the hall were still live after the siver paper fiasco. Obviously, the fault path had been via the live Echolette connection to the chassis, then through the output (signal) cable to the AC30, then via the amplifier's earth bus to the mains earth and back to the board. The earth bus then vapourised, instead of the fuse in the building's mains distribution!

Which, of course, begs the question of what the value of that fuse was! It was hardly a normal 30amp ring main fuse!

Anybody know the fusing current of 16 swg copper wire? I've found one reference that suggests 70 amps!

In the 60s, I had a table published in some magazine or other, which showed the colours used for mains leads throughout Europe. It "lived" in the back of my Wireless World diary and was carefully transferred to each new diary every year. Considering the wide variety, it seems a miracle that it was ever possible to produce a scheme which, using simple colours, could be employed in all those countries without the possibility of confusion with the existing standard in any of them.

Terry
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