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The 7864 transformer had, in addition of the main regulated outlet for controlling trains and a fixed 12v DC for plugging in remote controls and lights, a third socket available which provided 12v AC.

This third socket was blocked with a plastic plug, as seen on about every photo of it I can find, and I've never seen any reference of it being used in an official, LEGO-endorsed way.

So I wonder, what's the story behind this? Did LEGO have something else in mind with it, which was later abandoned? Or did they just toss it in because they had it internally available?

Retro-compatibility could also have been an answer, but the two AC socket on the blue transformer are apparently 16v (for switches) and 3,8v (for lights), so I'm thinking that's not it either.

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2 Answers 2

Is it possible it was for use with other sets, such as the LEGO #4561 Train Set Controller.

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I think it's unlikely due to the fact the 7864 was released in 1980 and the 4561 wasn't released until 1999. Also 7864 is 12v and 4561 is the later 9v system. –  mcqwerty Sep 29 '12 at 0:22
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Agreed, 4561 is 9v era so it wouldn't make much sense –  Joubarc Sep 29 '12 at 10:07

I had one of those, and afaik it was just used for the lights. I had like 2 light poles in my set and they where used for that. lights is about the only thing that could take AC current instead of DC in lego. Those weren't LED's at the time. I just can't find any pics online from the ones I have(had...).

Edit: Found it, it was powering this light brick: http://guide.lugnet.com/set/1140

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The socket could power lights, indeed, but as far as I can remember the official way to power lights was through the extension port of the transformer (as seen in the instruction manual for set 7867). My question is not really what one can do with the socket, but why did LEGO put it in there in the first place since there seemed to be no official use for it (unless someone can point to a link showing otherwise), and considering it was capped as to say "don't use this, folks". –  Joubarc Oct 29 '12 at 10:42
    
It was capped because it's AC and putting your trains on it would backfire. I found the set I have, there where 4, not 2 : guide.lugnet.com/set/7867 –  Glenn Plas Oct 29 '12 at 10:43
    
But I see your point, indeed the manual doesn't tell you to use it in the front but on the side, I do remember my dad (the engineer) telling me it was ok to use on the lights, knowing the plug wouldn't survive my curious hands :) –  Glenn Plas Oct 29 '12 at 10:45
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Unless your dad worked for LEGO, he doesn't count as official :-) –  Joubarc Oct 29 '12 at 10:49
    
That said, welcome to bricks.se! –  Joubarc Oct 29 '12 at 10:52

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