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The position of the black dot on the diagram shows the relative polarity of the windings, so if a primary wire with a dot is connected to one without, and the other two are connected to the mains that's correct for 230V. If the dotted wires are connected together, then to the mains and similarly for the wires without the dot then the primary windings are in parallel and in phase, so 115V is required - i.e. ideal for the countries with 115V mains (e.g. USA). Any other connection is not permitted since it will involve connecting the windings 180° out of phase, so they'll approximately cancel and you'll get nothing.



If you put the two secondary windings in series then yes you would get 70V, however we want them in parallel to get 35V at twice the current as if they were in series. Most transformers are wound with at least two secondaries to give you this option.

so secondry in series on the diagram would be red 70v+ , yellow and black joined. orange -v

sorry here is a good detailed page.. http://www.routoutcnc.com/PowerSupply.pdf