If you want to add turns to an existing transformer, which is easiest on a Toroidal type, BTW.
You would need to find out the Turns/volt, and the simplest way is to wind a small number on top of the existing secondary, say 5 or 10, and measure the resultant AC voltage, This will allow you to calculate the amount of turns you would need to increase the existing winding.
Remember that the resultant AC output will need to be .707 x your desired DC output.
Square lamination transformers are usually the hardest to increase because of the construction.
The other way is to get a closest value Toroidal type off ebay
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.