Ok, you seem to have made the right decision. I am trying to remember
what the transformer looks like. Is it a toroid or a regular
rectangular shape. I think the transformer on my big unit is a toroid.
I don't know what the core material is. Could be ferrite. I know there
was a bunch of tapped windings.

Are you in an area where you could throw it on a trash fire and burn
out all the insulation. Then you could see what is there. I used to do
this when I lived in Portland, OR and had a wood furnace. Still have
some BIG copper strap from a burned out transformer.

Paul