I have been using this old audio amplifier for some time now and recently it stopped working - music was playing and it suddenly went out, you could only hear the 50 Hz buzz, magic smoke coming out and the thing was warm.
After changing a bad fuse and powering it up, I found out that the transformer was heating up so much its wax insulation melted. I tested the output with a multimeter and it roughly checked out, but when I disconnected all secondary wingdings and powered it back up it still got hot, so I figured that it was faulty.
Since it was pretty specific (2x20V with center tap plus 16V) there were no replacements available online, so I decided to replace its wingdings manually. I measured the primary winding to be 70Ω, 0.2mm copper wire and calculated that it has to be roughly 130m long. After taking the core and old coils apart, I winded 960 turns of the same wire type onto it, then 10 turns secondary so I could measure its voltage per turn, and carefully powered it up with a fire extinguisher nearby. It didn't trip any fuses, but it buzzed pretty loudly and heated up to 80 degrees Celsius before I disconnected it.
My question is: what did I do wrong? Although I never built a transformer, I have been playing with electronics for over a decade now, so I thought I could handle this.
For reference, the sockets here are 230V 50Hz.