Almost all modern transformers are made with thermal protection, which prevents the transformer from overheating and fire.
This is good, but such protection is triggered if the transformer is heated only once, and then the transformer can no longer be turned on. Many then throw away transformers, but in most cases they are still working.
The transformer heats up under load, with poor ventilation and with a lot of dust. If these factors became the cause, the transformer can be revived in 5 minutes, or even in a minute!
Introductory photos of the device transformer
Caution, interturn circuit!
If there is an interturn circuit in a trance, after switching on the transformer will (may) warm up and will not produce the declared voltage. There is nothing to do here, you need to disassemble the magnetic circuit, remove the coils and look for a short circuit. Do not rewind the primary, there is a very thin wire sooo long. And you can rewind the secondary, and you can rewind it to your needs.
We remove thermal protection
So, on the primary coil of the transformer there is a small rectangular crap, you can feel it with your finger under the insulating film. This thing is protection, it does not release current after tripping already. You just need to remove it and turn on the wire directly. Carefully remove the insulation film, but everything must be done very carefully !! If you damage the primary wire, you won’t do anything. We pry the film with a knife, cut it and pull it up, solder the wire directly, fill in the solder with hot glue on top to fix the wire.
And ideally, you need to put a new one, otherwise the trance under load can completely burn out. But it is important to choose the same fuse for current and temperature!
Everything, trance will work if there is no break in the primary or secondary. Next, measure the output voltage and after a minute or two, feel the trance body, turning it off from the network. If it is hot, then there is an interturn closure.