The failure modes of transformers

[Glasgow]

Hello.

Other than shorting, do transformers fail in less obvious ways?
Do they wear out?

Thanks,
BrianB

[Enzo]

Shorting is such an inclusive term. They can short in a variety of ways and still work.

WIndings can short to each other, they can short to the frame. Windings can have one turn shorted to the turn next to it. In a PT this would make it run hotter but not likely kill it. And in an OT it will affect the sound.

Another short is the arc. You cannot find that with a meter, but under high voltage, a spark forms between two points and that arc might as well be a wire to electrical current. That might show up as blown fuses, loud noises, etc.

Many transformers have thermal fuses inside, and if the tranny gets to hot, the fuse opens. Seems dead, but a new thermal fuse will restire operation most times.

Then there are issues with the laminations. Others here know a lot more about that than I do. But when the laminations short together it affects the operation. And when laminations get loose, things happen as well.

The nice color coded wires that trail out of a transformer are not the same wires that are wrapped around the core - the windings. Each piece of wire that goes into making a transformer must be connected to other pieces of wire. Each of those connections is a potential problem if that solder joint or weld gets loose or fails.

Transformer windings can open too. Might be a wire burnt out, might be a mechanical failure in a junction, etc., but an open will kill the tranny.

But wear out? I don't think so. Age will make certain failures more likely, but that is not the same as wearing out to me.

[Alex/Tubewonder]

Many transformers have thermal fuses inside, and if the tranny gets to hot, the fuse opens. Seems dead, but a new thermal fuse will restire operation most times.

This is something you shouldn't do. Non resetting thermal fuses are usually dimensioned to open when internal transformer temperature reaches 130C. Interwinding isolation is considered compromised at this temperature. The thermal fuse is there to prevent fire hazard. Some transformers have resetting thermal fuses with 110C trip temperature (like the ones in hot air guns). If it repeatedly trips and resets you should search inside the amp for the cause of overload.

[Glasgow]

Other than measuring the resistance through the various taps, are there any other bench tests people perform on transformers?

[Glasgow]

Well, testing voltages comes to mind silly.

[rocket]

There is a check that involves a battery and a neon bulb that is supposed to detect a short - R.G. described it in the Pickup section.