Mains solder smoking?

[AmmoJammo]

And while we're here.

What are you trying to run off the transformer? How much current are you aiming for?

A 12volt AC transformer, once rectified, will be closer to 16volts.
However, a transformer that does 12volts ac with no load, will drop when a load is applied...

[Ian.M]

Transformer laminations *may* be insulated with lacquer, but are often insulated with a black or blue oxide layer. That's to avoid circulating currents round the perimeter of any core cross section normal to the flux acting as a shoted turn.

Welded transformers work because the flux at the outer surface of the core near an outer corner is a lot smaller - it concentrates towards the inner corner (unless there is a badly fitted gap).  Its probable that your weld went deeper than the original, (unless of course you used an ultra-thin slitting disk to cut the old weld).  That would increase the loop area of the partial shorted turn that each weld forms in the outer arm of the core, increasing the losses.  Its also possible that it was a noisy b----r to begin with.    I would try letting cheap runny superglue wick into the joint between the E and the I, as it may just be due to the I flexing under the forces involved.

[seancsnm]

Transformer laminations *may* be insulated with lacquer, but are often insulated with a black or blue oxide layer. That's to avoid circulating currents round the perimeter of any core cross section normal to the flux acting as a shoted turn.

Welded transformers work because the flux at the outer surface of the core near an outer corner is a lot smaller - it concentrates towards the inner corner (unless there is a badly fitted gap).  Its probable that your weld went deeper than the original, (unless of course you used an ultra-thin slitting disk to cut the old weld).  That would increase the loop area of the partial shorted turn that each weld forms in the outer arm of the core, increasing the losses.  Its also possible that it was a noisy b----r to begin with.    I would try letting cheap runny superglue wick into the joint between the E and the I, as it may just be due to the I flexing under the forces involved.

Thanks for the clarification. Also, you might be right in that it was noisy to begin with. I'll fire it up again in a couple days if I get a chance.