if the OP has the original failed transformer they could try looking for a thermal fuse buried in the primary wiring coil.

OP sghould look at the temperature where the transformer lives. perhaps the temp in the area is too hot, causing the transformer to fail.

OP could shop for a higher current transformer rather than 40 or 70 watts how about 150 watts.

the transformer with the obvious charring was likely a manufacturing defect.