[Nicholas Lindan]

I'd give the connector pins a very close inspection

That problem isn't so bad with PTH 2-sided boards, but is a big problem with single sided boards.

If a single sided board fails then 99% of the time the problem is a cracked trace or solder joint. Often the connector pin or the lead on a heavy component lifts the pad and the foil cracks where the trace meets the pad. The other problem is that there is not enough solder to form a good fillet between the pin and the pad and an annular crack forms where the solder bridges from the pin to the pad.

Examine the pads around the transformer pins for solder cracks, broken traces and lifted pads. The transformer pins should be clinched (bent over tightly) as there is no hardware securing the transformers to the board.