> Vibration etc is likely to make the pcb flex and can crack traces or
> delaminate the copper - or produce broken solder joints

On a couple of designs I've seen a plastic tie to hold the transformer
down. You could likely use contact adhesive too (eg Ados F2) or
double-sided tape. The transformer is probably the component that
expands/contracts the most and dry joints are common on PCB-mount
types. More than a few things around here were originally throw-outs
because of dry joints on transformers and connectors (like DC sockets)
which are seldom properly anchored

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