On Thu, 28 Nov 2019 13:43:15 -0800 (PST), Mathew Newton wrote:

Measuring the primary coil revealed it was open circuit.

Internal thermal fuse has given up.

Or could it just be age / poor design?

I'd tend towards age and border line design. It takes quite a while
for plastic to change colour under heat without getting smelly.
Especially when there is an air gap like for the cover. The
transformer case was blue but is now mostly and evenly dark.

As you have a working board how warm/hot does the transformer on that
one run?

--
Cheers
Dave.