On Friday, 29 November 2019 17:05:41 UTC, Dave Liquorice wrote:

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?

Good question; I'll do some testing. I did wonder about the location of the PCB as whilst it is underneath the combustion chamber it is never the less inside the casing and so it wouldn't surprise me if it got close to, or exceed, the 50C rating that I've seen printed on some replacement transformers.