Another quick update. I re-checked the original supply, and it was much the same.
However, I checked the attic, and found my Amiga 500 power adapter, and checked that. It looks horrible compared to the 1200's, really yellowed and dirty, but the voltages of the pins looked spot on
(plus it didn't have the high-pitched whine that the 1200's supply did when switched on.
The bad news is... the issue still remains.
So, as I see it, there are 2 options:
1. Ignore it for now - the keys do eventually work
2. Replace the CIA chips
I think eventually, assuming that the issue is (one of) the chips, then I would like to replace them.
So now the question is, does anyone know where I could get replacement chips and sockets? Is Ebay the best bet?
I'm hoping removing these chips may be made slightly easier to be removed by using kapton tape (to mask nearby components) and a hot air gun. I avoided using this method before, when replacing the capacitors, as I've heard they can explode, but a chip shouldn't be as dramatic, hopefully...