Once @sjdavies and another guy helped narrow down the likely culprits, I found this under the main board of a 1990s synth…
So, I thought a little bit of cleaning and all fixed…
Nope that middle pin just disintegrated with a light touch, including all the solder from the pad and of course the track is also broken under the IC. Notice there is absolutely NO pin left, can’t even see any at edge of encapsulation!
Well, some searching showed the way forward…
Now it works!
But I do want advice on how to stop the corrosion continuing over the next 20 years or even if should desolder, dump content and replace chip with an EEPROM.