> It had some kind of hardware > issue which I don't remember the details of because I was 5 at the time, but which my > dad managed to overcome (overheating TED chip? I remember him opening it up and > smearing grease on some component)
The CPU was known for overheating. There was a mystery batch made by CSG in 1990 that solves the overheating problem. I can only imagine that someone in charge of production had a plus/4, because I don't think the 7501/8501 was used for anything else. Supposedly 50% of all plus/4 faults are a dead CPU & 40% are a dead TED.
It appears the NTSC C16's may have ended up here...