How about an old Commodore 64 if I plug a 56kbps modem in?
Anyone got bitcoin core on cassette tape?
How long before I can mine 1 BTC?
It's not a Commodore 64 but someone did manage to find a way to mine with a NES from 1985:
http://www.retrominer.com/Unfortunately, I couldn't find a hash rate figure. Modern smartphones are MUCH more powerful than anything from the 80's though. Seeing as though someone mined hundreds of bitcoins back in 2009 with a Pentium 4, it probably would have been profitable with today's prices to mine with a phone back then too.