It could be possible to build a digital tuner but it would be pointless, since buying one would be cheaper. Now modifying a TV so it has a digital tuner, now that would be something. I’m not just talking about hiding the digital tuner inside the TV, I’m thinking more like modifying it so not only it has a DT, but also the front controls are fully functional.
Like for example, taking a Sony KV1946r (the one with the 14 channel neon displays) and modifying it so not only it has a digital tuner, its using the neon display to indicate what channel your on. Or taking a TV with a rotary tuner and modifying it so you can still use the same controls to change channels with.
Now that would be one hell of a project, don’t you think?