I had posted this info on dc restoration back in August.

The dc restoration is an interesting issue. I have not played with many pre 1950 sets, but from what I have seen, some of the early sets included the restorer diode, but once the larger-screen sets of the early 50s appeared, the dc restorer had been abandoned (for cost reasons I assume). I don't recall seeing a B&W set with dc restoration until I saw a Sylvania circa mid to late 60s perhaps. Some later sets, including color, had partial dc coupling. It wasn't until Consumer Reports Mag. made an issue out of dc restoration in about the early 70s that all of the manufacturers got on board and put the dc restoration back in the sets. The CR article was prompted by Sony (and perhaps Sylvania) being the only manufacturer having 100% dc restoration at the time. The picture quality of the Sony was rated the highest of the sets tested. I had just purchased a 1973 Zenith, and that or a similar Zenith was rated as having only 10% dc restoration. I bought the Zenith (hybrid) because I liked the way it was built, and would not have touched a Sony. Shortly after I read the CR article I set about eliminating all of the coupling capacitors in the video signal path and adding appropriate level-shifting transistors to achieve 100% dc coupling, which give the same result as restoration. I have done a similar operation to a number of other sets, including a vintage CBS Columbia that I used in my bedroom for many years.