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I'm looking strictly at the chassis, since that's what makes the set. After all, we refer to them by chassis designation not the CRT paired with it.
From that point of view, you can see that the CTC-2(B) faithfully stuck to the original signal decoding scheme outlined by the NTSC. The CTC-4 was the first departure from the original plan, having a 'much simplified chassis' as RCA put it (generational). Gone was the original decoding matrix and driver tubes, in it's place a circuit which was a little strange. Instead of separate matrix/output tubes, the 4 chassis uses a single section of a 12BH7 to do both jobs (one for red one for blue). Kind of genius when you look at it. They fed chroma and 3.58Mhz into a tube, and the output signal drove the CRT grids directly.
When you get to the CTC-5, you finally had the circuits that set the tone for all remaining chassis (well, with the Deluxe 5 anyway). Here they got to X and Z demods, and the familiar circuits that became standard thereafter. From the CTC-7 all the way to CTC-15 (I think, these slow ship computers gag trying to download the manual) the story remained the same. These were the last generation of sets before a whole new string of changes came along, so I lump them all into the same category.
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Evolution...
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