Why Did They Change?
 

We know that the 21-CT-55 is basically a CT-100 with a 21AXP22 CRT.

Why did they change the color demodulators from I and Q (CT-100) to R-Y and Q (21-CT-55)? Cost? Patents? Better match to CRT phosphors? Easier to align?

This has been discussed somewhere here before. I don't recall if there was a clear conclusion, but some patent issue would make the most sense. It had nothing to do with matching CRT phosphors, as this would be determined by the following matrix stages in combination with the demodulator angles and gains, so one could compensate for the other. The only practical effect on the circuit seems to be the elimination of one resistor from the input to the red adder tube.

Here is a discussion on the changes made to the 21AXP22 and circuitry.

https://visions4netjournal.com/wp-co...1APX22-PDF.pdf

Edit: One more article. https://visions4netjournal.com/wp-co...w-1955-Mar.pdf

Thanks for all the inputs.

Let me slightly expand the question: Why did RCA (and others) go to the X-Z demodulators? Was it something about better flesh tones? Or less sensitivity to phase shift with the color sub-carrier?

Neither. The resulting grid drive signals were exactly NTSC (at least at first).
The X-Z demods with following matrix/amplifier stages was a clever design to maximize the DC stability of the color difference signals. The three amplifier stages are coupled at their cathodes, which generates G-Y in the third tube. But at the same time, some minus (R-Y) is coupled into the B-Y output, and some minus (B-Y) is coupled into the R-Y output. This is compensated by changing the demodulator axes to "X" and "Z." By changing the angles, the X demodulator output is mostly (R-Y), but with a little (B-Y) added in to counter the minus (B-Y) in the R-Y amplifier. Similarly with the Z demod and (B-Y) amplifier.

The improved DC stability of this circuit comes from having the amplifiers also act as DC restorers as well as blanking amplifiers. A negative blanking pulse applied to the three cathodes causes a negative blanking pulse on the CRT grids. At the same time, it draws a bit of grid current in each amplifier tube to set the DC voltage on the coupling capacitors that feed in the X and Z signals. If you look at the G-Y amplifier, you see an input coupling capacitor, which doesn't go to a demodulator but just sets a constant DC voltage.

The X and Z angles were changed in later years to provide some compensation for the non-NTSC phosphor colors.

Simplification, stability and linearity.

The way I understand it QAM which is the transmission scheme for the I Q signals was invented by Philco...They probably wanted to charge RCA patent royalties and RCA didn't like to pay others for patents.
RCA was experimenting with dot sequential. Switching which primary was being fed down the monochrome carrier at very high frequency and trying to have the reciever switch which gun was driven by the monochrome signal in sync. I don't think they ever got it to work well and jumped on QAM when it was invented.

I Q to R-Y Q
 

There is a tiny note with a plethora of patent holders pasted to every CT-100 cabinet. I'd guess it is in 2-point type! or so. AT&T, Farnsworth, GE, Hazeltine, N.V. Philips, Philco, Westinghouse, and it says "others"...

So it is likely patent payment may well have been a factor.

To decode the R-Y signal, the quadrature transformer changes from a TRV365T in the CT-100 to a TRV-370T in the 21-CT-55.

Pete