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Thanks to a tip in a PM from one of our friendly VK members, I was able to get the purity much better (see first picture). What I had not yet tried was changing the position of the yoke along the neck of the CRT, and that helped a lot in terms of cleaning up the top edges of the raster for red purity. Now the color rendition is acceptable over the entire screen. The small amount of pink in the upper right corner is not easily noticeable in a normal picture.
I tried using the field cancellation coil as a degausser to see if any further improvement could be made. After all, the cancellation coil does bear some resemblance to the automatic degaussing coils built into newer sets. The wire size looked like it could handle an amp with no problem, so I applied AC current up to 1 amp (about 40 volts AC) and then backed off slowly. This had no effect on purity afterwards. It probably takes a lot more current than what I applied (automatic convergence coils give a nice buzz for the moment they work, and this coil never got into that regime). In any case, I'm not interested in testing the limits of the cancellation coil, since I might damage it, and the purity is actually pretty decent now anyway.
Worked on the convergence a bit. Horizontal convergence is working properly, and I was able to get good convergence across the horizontal axis (see second pic). The vertical convergence is not working properly -- adjusting both the vertical convergence amplitude and the vertical convergence shape controls has very little effect on the screen. Also the "smile" when probing at the plate of the convergence amplifier looks possibly inverted. I wonder if I have the phase of the new convergence transformer reversed? There may be some other problem in the vertical convergence circuitry, so I'll look into that further next time I get a chance to work on the set.
I also mounted the new convergence transformer inside the housing in which the old transformer was potted, and have positioned the solid state voltage tripler module inside the HV cage (not permanently mounted yet, but it soon will be). Also adjusted the range of the DC convergence control by adding 5 megohms on the top side (for some reason my control is only 10 megohms as opposed to the 15 megohms on the schematic; adding this resistor get the sweet spot properly within the range of the control).
I also made a slight modification to the focus control circuit. I'm still having loss of focus on very light or white screens. There isn't any evidence that this is caused by loss of regulation of the HV, so I think it is the focus voltage that is sagging or otherwise changing in high brightness scenes. I adjusted the range of the control (it is only 1 megohm as opposed to the 5 megohms on the schematic, so its range is really limited) and also increased the resistive load on the focus rectifier to both improve regulation and pull the voltage down into a range where good focus in properly centered in the range of the control. This may have improved focus stability, but I am not sure yet. It still won't stay in focus on the very brightest scenes. Nonetheless, it is watchable at this point.
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