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I don't think the chroma could be completely missing, it's likely very attenuated after passing though the IF. I'd bet if you used the scope to zoom in on one of the luma bars, you'd still see a bit of 3.58 modulated on them. That's why there's a band pass amp, to bring the 3.58 back up to the point where it can be processed in the set. Same principle as attenuating sound before it messes with the video, then amplifying it in the audio section in intercarrier sets. Does your pattern gen have a means of switching off the chroma when bars are on screen? If so you can probe the band pass amp or demod driver to see if the chroma signal is getting though, switching chroma on and off while you probe gives you a way to make sure you're not just looking at noise. If it isn't getting through, you're probably looking at a slope adjustment to the IF at the very least because the 3.58 has likely slid off to one side and is not being seen by the set anymore. If the chroma is still there, it's likely the color AFC needs attention. You were able to get solid color raster before, but that is a very strong signal and is easy to lock onto. A weaker scene type signal might be throwing it off, so see if doing the zero-beat procedure does anything to improve it. I don't have many more ideas, besides a full on alignment. You could mail me your chassis I guess, I have the rig to do it...
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Evolution...
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