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There is one more thing I remember about the HV rect in my first round-tube set, a RCA Danish Modern CTC16XL, a "most glorious tv" I got when I was 15.
This TV had a rebuilt Channel Master 21FJP22, had no red emission and NO horizontal sync, just the picture scrolling horizontally with the control near the center. Of course I was fumbling around in the oscillator and afc stage area. Out of desperation, I started replacing tubes elsewhere; 6JE6, 6KA8, 6DW4, 6BK4, 2AV2......THEN I found a 3CU3 in the HV cage and, when I replaced it with a 3A3, the horizontal locked solid. I borrowed the CRT tester from the HS shop, brought the red back, and transported it there learned how to do a marathon purity and covergence session that lasted a week. The picture on this RCA looked as good as the 1971 Zenith downstairs!
Years later, a former RCA factory-service tech confided to me they found alot of sets this way (after other attempts failed or even introduced this new problem) because everyone wanted to stock the fewest HV rect tubes in thier caddys, and a 3A3 was an old-school type that was "improved" in the 3A3C and some others.
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