Need help with purity adjustments

[matt_s78mn]

Ok, so i'm trying to tackle some half finished projects that have been sitting around for a while. One of the projects is to do convergence and purity on my CTC-31. I spent over 3 hours on it yesterday and got the convergence aligned pretty good, but I've got a real problem with the purity. No matter what I do with the purity rings, I cannot get rid of the color blotches on the screen (they move around when the rings are turned, and are not permanent, so i know there isn't an issue with the shadow mask) I've followed the correct procedure in the service manual, where you slide the yoke back, and adjust for a red spot on the center of the screen, but when I slide the yoke back into position, it the color blotches come back. I've even degaussed it several times.

Do any of you have any suggestions, or tips and tricks that I could try? Also, the blue lateral magnet and purity rings are not original, but are off a newer set (ctc-68.) Is it possible that perhaps the magnetic rings are either not strong enough or too strong?

[Einar72]

Two things: What are the CRT numbers and what are the RCA part numbers for the parts you have interchanged? I'm assuming the CTC-31 is all-tube, and the CTC-68 is solid state. There may be a slightly different gun structure between the two tubes. More importantly, though, I'd check to see if any surrounding metal is still magnetized, I've had trouble with residual magnetism in steel-cabinet setups.

[Chad Hauris]

I haven't had any trouble interchanging any of the purity/blue lateral units among any of the delta gun sets whether tube type or solid state.
You might check that the entire purity unit is positioned at the right place on the CRT neck (in a forward to back direction)...also turning the two rings as a unit causes a different effect than moving the rings one apart from another. Turning them together tends to move the center red area in a circular motion.

The way I would do it is to slide the deflection yoke all the way back, adjust for the best red purity in center of screen, then slide yoke forward till the purity is the best you can get it. This is not necessarily all the way to the front. Then you can do some slight readjustments of the purity magnets to help get rid of remaining off-colors.

This may be a silly question but you are not sliding the entire plastic housing back (including convergence yoke) are you? Only the deflection yoke should be moved inside the housing by using the slide adjustments with locking screws on the sides.

[vintagecollect]

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