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Use the following purity adjustment procedure as a general guide only.Depending on the particular model TV, the following purity adjustmentprocedure may substitute green for red depending on the arrangement of theguns in the CRT. This description is based on the Sams' Photofact for the RCACTC111C chassis which uses a slot-mask CRT. The procedures for dot-maskand Trinitron (aperture grille) CRTs will vary slightly. See you servicemanual!
Obtain a white raster (sometimes there is a test point that can be groundedto force this). Then, turn down the bias controls for blue and green sothat you have a pure red raster. Let the set warm up for a minimum of15 minutes.
Loosen the deflection yoke clamp and move the yoke as far back as it will go,
Adjust the purity magnets to center the red vertical raster on the screen.
Move the yoke forward until you have the best overall red purity.
Now, move the yoke forward until you have the best overall red purity.Tighten the clamp securely and reinstall the rubber wedges (if you sethas these) to stabilize the yoke position. Reset the video adjustmentsyou touched to get a red raster.