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802BT/802R Video Test Generator User Guide 611
Purpose To produce correct colors in a displayed image, the electron beams from each of the three
(3) guns in the CRT should strike only their matching phosphors. A white image shows
patches of various colors on a monitor with bad purity. The purity adjustment(s) should be
performed before doing any brightness or color tests. In some cases, purity adjustments
involve loosening and repositioning the yoke, in which case purity should be adjusted prior
to doing any geometry tests.
Method The methods used for adjusting purity on a color monitor depend on the type of monitor
and CRT (for example; Delta, In-Line or Single Gun). In most cases, the first step is to
degauss the CRT.
Note: For a Delta Gun CRT, turn on only the red output. A solid uniform field of red should
appear. If the color is not uniform, adjust the yoke and the Purity Tabs assembly.
If purity cannot be corrected to acceptable limits, the monitor may not have been properly
degaussed or there may be a defect in the CRT or purity assembly.
Test Shadow mask warping
Purpose The purity characteristics of your CRT can change over time if you leave it on with a lot of
video being displayed. This may be due to the CRT’s electron beams striking its shadow
mask with enough energy to cause the mask to heat. This internal heating may be enough
to cause the shadow mask to warp and give bad purity.
Method Set the purity image to white and allow the monitor to run for a few minutes. Any mask
warping shows up as a change in purity. You can use a color meter to measure the
change. The BriteBox pattern may also be useful for measuring shadow mask warping.