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Signs of a bad CRT include but are not limited to (or always caused by the CRT): Dim picture, shift in gray scale tracking, video smear (especially if associated with only a single primary color), and poor focus. Different tubes fail differently, and manifest different combos of symptoms. The best test is a CRT tester and a working set that uses the tube....Some tubes test weak-bad but have decent life left at reduced brightness, others test mediocre to bad but preform lousy in an actual TV (60's-70's small low voltage focus tubes are notorious for having mediocre to bad focus once the emission drops below like-new status, and 50's monochrome tubes tend to let you milk a good picture out of them with a brightener until the cathode is totally gone).
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