A dim screen usually is due to a lamp nearing the end of its life span. If the lamp is 2-3 years old chances are a new lamp will bring the screen back to its original brightness. A weak ballast usually is able to power an old lamp since most of the mercury gas has been consumed. If a new lamp isn't igniting that can be due to a weak ballast. A new lamp since it has all the mercury gas it comes with it will require more power from the ballast to ignite than an old lamp. Let us know how it goes.
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