Have you tried a greater range? If a low range is selected a reduced test pattern is displayed.
When you select target alert mode does the entire red display flash?
Do you have bands where the other colours should be but are filled with red.
When you adjust the brightness how does the display change?
When in auto gain detent do you see small green specks on the screen? 10 to 20 specks are ok. Look real close and give it sufficient time to warm up.
Really need to know the model of the display and RT to go much further. Even the aircraft type may be helpful. Let us know how you get on
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Curiousity killed it