The polarizer film went bad, its a characteristic of some vintagem LCDs from 90s, just replace both (easily found online).
Carefully disassemble the display, peel of the old polarizers
Clean the glue residue, try rubbing alcohol, then acetone and last gasoline
Make sure the screen is totally residue free, no oils or dirt
Play with the film to find the best contrast angle. Use a scalpel to cut the film, not a scissor (they crack the film)
That was my THS720P before:
![](https://www.eevblog.com/forum/repair/tektronix-ths720p-bad-screen-lcd-lm32p10/?action=dlattach;attach=168119;image)
![](https://www.eevblog.com/forum/repair/tektronix-ths720p-bad-screen-lcd-lm32p10/?action=dlattach;attach=168121;image)
And after replacing the polarizer. You can see I missed cleaning the backlight diffuser.
![](https://www.eevblog.com/forum/testgear/tek-ths720a-portable-scope-teardowndiscussion/?action=dlattach;attach=852746;image)