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I will be happy to assist you with this question.
the issue you describe is not a tv malfunction,rather a wrong setting for the closed caption feature.
If you check the CC(closed caption) feature,you will probably see that is set for TEXT(T1,ot T2 on some models).
By scrolling throug the cc menu,you will need to either turn the CC off,or set it to CC1(or caption 1 on some models).
Once the CCis OFF or set for caption 1,then the issue with the black letterbox will be gone.
I hope this helps,please,accept my answer if you are satisfied and will be able to leave a positive feedback.
Thank you and have a good evening!