On certian channels a big black square pops up and random letters start going across the screen.

It is a text box. You can disable it within your television's closed captioning settings. You have to go into your menu and go to closed captioning and enter the category and go to mode off

How big is this black square? Is it like 1/4 to 1/2 the size of the screen? If so, you probably have Closed-Captioning turned on, and turned on to TEXT mode. Either change it to CC1, or else turn it off, and your black square will go away!

keypad mat of your might have rust or deteriorated.
does your wet?

It is most likely related to the Closed Captioning option on your TV. Go into settings and make sure that CC is turned off, or make sure it's set to CC1. What you are probably seeing is the "text" option included in the CC options.

turn off the closed captions - you have it in text mode that will have a big black box with video on the sides
thanks
scott

You have the closed captioning turned on to "Text" mode. Turn off the CC and it will go away.

in menu, turn off cc-closed caption

shut off close caption. use the CC button onthe remote or go in to the menu and change close caption to CC1. this box is the close caption in TEXT(1,2) which will put on a black box whenever there is caption present.

Shut off the closed captioning and your problem will be gone.

I’m happy to help further over the phone at https://www.6ya.com/expert/dewayne_4a10eab6caecdb92

if your big black square starts about the middle of the tv & goes down close to the bottom, you have closed caption text turned on. normal closed caption is small, but test caption, is just a big black box with no writing in it. go into your menu & shut it off, it could even be a button on your remote called, "caption", or "cc". good luck.