I get a large black rectangle covering 1/3 of my

The picture position is misaligned. Adjust it. For more details, check this link. http://electronicshelponline.blogspot.com/
Pull up previous posts by its left sidebar, or type in the brand name and model number to of your set, to its search box, and press 'Enter' or view it in Mosaic / Timeslide option.

faulty caps on power supply board......
replace caps or replace board.......
cheers

Used screen is about $100 plus shipping

I have a similar problem using an older 20" flat screen standard definition TV. (not High Def). I can see a little of the picture but more than 2/3 is covered by the black. My model is LC-20SH7U.
I used sharpusa.com "contact us" section.
Hopefully I get some kind of reply to report what they say.

Its in the user's manual. Press menu, setup and find 'reset' to factory settings. It will require inputting all changes you made...

You should check the set with another video source to rule out the set from being defective. The process of elimination is the only way to find out where the problem is.
Jerry G.

http://www.sharp.ca/manuals/default2.asp

The Audio cable is not pluged in the right place.

Did you go to Menu - Setup - EZ Setup - then perform the Channel Search? It programs the channels that way.

The Sharp LC-19SB25U 19 in. HDTV LCD TV has both a NTSC and an ATSC tuner.

The NTSC tuner will stop working in Feb. 2009 but the included ATSC tuner is what you need for after the digital transition.

If this is not the model number or manufacturer of your TV, then please post them.