What is the best way to clean the TV screen ?

Defective LCD panel, it is not worth to replace it.

I think it is more than a Kool-Aid stain. likely it was some kind of heat applied to the screen causing physical damage. How does it appear when the screen is white?

If you wish to get some details; visit the site linked here. Pull up older posts.
http://electronicshelponline.blogspot.com/
faults & possible causes to LCD & Plasma screen are given with illustrations. You can get an idea about the fault to an extent. Faults to all LCD & Plasma screen are similar, irrespective of its brand and screen size.
Search the site by type in the "BRAND NAME" of your device; (Exclude the model number), in the "Search box at the top right of the display window of the Homepage" to get gathered related posts.

If the LCD panel has broken and the tv is not covered by accidental damage repair insurance, then your only realistic option is to sell your tv as a spare parts donor and to buy another.

The LCD panel is overwhelmingly the single most expensive component of your tv; it actually costs more to buy a new panel than it does to replace the complete tv. If you do have insurance cover the company will usually just buy you an equivalent replacement model rather than repair yours.

Sorry, but not everything is realistically fixable.

Need s to be replaced the main board.

sanyo lcd dp42647 turns on and has sound no picture black screen

Hi there! The panel may be in protection mode. The Inverter board is the most logical area to start after checking power supply Voltages.(5,6,24 and 13Volts). There may be coils on the Inverter Board which have bad soldering or faulty. Other than this it may be main board which controls switch on or off. If all else fails, refer to a repairer. Beware of High Current!!! Best Wishes and Good Luck, Jus004

This is the best solution for your problem as a 40 year experience with wires etc. Go to the source of your cable signal connector . Open it and clean the terminals with emery paper , sand the end of the cable shiny clean from corrosion , get a new barrel connectors with gold terminals if possible lengthen the ends of the cable , insert and tighten good, use thick rg-16 or rg 40 cable with good insulation ends crimped professionally with a crimpier or buy ready made , not from dollar store. Get a gold plated professional splitter if you need one or two , it will cost about 15 or more each . Make the cable runs as short as possible, using the thick good cables. If needed get a signal magnifier. From radio shack or the source etc. Connect them good and tight . Ground the splitters . Problem solved , you will get crystal clear channel 2 to 7 including 3 and 4 . Isolate the TV cable from other cables and power wires. Vitally

hello,
this tv is 720p heres a link to your manual info

http://tv.manualsonline.com/manuals/mfg/sanyo/dp42647.html

hope this helps