Hitatchi tv i have a problem with the screen yellowing in a large oval shape which occupies like 70% of the tv.

Most likely your "light engine" needs complete replacement and it is quite an installation...If not under warranty, usually Hitachi will NOT charge you for the light engine assembly (about as big as a bread box) for this particular V500 series because of all the failures they are aware of..However Hitachi will require a technician to do the installation and first verify that the light engine needs replacement...Hitachi will NOT pay for labor unless the tv is still under warranty. Labor varies. I know that for this 3 hour procedure or more, including calibration after replacement as necessary, I charge $400 or more..Above all pick a highly recommended technician.. Sorry for the bad news.. this happens alot to the V500 series. colors vary..sometimes red and sometimes blue ...this problem due to overheating. The replacement should be an improved assembly, sometimes rebuilt, sometimes new and both usually upgraded so this does not happen again. Do not attempt repairs on this yourself as it requires great expertise; and disassembly and reassembly of top portion of TV requires 2 people. And, on top of that, ever so often the lamp for this tv has to be replaced after a certain amount of hours of play (hopefully 2 years depending on hours of use) and just the lamp alone which you CAN replace yourself is $260 give or take more or less money. Always research any NEW TECHNOLOGY TV's thoroughly before you spend mucho bucks so you know what you might be in for. I try to avoid buying cutting edge technology myself as it's not fully tried and true and tested by any means. It cant be avoided if you want to buy the latest and the greatest motherboards for computers and once in a while I get burned like anybody else. But motherboards are usually only a couple hundred bucks, not thousands of dollars!