I'm trying to locate where to buy a picture tube for a mitsubishi 1080p 73" tv

Replace the lamp ballast.
http://electronicshelponline.blogspot.com/

Try checking for a blown fuse where the power cord enters the TV. Please unplug first.

Try replacing the lamp. part# 915B455011
see website: http://www.shopjimmy.com/dlplamps?tv_brands_select=Mitsubishi&tv_models=Mitsubishi+WD-73740

means u need a repair guy,lol, they are usually worth their time.

Contact the authorized service center for Mitssubishi. It can't be purchased from open electronic spare part market.
Electro help

http://www.tomsguide.com/forum/57674-4-white-dots-mitsubishi-screen

Take a flashlight and at a angled view see if you can see the picture, if you can the lights burned out in the back

turn down volume see if you hear a noise from back where color wheel is located

U can't just do it on ur own setting like 480i to 1080p when ever u want.No,u only get what ever the tv stations modes they set and send out only.Ur tv have the 1080p mode capability mean ur tv can pickup that 1080p mode.Dosen't mean u can change 480i to 1080p by ur self anytime u want,if the tv station is broadcating tv signal to u only in 480i u get only 480i.