There is a thread on this problem a few pages back. It was from myself... It is a common problem. More than likely it is C-18 on the display board. It is easy to replace. However, there is a good chance that C-18 has "leaked" its electrolytic compound onto the PCB and caused some trace damage. This was the case in mine. It is somewhat easy to repair with a "jumper" to deliver the voltage. Be careful in removing the bad cap. The holes for the mounting are very delicate as they are "feed through" type and require solid connection on both sides of the board.
If you are not "comfortable" in doing this kind of work find a ham in your area who is good with technical stuff. Sending it to a service center will be costly (with shipping and all about 175-200 bucks). That is a high price to pay for a 50 cent part replacement.
Some say the whole face needs removed. That is simply not the case. Removing the top and bottom covers and loosening the top two screws that hold the front on and completely removing the bottom two screws facilitates the "flipping up" of the whole front unit. Then there are 5 screws that hold the display unit on and only 1 ribbon cable to disconnect. C-18 is right there and ready for you to replace it.
The process takes about 1 to 1.5 hours to perform if the solder traces are damaged. "ring out" all the paths and connections once you remove C-18 to determine if there is trace damage. Refer to the service manual PCB views to see where all the traces make their paths.
GL and if you have questions just ask.
The TS850 is a great rig and worth keeping on the air.
Here's a link to the "previous display thread" I mentioned earlier.http://www.eham.net/forums/Elmers/190513