it seems that if you do not change the internal battery (after the TEX tells you to do so), after a while the TEX just completely looses its internal operating system. during boot up the machine is simply lost and signals this by a blank screen that flickers a bit every second or so.
the easiest way to get the machine back working: first replace the old battery with a new one. (the TEX won't work afterwards because of the still missing OS). then get a CF card, and copy the necessary OS files on this card (see instructions at the korg homepage where you can also find the newest OS). CF card into the TEX slot, then turn on the TEX while holding [ENTER] + [LOCATE] and the new OS will be written in memory (takes about 3-5 minutes), then the machine restarts and the OS should run smoothly. Restore the factory data under [GLOBAL].
Then simply leave the TEX turned on for 2 days or so (I choose a preset without the tube, to prolong the lifetime).
If you directly turn off the TEX after you installed the new OS it is most likely that it will loose it again. I assume there is a small capacitor that buffers the OS memory which needs to be "charged" - that's why I left the TEX turned on for 2 days.
Since then everything works fine...
hope I could help
