There is a small led display on the rear side of machine.
What code you have on the display?
2 is error for system memory
5 is for sd card
8 is also for SD card.
Have many 58xx machines with white screen and new sd card always was a solution.
If you have another 5890 create an image of the SD card and put it on a decent SD card, then try again with the altboot.
Also try a forced altboot rather than a normal one, as this will overwrite every firmware slot regardless whereas a normal altboot will not update everything.
To do this create a folder called 'altboot' within that folder load the firmware file, and also the 2x extracted files from the below zip file
Dropbox - Xerox.zip
Bookmarks