[Test-Equipment] HP 34401A DMM display problem

[DaveC]

Bought recently, tagged as "needs new display". When I first powered 
on this unit the display worked. After pressing a few function 
buttons, display went dark.

Now when powered on the display goes all-segments-on and stays like that.

I swapped in a known-good display and the DMM works as expected: no errors.

Anyone have any experience with the display PCB on this or other 
unit? The fluorescent matrix clearly works; it's something to do with 
the on-board support circuitry.

Thanks,
Dave