Many of the old meters in 34401a series are still in great working condition, however their aging VFD displays are failing in various ways. Low contrast, leaking current in segment drivers, shattered glass are frequent cause of these excellent devices being written off.
While replacement VFDs are still sold on some markets, they are not cheap and getting them intact in mail is a lottery. Also replacing the VFD itself does not always resolve the issue since in some cases it’s the driver IC that is at fault.
On older issue 34401a meters (fw version 06-04-01 and prior) have a custom HP designed micro controller with integrated high voltage VFD driver which is not obtainable, unless bought as a whole front panel assembly, which again is not cheap.
This mod sniffs on the SPI bus of the front panel and draws the display data on a modern OLED display. It is a direct replacement for the hard to source original VFD displays.