HP / Agilent 34970A: Faint display solved.

[Sören_Marodören]

Some months ago, I purchased an Agilent 34970A Data Acquisition unit.
(More about this unit can be read here: https://www.eevblog.com/forum/repair/hp-agilent-34970a-error-614-easy-fix-for-one-unit-at-least/)

When I received the unit, I noticed that the display (Vacuum Fluorescent Display - VFD) was dim or faint. When taking the unit apart I could see that the VFD was replaced at least once. So, someone had tried to fix this problem, but with no success. (The instrument is manufactured at the beginning of 2009 and the new VFD is from 2017.)
Luckily, I had another 34970A that I could switch front panel with. Interestingly, the front panel from the working 34970A also got a faint when connecting to this unit.

After some measurement it turned out that the filament voltage was faulty and not at the level it shall be. Firstly, the controlling Op-Amp (LM393D) was replaced but with no improvements. Secondly the dual MOSFET, MMDF1NO5E was replaced to an IRF7303. (See attached picture from schematics.)
This last change made the VFD bright and clear.

This problem was also an easy fix.
I don’t know if there are other 34970A with faint display that can be fixed in this way. But it is always worth a try.

[coromonadalix]

you may have got a more recent vfd ....     since  the problems was not vfd wear ...