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Agilent 34401A VFD replacement
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coromonadalix:
do the short display test



Hold shift and power up the meter and release shift,  it will briefly light all the vfd sections ..
Veteran68:
Ok, so further observations of interest. Now that it's been powered up for over 2.5 hours, those leaky extra segments have gone, come back for a few minutes, and are now gone again. But when gone, multiple segments appear "hot" or over bright, particularly the slant segments both top and bottom, but only while in the menu. At the DC voltage measurement mode, for example, brightness is consistent across all segments including the slants in the V in VDC.

Looks like some thermal variation causing segment lines initially shorted to break contact, and voltage or current to fluctuate. That or the brightness variations are just a characteristic of an aging VFD, but not sure why only when in the menu. Maybe because the menu display is static vs refreshing like in measurement mode? The brightness difference doesn't show as significantly on camera as it does in person, but this pic captures some of it. Notice how bright the slant segments on the R and V are:




--- Quote from: robert.rozee on November 30, 2022, 02:02:23 am ---hmmm... when one lower diagonal is on, so is the other. looks like it could be a short. do you see ANY characters displayed where one lower diagonal is ON, while the other lower diagonal is OFF?
--- End quote ---
Nope. When it happens, both lower slanted segments show whenever either one should be displayed. I agree it looks like a short on those segment lines.


--- Quote from: robert.rozee on November 30, 2022, 02:02:23 am ---i'd be checking the display PCB for the dreaded tin dendrils. these are fine whiskers of tin that can grow between electrical contacts, developing over a period of many years. microscopically thin, but can grow up to half an inch or so long.
--- End quote ---
That's an interesting notion I hadn't considered. If such a thing occurs over time, it would certainly make sense in a 20 year old instrument. And with such a delicate short, thermal expansion could easily break the short.

I have a sneaking suspicion it's a faulty component or driver IC, but I will look at everything suggested and will give it a good brushing just to cover the bases.


--- Quote from: coromonadalix on November 30, 2022, 02:07:34 am ---do the short display test

Hold shift and power up the meter and release shift,  it will briefly light all the vfd sections ..

--- End quote ---
Yeah I've done the tests. All VFD segments light up, but that's to be expected -- I'm not having any issues with segments NOT displaying, just those displaying when they shouldn't.
coromonadalix:
you could hold for a long period the shift button and see over a long long time  how the vfd would react, it would be the maximum stress given to it ?

once again i have my doubts over the vfd drivers,    i had an 22 ish  year old meter, no dendrils  but the unobtanium driver ic  with some of the symptoms

and put an aliexpress clone  and everything went fine  .. the vfd as you know  have filaments, plates, heater  etc ...  it can create deposits inside the vfd

You have some trick like overdriving a bit the filaments to clear some internal residues   ...


another thread of vfd  here

https://www.eevblog.com/forum/repair/vfd-unwanted-segments-glowing/

some even tried this

https://www.eevblog.com/forum/chat/vacuum-fluorescent-display-rejuvenation/
james_s:
Your VFD looks great, I really don't think that's the problem, so you already have the hard part taken care of. I suspect a problem with either the driver or the digital signal to the driver, maybe as simple as a bad connection. As far as displays go, VFD is one of the most robust and longest lived there is, second maybe to LED but some of those are not as reliable as you'd expect.

OLED displays look great as far as modern display tech goes, but they have a spotty reliability record, there are many cases of failing OLED displays.
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