Products > Test Equipment
Agilent 34401A VFD replacement
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Veteran68:
Thanks all for the responses!

I like the idea of an OLED hack but I'd need to see some trustworthy reviews to risk such an invasive mod.

When I get home I'll dig a little deeper into what specific revision I have.

Yes, the VFD is still plenty bright, and as far as I can tell it's still reading normally. I have had occasion to misread at a glance and have to do a double-take to realize there were extra segments. Not the end of the world, just annoying. Especially to my OCD.

If this proves too difficult or risky, I'll just live with it as long as it doesn't get worse.
coromonadalix:
the vfd  will eventually deteriorate over the years  ... gradually dim ...  etc ...  it's a sure fact

Same problems for many vfd based instruments, since some of them are powered for very long periods ....


There was a project here for a remote display for the 34401a,

https://www.eevblog.com/forum/testgear/external-display-for-agilent-34401a-(or-any-dmm-with-rs232-output-stream)/25/

You put the 34401a  in talk mode over serial port  and use an external display  to help visualize the meter ???   it could be more practical for you ???

It depend of your electronic knowledge / experience(s)  you can add anything for a display  with arduino's

I tried it  with an 4x40 display  loll   but it is kinda a bit slow refresh rates  vs the VFD ...  you have some arduino libraries to double de digits sizes over 2 characters lines

But enough for normal use,  @CellularMitosis  give the arduino code


pqass:
GOOD NEWS..... 
I was reading this thread https://www.eevblog.com/forum/repair/hp-34401a-dmm-with-leaking-segments/msg2690097/#msg2690097  and saw the attached image (see below).

Notice the VFD driver is a HV518PJ which is still available from digikey!   The function and pinout appears to be the same as the original SN75518FN.
See: https://www.digikey.com/en/products/detail/microchip-technology/HV518PJ-G/4902550

wasedadoc:

--- Quote from: coromonadalix on November 29, 2022, 03:44:00 pm ---the vfd  will eventually deteriorate over the years  ... gradually dim ...  etc ...  it's a sure fact

--- End quote ---
Don't count on that not applying to OLEDs too.
Veteran68:

--- Quote from: coromonadalix on November 29, 2022, 03:44:00 pm ---the vfd  will eventually deteriorate over the years  ... gradually dim ...  etc ...  it's a sure fact
--- End quote ---
Of course. I realize that, but the issue with mine isn't dimming. As you can see in the photo, mine is still very bright. It just has these random spurious segments that come and go. And when they appear, they seem to be brighter than surrounding segments, as if they're being overdriven (or driven by a short).

This meter looks to have been very lightly used, as it was in largely pristine condition when I bought it 2-3 years ago and it's seen very light use since then. So I believe the VFD itself still has a lot of life left in it, particularly as it seems from the responses and further research that this issue is likely not a VFD issue, but a driver or supporting component (bad cap?) issue.


--- Quote from: pqass on November 29, 2022, 03:57:24 pm ---GOOD NEWS..... 
I was reading this thread https://www.eevblog.com/forum/repair/hp-34401a-dmm-with-leaking-segments/msg2690097/#msg2690097  and saw the attached image (see below).

Notice the VFD driver is a HV518PJ which is still available from digikey!   The function and pinout appears to be the same as the original SN75518FN.
See: https://www.digikey.com/en/products/detail/microchip-technology/HV518PJ-G/4902550
--- End quote ---
Thanks for the follow up! I'm at work right now but am gambling on that being a compatible driver for mine. Since I had some stuff in my DigiKey cart already that I hadn't pulled the trigger on yet, I just added one before placing the order. If it's not the issue, it's only 7 bucks, and I'll toss it in the spare parts bin for a potential future project. :)
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