Electronics > Repair
HP 34401a DMM with leaking segments
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bitseeker:
@qu1ck: Good to see that it's SPI. Substituting the VFD is still an interesting solution in the long run.

@floobydust: Interesting. I hadn't seen before that replacing the VFD fixed the phantom dots.
floobydust:
This thread, member says it's the display: https://www.eevblog.com/forum/testgear/hp34401a-display-with-dots-after-every-digit/msg1228322/#msg1228322
This thread, member says it's the driver MCU: https://www.eevblog.com/forum/buysellwanted/ebay-many-faulty-hpagilent-34401a-multimers/msg811170/#msg811170


I was looking at what's involved making an OLED conversion, as this big VFD and driver scheme was used (at one time) across the HP/Agilent product line and is going obsolete for parts.

There are a few flavours of VFD's with special annunciators being the difference.
Universal counters: HP-58503, 53131A, 53132A, 53181A
Power Supplies: E3631A, E3649A, E3642A, E3643A etc.

It looks not bad to convert, but a big OLED display is as expensive as a replacement VFD ~$60 so I didn't think there would be interest.
qu1ck:
@floobydust
In my case it's 100% the display. When I disconnected that segment in my experiment it was still lighting up, with the pin flapping in the breeze. And driver was sending perfect square wave +-18v as expected, very different from what I was seeing on the scope before (see my first post). Connecting my bjt output stage which is similar to your schematic with fets did absolutely nothing.

You don't have to buy 5" oled, it won't even fit because it is higher. 3.2" oled is $30, add an mcu board and some mounting hardware, that's $35 tops. Half the price of a VFD, much more value in terms of longevity and repairability. I bet you can even make the digits the same size, just make the font more compact, VFD wastes a lot of space.
DC1MC:
@qu1ck:

IMHO I will still try first an "analogue repair", the segment has to have a cut-off voltage somewhere and if there is a little drive current more, it wont hurt. But if you try to go on the OLED display route, the message structure could be simpler maybe, but is hard to think how. The values are in nice ASCII and the non-printable chars have most likely the bit fields with the status of different aux indicators around. I can't think of a controller that will not be able to decode this message. What is the clock frequency, I'm too lazy to launch Sigrok now ?

 Cheers,
 DC1MC
 
qu1ck:
@DC1MC

The thing is to shut the segment off you just have to put it at low potential, there is no current. At least that is my understanding of how VFD works. Both grid and segment have to be at positive voltage relative to filament for segment to light up. If one or the other has low potential electrons won't flow to the segment.

Since in my display driving one segment high also induces high potential on another pin it leads me to believe there is some short in the tube itself. I don't want to force that segment low, that might increase the current significantly and those thin threads connecting the dots won't take much.

Maybe someone with old display that had similar issue will try it but I'm reluctant, at least until I have alternative solution in place.

SPI clock is around 93khz. Any controller will be able to decode, but you need something with at least full 8 pin port to quickly push image to the 8080 oled display. Blue pill stm32f103 is super cheap and has plenty of pins, I will likely use that one. Arduino pro mini should work as well and won't require level shifters.
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