Electronics > Repair
HP 34401a DMM with leaking segments
DC1MC:
No display has such a thin thread that will not take 1mA.
floobydust:
If a row of segments stays lit with the VFD pin lifted, then there has to be some leakage current.
I don't know of any corrosion or ion migration mechanisms here, occurring inside a VFD. Only a gassy (some air ingress) tube shows leakage currents, the Getter can sometimes show that.
Instead, it might be the VFD stray capacitance to adjacent segment (internal) traces, that you are seeing with the pin lifted, enough to light a segment.
Normally the driver IC has weak current-sink ability to discharge the anode (capacitance and gas leakage current), and a small time delay with the mux driver between selecting digits, to prevent ghosting.
HighVoltage:
You can test a VFD manually by applying power to the VFD and then apply a voltage to each segment and look what is lighting up.
bitseeker:
--- Quote from: floobydust on May 08, 2018, 04:54:44 am ---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.
...
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.
--- End quote ---
In the long run, though, it may still be beneficial. As you said, with parts going obsolete, even if the price is the same for the VFD, it eventually may not be an option. On the other hand, with a custom front panel controller, one could use other available VFD, such as dot matrix, as well as other display technologies. Anyway, interesting stuff to explore.
qu1ck:
Update:
With a strong pull down (2.7k) I got the ghost segments to finally disappear. But I did it on the disconnected pin, I don't want to try it with pin connected to driver as well, I'm not sure the driver will take it and I don't want to kill the front panel driver because it does input processing too.
On the other front:
It's just a proof of concept at this stage. It doesn't decode lower symbols, it skips bytes sometimes and shows gibberish, but it kinda works.
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