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Fluke 8840A/AF 8842A VFD Display Replacement

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bdunham7:
Thanks for the thoughts.  I've seen the previous attempts to make replacement displays for these meters and I have two additional goals for the project.

The first is to keep it simple and in keeping with the original design--so no MCU/LCD type solutions.  The original VFD is driven by 3 NE594 drivers, all powered from the +30V regulated supply.  A VFD drives both the segments and the digit select lines positive to turn the display on.  Obviously an LED display needs one of those to be negative.  I figured out that I could  replace the digit select NE594 with a ULN2803A, which is pin-for-pin compatible but switches the outputs to ground instead of V+.  This worked on a breadboard and the remaining NE594 drivers are stout enough to drive the LEDs at 15mA (so far) but the current was too high for the +30V supply.  So the fix for that was to nip the V+ leads of the two NE594s and bodge them over to the pre-regulator side of the +5V supply, which is a bit over 10 volts and can supply plenty of current.  That's all so simple and works so well that I just can't see going for a more sophisticated approach.

The second was to make a display that is as good or better than the original VFD--and that's not easy because a good unfaded VFD on these looks fantastic.  There's no way to replicate that VFD color, so I tried some other colors.  I know a lot of people think the blue LEDs look good, but I find them difficult to focus on and bothersome to the eye in the long term.  I could not find decent green versions, so I got these AVAGO deep red models and they look great with better visibility than the original VFD, including the maximum viewing angle.  My remaining challenge is to get the rest of the display to look as good as the digits.  I want to avoid the 'light through a hole' appearance and any hint of bleed-through or shadows.  I'm going to use a laser-cut mask in a 1.0 or 1.5mm ABS black sheet, but what would really finish the job is if I could find a hot glue or resin or something that matched the milky-white appearance and properties of the material used in the digits themselves.

The prototypes function perfectly so if I can work out the appearance and layout issues, I'll make these if anyone wants them.  The only issue I see is that I don't think my version will be cheap, so I'm not sure if there is more demand for a thrifted version at a low price that works but just looks OK or for a great looking one that costs more.

bdunham7:
So I drew up the annunciator mask and just printed it on paper with a laser printer for now to test it out.    One issue is that in order form me to get it lasered into a thin black plastic sheet, I have to use a line stencil font.  The same issue occurs if you use a PCB with text cut out.  I really would like to find a two-layer acrylic like they use for making placards where the laser or CNC just removes the top layer.  I would need black-on-translucent and I simply cannot find it anywhere.  I think it I'd be able to make better looking letters, but perhaps what I have is good enough?  Or is there some other process, like screen printing on a translucent material that might work for this?

H713:
If you ever get this project finished, I think there are a lot of of us with 8840s / 8842s with dim VFDs who would love a replacement option.

robert.rozee:
personally, i'd like to see a small adaptor board that plugs in between the main PCB and the front panel that handles the level translation without any need to modify/change any parts on the main PCB. this is simply because many owners likely do not have the skills necessary to carry out such modifications.

the display section looks extremely good, with the only real difficulty being the 'hand crafted' annunciators. my personal preference would be to:
- have a single 7-segment module dedicated to the '-' sign, and drive this with the inverted vertical bar of the '+'. this is far simpler than trying to build a '+' out of several separate LEDs.
- use DIP bar graphs (commonly used for VU meters) for the annunciators, arranged vertically, one at each end of the display window. the user can then choose to either just write the functions beside the bars externally, or produce a mask with the text labels to be fitted in front of each bar.


just my opinion, mind you!

cheers,
rob   :-)

robert.rozee:
looks like someone on ebay is selling a complete solution, with no need to swap the drivers:

https://www.ebay.com/itm/154545708329
"LED Display Module FLUKE 8840A/42A (Diy Kit) replace broken, damaged, etc. VFD"

cheers,
rob   :-)

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